sail aerodynamics [Archive] - Boat Design Forums

brian eiland

01-01-2003, 01:59 PM

....while we're at it Erik, don't get sucked into the old slot theory explanation

Subject: How Sails Work, the slot effect

I noted that this forum (another one) has had quite a bit of discussion recently of “lee side air speed”. Well, I couldn’t resist bringing up the closely related, and also much maligned subject of ‘air flow thru the slot’ created by the mainsail and the headsail.

I’ve excerpted a portion of a proposal cover letter I wrote most recently….

[Now look at the rig’s aerodynamic configuration. We’ve had many years of controversy over the flow of air in the slot between the jib and the mainsail, and many incorrect explanations. We now know irrefutably that the flow between these two sails is slowed rather than speeded up, and that results in a higher pressure on lee side of the jib and on the windward side of the main; ie, the jib’s drive is improved, and the main’s drive is degraded!! Put another way, the mainsail provides an upwash for the jib that makes the jib both more efficient and able to point higher (its operating in the safe leeward position). The jib meanwhile creates a downwash on the mainsail that decreases its efficiency. THEN WHY do we continue to make the mainsail bigger than the jib??? As if this wasn’t enough, we hide the mainsail behind a mast, and we don’t hoist it at the most favorable angle to help the jib.]
[I’ve reversed the size of these sails in proportion to their relative efficiencies. The parallel nature of the slot between my mainstaysail & its jib/genoa should further enhance the supporting role of the main and….. Still questioning the efficiency of the traditional mainsail? Look at all of the past America’s Cup boats on which $millions have been spent on sail research; going up wind you often detect huge areas of the sail right in the prime draft zones that are right on the edge of ‘backwinding’. These ‘soft’ areas are certainly not providing any significant driving force (not very efficient).]

Now lets see how many people are going to tell me I’m wrong-- that like a restricted water hose, the air is speeded up in the slot. This is another of those axioms that the textbooks have got wrong and have taught us wrong for so many years. But as Tom Speer noted, “there’s no way to finally put a stake through the heart of that old explanation—it just keeps coming back to life”.

More recently I ran across a news article in the Sept issue of Seahorse magazine which discusses the very interesting full scale prototyping work being carried out on a J-90 class boat by Eric Hall of Hall Spars. Eric is now on his third-generation, free standing ,carbon wing rotating mast, with a una-rig mainsail. His “ thought process (and maybe not entirely logical) was: If biplanes became monoplanes and monoplane wings shed wires, why not an unstayed una-rig upwind” Boy, you would surely think this was the ideal upwind rig. In responding to an inquiry on upwind performance, Eric responds, “ first, of course, the boat would be improved upwind with a No.1 jib. Generally, we could not point as high as the others here (Block Island) and therefore had difficulty holding lanes.” He goes on to say, “this is a very interesting project that we especially want to succeed. I have been accused of a missionary zeal, which frankly keeps moving it along. It’s a real problem sometimes keeping focused on what we are trying to do in view of all that is ingrained in our minds about what makes sailboats work. Anyway we are having fun…

DJB

01-01-2003, 04:25 PM

Brian,

Your post confused me somewhat, with the phrase:

"the flow between these two sails is slowed rather than speeded up, and that results in a higher pressure on lee side of the jib and on the windward side of the main; ie, the jib’s drive is improved, and the main’s drive is degraded!"

Is this the wrong way round? A reduction in velocity of the flow in the slot will result in an increase of pressure on the WINDWARD side of the jib and on the LEEWARD side of the main (the two sides that face each other when the jib is overlapped). I agree that this results in the jib's drive being improved, and the main's degraded.

brian eiland

01-01-2003, 05:46 PM

Sorry about that mistake, you are exactly right. Thanks for bring that error to my attention. I had changed the order of listing the sails from one document to another and failed to change the windward/leeward designations correspondingly.

tspeer

01-02-2003, 12:58 AM

Originally posted by Guest
I am a bit confused with the explanations of how the sail actually pushes the boat forward. Do anyone have a good scientific explanation? Not just "The lower pressure on the leeward side sucks the sail forward".
/ErikW

The net forces on the boat are equal to the net change in momentum of the two fluids - that's conservation of momentum. The flow also has to satisfy conservation of mass and conservation of energy. Any description of what's happening in the flow has to relate these three conservation laws in a way that is consistent everywhere. What's cause and what's effect is largely a matter of what you know or control, and what you want to know. The whole picture has to hang together, regardless. So here's the big picture, and I'll fill in more details as I go along. I hope this doesn't get too long-winded.

The sail bends the wind sideways, aft and to windward of the apparent wind direction. The change in momentum of the wind results in a force on the sail that is oriented to the side and ahead of the beam. The hull/keel/board bends the water flow toward the leeward side, and the change in the water's momentum results in a force that is oriented to windward and aft. When there's a change in the force due to the wind, the boat accelerates (both sideways and ahead) until leeway angle and boat speed arrive at a hydrodynamic reaction force that just balances the aerodynamic force applied by the sails. That's Newton's law (F=m*a), which is another form of conservation of momentum (in this case, the boat's momentum).

If you want to model the forces for engineering purposes, there are a lot of other equivalent ways of looking at it, but the conservation laws are the basic principles.

For example, in order for the wind to bend, there has to be a decreasing pressure toward the inside of the curve because if each blob of air had the same pressure on each side of it, it would just go straight. This pressure gradient provides the centripetal force to change the direction of the wind's momentum. Far away from the sail, the pressure is the ambient pressure reported by the weatherman's barometer. As you approach the sail from the lee side, you're moving toward the inside of the curve, so the pressure is dropping and is at its lowest when you arrive at the lee side of the sail. If you started far away from the sail on the windward side, you again start with the barometric pressure and as you move toward the sail you're moving toward the outside of the turn of the wind, so the pressure has to be increasing; again due to the force required to accelerate the wind to the side (higher pressure on the outside, lower pressure on the inside of each blob of air). So when you arrive at the windward side of the sail, the pressure is higher than the barometer says. The high pressure on the windward side and the low pressure on the leeward side provides most of the force acting on the sail, and when you add up the pressure difference over each part of the sail and compare it with the forces needed to divert each blob of wind, you'll find the two are equal and consistent.

For most of the flow, there's no change in energy. So if you consider all the forms of energy - potential, represented by the pressure; kinetic, represented by the velocity; thermal, represented by the temperature, and rotational (like kinetic energy, but spinning) - they all have to add up to a constant. At the speeds we're dealing with in sailboats, there's not much change in temperature or density as a result of flowing around the sail, and except for the flow very near the sail individual blobs of the air don't rotate although they do travel in a curved path (think of people riding a merry-go-round but always facing North while they do it). So a good approximation of the energy in the flow is that the potential energy has to be a constant, and this constant is called total pressure. It's the pressure you get when the flow is brought to a halt, so that there is no kinetic energy and all the energy is in the pressure term. This is the highest pressure you can generate with a given amount of wind. Where the pressure is lower, the kinetic energy has to be higher to make up the balance. So low pressures mean high velocities and high pressures mean low velocities. This is where Bernoulli's law comes from - it's an approximation of the law of conservation of energy. But it's only good where the flow is not rotating or being heated/cooled, etc. Then you have to take into account more of the thermodynamics.

Ironically, one of the areas where the flow is rotational, and therefore Bernoulli's law isn't a good approximation, is in the flow right next to the sail. In this boundary layer, the fluid actually touching the sail doesn't move relative to the sail, and there's a rapid change in the velocity as you move out away from the surface. But there's very little change in pressure because some of the energy is going into rotational energy and some of it is being converted into heat, resulting in a loss of total pressure. So as the air in this boundary layer exits the leech of the sail, it's been slowed as well as had its direction changed, and this loss of momentum is the skin friction acting on the sail. But since the pressure doesn't change very much across the boundary layer, the flow acts much like it would about a body that had no boundary layer at all, but whose shape was like that of the sail plus the boundary layer thickness.

Because the boundary layer effectively changes the shape, almost everything else about sail trim and sail shape is all about keeping the boundary layer thin so that effective shape of the sail is much like the physical shape. When you put telltales on the sail, you're looking to see what's happening in the boundary layer. If the yarn lays back flat, then the flow in the boundary layer is smoothly following the sail's shape, and the air is being bent the way you want it to and you're getting the desired force on the sail. If the yarn is going nuts or pointing foward, the boundary layer is doing something quite different, and you have to go some distance out from the sail to get into the outer flow - so the effective shape doesn't look anything like the physical shape of the sail. This is what happens when the sail stalls and the effective shape looks like a big fat wedge whose lee side isn't bending the wind near as much as you'd like, so the lift (component of the force at right angles to the apparent wind) is much less and the drag (component of the force parallel to the apparent wind) is excessive. The combination of the two is smaller and points aft, robbing the boat of the drive you'd like to see.

I think I'll stop there. If this makes sense to you, I can go into more detail on the care and feeding of the boundary layer.

01-04-2003, 12:07 AM

While we are on the subject of old theories that didn't pan out like the slot theory (and the flat earth theory), I would like to get other peoples opinions on some other theories that really bother me. The more I read the more irritated I get with some of the vortex and circulation theories I hear.
The Helmhold vortex theorem states that "vorticies end on boundries or form a closed path". This theory is just plain wrong. A funnel cloud is a vortex that does not end on a boundry or form a closed loop. The vortex I create with a boat paddle does not extend to the bottom of the lake. I know that most text state that this theorem can be proven mathematically, but all you can prove is that the mathematical model you have chosen to represent the vortex does not accurately discribe the phenomena. From this misguided theory came another bizarre theory that "the circulation around the wing, the vortex shed off the wing tips and the start vortex back on the ground are a closed path therefore allowing lift to be generated. PUT DOWN THE CRACK PIPE. The start vortex has nothing to do with lift. What would hapen if a seagull flew through the start vortex after the plane was 1000ft in the air. Would the plane fall to the ground because the closed loop was broken?
But we aren't done with the stupid theories yet. The Kutta-Joukowski hypothesis about the start vortex acting like a gear to start a circulation around the wing that creates lift. These guys are some real glue sniffers. There is no real circulation around a wing. I know that by subtracting the average velocity vector it then looks like there is circulation, but that same trick can be used to argue that the cars in the right lane of the interstate are going backwards. The cars in the right lane aren't really going backwards and there isn't really any circulation just upwash down wash and a slight difference in velocity above and below the wing. I would venture to say that never in the history of aviation has a single particle ever traveled a circular path around a wing. Even if the start vortex did create this imaginary circulation, once the start vortex is shed what would sustain the circulation? is it some sort of pepetual motion?
I guess I'm just a slow learner but I have trouble accepting ideas that contradict all common sense (and sometime the laws of physics). Does anyone out there also have trouble believing the earth is flat or vortices are infinite.

tspeer

01-04-2003, 05:03 PM

Originally posted by Guest
...The Helmhold vortex theorem states that "vorticies end on boundries or form a closed path". This theory is just plain wrong. A funnel cloud is a vortex that does not end on a boundry or form a closed loop. The vortex I create with a boat paddle does not extend to the bottom of the lake. ...

I agree that any description of fluid mechanics has to agree with what we all observe in practice. However, the vortex theories with which you take issue have been well substantiated by experiment and form the basis of most practical fluid dynamic engineering calculations. I respect your legitimate questions, and my appologies to the rest of the board if I'm responding to an obvious troll.

When I was in college, one of the professors, Dr. C. T. Tsu, was doing research into tornadoes. He had made an experimental rig with a blower that exhausted through a duct with a spinning honeycomb disk inside, followed by an exquistely machined plexiglass nozzle. Below the vertically oriented nozzle was a ground board that could be moved up and down to vary the distance to the nozzle. To make a long story short, he found that he didn't need the nozzle, and he didn't even need the blower - all that was necessary was the rotating disk. It turns out tornadoes are a boundary layer phenomenon, and all you need is a rotating air mass in close proximity to a surface. That's why so much of the damage in hurricanes is actually due to tornadoes spawned by the hurricane's rotation. His rig produced tiny little tornadoes that had all the characteristics of the real thing, including the sheath at the bottom where debris (ground walnut shells, for the model) is picked up. So it's not correct to say that the tornado doesn't obey Helmholz' laws. One end does end at the boundary to the fluid and the other end is the concentration of the vorticity already contained in the airmass. You need both - the surface and the rotation. If the surface is too far away from the rotating airmass you don't get tornadoes. If this still seems to contradict the Helmholtz theorem, remember that the theorem specifically assumed an irrotational fluid in which the net vorticity had to add up to zero. That assumption is violated by the conditions that spawn tornadoes, just as it is violated inside the boundary layer of the wind flowing around a sail.

As for the paddle, the two vortices you see at the surface are in fact the ends of one vortex that goes down the side of the paddle, wraps around the tip, and comes up the other side. Prandtl even used the analogy of the flow about a downward-moving paddle to describe the vortex wake left by a wing:

"Instead of a downward acceleration of the wing itself in its various consecutive positions we shall consider an instantenous acceleration (impulse) of the whole surface of separation [meaning the depressed wake behind the wing - TS]. In other words, this surface of separation is momentarily solidified into a 'board,' and this board is given a downward impulse (In order to take care of the variation of w1 with x, the 'board' may be considered elastic)." (Prandtl, L.and Tietjens, O. G., "Applied Hydro- and Aeromechanics," Dover Publications, Inc., NY, 1934.Section 111, pp. 192)

So, you're right - the vortices don't go all the way to the bottom of the lake. They are a continuous loop and only have ends at the boundary of the fluid, just as the Helmholtz theory says they must.

...There is no real circulation around a wing. I know that by subtracting the average velocity vector it then looks like there is circulation, but that same trick can be used to argue that the cars in the right lane of the interstate are going backwards. The cars in the right lane aren't really going backwards and there isn't really any circulation just upwash down wash and a slight difference in velocity above and below the wing...

I think it would be perfectly valid to describe automobile traffic by a mean velocity and a perturbation, both positive and negative, with respect to that mean velocity. So relative to the mean flow, yes, some cars would be moving backward and some moving forward. That doesn't mean that their total speed is negative any more than it does for the flow on the windward side of a sail.

The vortices used in the Helmholtz theorem are one solution to the approximation of the flow phyics that result from assuming an inviscid fluid. It's one component used to describe the flowfield by superposition, in addition to a uniform flow and sources/sinks. The circulation about a wing and the Kutta condition used to determine its value are really just approximations used to model the vorticity generated in the boundary layer. By combining all three of these elements, one can put together a very realistic approximation to the flow that accounts quite well for lift and induced drag.

Presumably your argument about interrupting the starting vortex means that the entire vortex structure would have to fall apart. But that belies a misunderstanding of the starting vortex. First of all, there isn't a large, distinct starting vortex in most cases because the lift comes on gradually, generating a diffuse vortex sheet instead of the single large vortex of an idealized impulsive startup. And this diffuse sheet decays away because of the viscosity in the air. Large vortices also spawn smaller ones through instabilities in the flow. As Lewis Richardson put it,

Big whorls have little whorls,
Which feed on their velocity,
And little whorls have lesser whorls,
And so on to viscosity.

(see, for example, http://mixing.oce.orst.edu/people/jmoum/courseinfo/3Dturbulence.pdf)

Second, under the assumption of an inviscid irrotational fluid, the flow about your proverbial seagull would be the sum of both the flow about the seagull in the absence of the vortex and the vortex itself, so the seagull would not destroy the vortex just by flying through it. The seagull would experience quite a jolt going through the core, but the lifting surface that generated the vortex would not be affected.

...Does anyone out there also have trouble believing the earth is flat or vortices are infinite.

The infinite vortices are just an approximation that simplify the flow physics so to be able to make decent engineering calculations with a reasonable effort. Just like most of us function quite well using the flat earth approximation for our maps and just about everything we do in everyday life. It's only if we have to concern ourselves with more extreme situations like orbital mechanics or the difference between the rhumb line and great circle route of a trans-oceanic crossing that the flat earth assumption proves inadequate and we have to assume a round earth. Which is itself an approximation that doesn't give good enough results in even more demanding situations.

brian eiland

01-05-2003, 02:21 PM

Erik wrote:
I am a bit confused with the explanations of how the sail actually pushes the boat forward. Do anyone have a good scientific explanation? Not just "The lower pressure on the leeward side sucks the sail forward".
/ErikW
_______________________________________
I would encourage anyone interested in this subject, and that is looking for a simplified explanation of 'lift' to read these two articles:

1) Taking Flight, New Scientist magazine 5 May 2001

http://www.steamradio.com/pipermail/multihulls/2001-June/003583.html

2) The Physics of Airplanes---Why We Go Up, Discover magazine Apr 2001

http://www.discover.com/apr_01/featphysics.html

Enjoy, I did, Brian

sailwave

08-07-2003, 03:27 PM

Hi Brian,

Put another way, the mainsail provides an upwash for the jib that makes the jib both more efficient and able to point higher

Can you expand on this a bit. I'm reading you as saying that a sloop rig can point higher than the equivalent uni-rig... Small cat context.

Regards,
Colin
www.sailwave.com

brian eiland

08-08-2003, 01:21 AM

Colin wrote: I'm reading you as saying that a sloop rig can point higher than the equivalent uni-rig

Yes, the very existance of the two sails means there exist a slot between them. This slot will normally be of such a nature as to restrict a full flow between them; so it diverts some of this flow around the outer sides of the two sails. If one looks at the fow patterns of this diverted flow, it is termed an upwash on the leading sail.....this upwash is a slight change in the incident angle of attack of the leading sail, or in other words the sail can be pointed slightly higher into this 'new wind direction' that it sees.

The trailing sail (mainsail) experiences a downwash which makes it more difficult for it to point as high. And in some cases can 'appear' as backwinded.

Is that understandable? or should I try a different wording?

sailwave

08-08-2003, 03:58 AM

Hi Brian,

Is that understandable? or should I try a different wording?

Thanks very much for the reply; I think I see the argument...

But just to clarify: consider a uni-rig sailing up-wind such that at that angle its VMG is better than any other angle. Now consider that same boat as a sloop also sailing up-wind at an angle that maximises its VMG. In the right circumstances (sea-state, tuning, etc) it's possible that the sloop is pointing higher than the uni-rig.

And the reason is that the advantages provided to the jib by the angle of the upwash from the main (and high pressure from the windward side of the main?) outweigh the disadvantages provided to the main by the downwash of the jib.

So what about the 'slot effect' everybody seems to assume in our club - i.e. that the jib accelerates air over the lee of the main, reducing pressure and increasing left. Is that happening as well further away from the sails?

I've seen the "slot effect" referred to as a "myth" recently but folk seem to sail fast by using it as a mental model... Is it a myth?

Please excuse my ignorance here; two weeks ago I looked on-line to confirm my understanding about how aero/airfoils + slot worked and got hit by a maelstrom of different arguments; all of which I suspect have an element of truth to them...

Regards,
Colin
www.sailwave.com

Stephen Ditmore

08-08-2003, 04:33 AM

gonzo

08-08-2003, 10:37 AM

The problem with many theories, is that the proponents of them refuse to observe reality. Seems that the theorethical "proof" takes the place of experimenting. If the Wright brothers had payed attention to the scientists of the time, we'd still be ground bound. I grew up listening to the wind slot theories, but they didn't make sense to me. However, just look at the telltales on you leeward shrouds. They don't show more speed. Sit between the jib and the main, and the wind feels less not more. I agree there is a modification of flow, and it helps to point higher. It makes the air flow separation on the leeward side of the main shift aft.

tspeer

08-08-2003, 06:03 PM

Yes indeed. So many aerodynamic explanations are like one of Kipling's "Just-So Stories".

The best explanation I've seen of high-lift aerodynamics and the interaction between multiple surfaces is A.M.O Smith's 1975 Wright Brother's Lecture, "High Lift Aerodynamics", AIAA Journal of Aircraft. (Don't have the precise reference handy- Sept. if I recall correctly)

brian eiland

08-09-2003, 11:06 AM

Dear Sailwave (Colin),
Don't try to mix in the VMG factor with the aerodynamics of the slot. VMG is dependent on many other factors including the hull characteristics.

Colin wrote:
So what about the 'slot effect' everybody seems to assume in our club - i.e. that the jib accelerates air over the lee of the main, reducing pressure and increasing left. Is that happening as well further away from the sails?

Brian responded:
No, the air is not speeded up in the slot (on the lee side of the main). The air is speeded up outboard of the sail-plan (the lee side of the headsail). This speeded up air on the lee side of the headsail must eventually slow down to the free stream conditions as it leaves the sail-plan as a whole...but trhats another topic that Tom Speer can tell you much about.

I'll repeat an excerpt from my previous posting,"....that like a restricted water hose, the air is speeded up in the slot. This is another of those axioms that the textbooks have got wrong and have taught us wrong for so many years. But as Tom Speer noted, “there’s no way to finally put a stake through the heart of that old explanation—it just keeps coming back to life”.

I was in a library in Annapolis the other day, and decided to take a quick look thru a few text books to view their explantions of the slot effect. I was surprised to find that a few of the 'older' books did now have it right. The book, "Sail Power" by Wallace Ross is one that I would recommend to anyone seeking the basic knowledge of how sails work. It is very straight forward without a lot of technicalities, and it presents some nice simply graphics to accompony the text. And it presents a lot of pratical information on trimming sails....real nice reference to have around.

sailwave

08-09-2003, 11:23 AM

Thanks, Brian (et al) for your replies; I'll try and get hold of the book you recommended...

Regards,
Colin
www.sailwave.com

Guest

09-04-2003, 02:34 PM

I think that mainsails are usually cut to work with overlapping sails, and so aren't at their best alone. This might lead some to believe in the slot effect, as the main does better when there is a genny working with it.

A Guest

09-07-2003, 10:14 PM

Brian, re

THEN WHY do we continue to make the mainsail bigger than the jib??? As if this wasn’t enough, we hide the mainsail behind a mast, and we don’t hoist it at the most favorable angle to help the jib.]

What about the fact that many IOR boats until the '70s and '80s
had very tall, high aspect mains and masthead genoas? They were rarely vastly faster than the fractional rigs. The trend, of course, moved to fractional rigs, but we did NOT "continue" to make mains bigger, it was a trend because the fractional went faster - DESPITE the fact that they were penalised in some ways.

In some instances (ie Dubois 42s Police Car / Winsome Gold; S&S Prospect of Whitby 1973 / Saudade AFAIK; Hawkfarm 1/2 tonners; Holland 40s of the early '80s (Swuzzlebubble/Regardless/Spritzer/Flirt etc, the J/36 and J/35, the J/29) the same (or almost identical) hull was used with fractional and masthead (small main) rigs, and there was rarely - in fact almost never - a vast difference between the two. This was despite the fact that for an equal rated sail area, the big-main fractionals had less physical sail area. So are we not talking about very small fractions of efficiency?

Re

"Still questioning the efficiency of the traditional mainsail? Look at all of the past America’s Cup boats on which $millions have been spent on sail research; going up wind you often detect huge areas of the sail right in the prime draft zones that are right on the edge of ‘backwinding’. These ‘soft’ areas are certainly not providing any significant driving force (not very efficient).] "

When you say "often", isn't it because in the situation you highlight, the boats are depowering? They don't need extra driving force, they are already suffering from too much.

How many % are we talking about with the drag of a conventional mast? Wing masts have been tried here (with fat-head, fully battened mains) since at least 1958-ish. They only survive on fast cats (where gust response is less important, the boats are easy to sail, and low drag is vital) and dinghy classes that are easy to handle and have small rigs (Tasars, MG 14, NS 14) where the efficiency gain is vital; in other boats (12' skiffs, old Gwen 12s, R Class skiffs, 18 foot skiffs) the wing mast has been discarded although development in one case continues.

Guest

09-13-2003, 04:55 AM

I have been asked this question ad infinitum, Hold your curved arm out. Turn facetothewind, then turn 90degrees. During this, you will experience a law, best explained by bernoulli - that the inner will be negatively pressurised wrt the outer. Why complicate the deal with experience? I have sailed (& studied physics) for almost 30 yrs without needing to analyse the proceudre.

Slot - talk, etc is only that. There is vast experience to be gained just by getting out there, & doing it...

Schoonertack

10-31-2003, 07:30 PM

I can not resist throwing something into this. Bernouli? I think the most basic statement of wing lift that I have heard is , it is not suck and blow, but a deflection of a mass of air equal to the lift. In short a plane vectors air down to generate lift equal to its own weight, airfoils are the most effective way to do this. a sailboat redirects the surface flow ( their is a boundary called water) to generate thrust. To really put a spin on this I have read that below 30 miles an hour slots and gaps in airfoil are ineffective, the air regards them as closed.

SailDesign

10-31-2003, 10:36 PM

Schoonertack, you can't explain upwind sailing by that analogy. It just isn't possible, so there has to be more to it. Bernoulli may not be a perfect explanation, but he works well enough to explain the phenomenon.
Steve

Schoonertack

10-31-2003, 11:17 PM

Maybe not the best, but boats still sail up wind, might be touchand go at 30 degrees true but at 40 to 45 works nicely.

tspeer

11-01-2003, 12:35 AM

Please - if you want to understand the interaction between main and jib, go to an engineering library and look up:

Smith, A. M. O., "High Lift Aerodynamics", AIAA Journal of Aircraft, Vol. 12, No. 6, June, 1975, pp. 501-530.

Invoking "Bernoulli" is the same as saying "energy is conserved". It is meaningless as far as getting more lift or less drag out of a given amount of area is concerned, or understanding what's going on between main and jib. What matters is the care and feeding of the boundary layer. Without paying attention to what's happening in the boundary layer, any explanation of aerodynamics one puts forth is not much better than one of Kipling's "Just So Stories".

Schoonertack

11-01-2003, 01:09 AM

Tspeer; your absolutly right, and even in Marajah I was wondering what the different Reynolds numbers are for each rig a schooners inefficiency could easily be described as short cord multy panel with no boundary layer at all. I think one big sail does better than multipanel aerofoils mainly because of boundary layer preservation, all flow not being laminar.after the seperation point dependant on reynolds number.

tspeer

11-01-2003, 01:47 PM

For lifting surfaces that have a moderate to high aspect ratio, it's a good approximation to break the 3-dimensional aerodynamics into two two-dimensional problems - one that's a cross section through the surface in a plane that is perpendicular to the span, and the other is a cross section of the wake taken perpendicular to the apparent wind ( http://www.tspeer.com/Planforms/Fig01.gif ). The boundary layer is largely controlled by the pressure distribution around the section taken through the surface, and from this you get the maximum lift and the parasite drag. The cross section through the wake gives the induced drag and the lift distribution along the span.

I think the surfaces on a schooner are sufficiently far apart that there's not that much interaction between the two-dimensional sections. Instead, I think you have to take into account the wake shed by the sails and the local changes in angle of attack induced on one rig by the other. It's a 3D wing & induced drag problem, not a 2D section problem.

BTW, our sail rigs are low enough in aspect ratio that the approximation above isn't all that good. You really need to consider the 2D section and 3D planform aspects together.

SuperPiper

11-02-2003, 06:57 AM

I suppose that the tension in the shrouds and the sheets is what drives the boat forward. Does the considerable downward mast load push the boat lower in the water?

Schoonertack

11-02-2003, 08:43 AM

only if the shrouds and stays don't pull her out

Ian

04-07-2004, 06:34 PM

I love this discussion! When I first started reading about sailing and aerodynamic at about the age of twelve, I found the standard explanation of lift very unconvincing. The bit about faster moving air producing a lower pressure and vice versa (with no explanation) seemed to me an obscure tautology. The venturi effect comparison seemed hightly suspect (the venturi effect, even as it occured in an actual venturi tube, was never explained). I independently came up with the force vector explanation offered by James R. Eventually I arrived at the more advanced but similar concept that is explained by tspeer. I was astounded and delighted to discover, years later in an article (it may have been the Discover Magazine one that Brain Eiland mentioned--I don't remember), that the standard explanation was equaly unsatisfactory to airospace engineers--and that they had also had it stuffed down their throats in introductory aerodynamics. I heard about the circulation (vortex) theory some years before that but never encountered an expanation until I read Tspeer's post just now! Thanks for a series of very informative posts!

Javelin

04-16-2004, 08:20 PM

I race a European Javelin, 17'6" Trapeze dinghy similar to a Flying Dutchman.
Getting the slot correct is vital on the Javelin, as I guess it is in most racing boats with overlapping jibs.
At last years European Championships a Dutch team were using a jib with a very fine entry, and they also seemed to be able to sheet their jib clew some three to four inches further inboard.
They pointed higher than anyone else and their boat speed was no slower than the rest of us so they simply left us for dead in a range of conditions from 5knots to 18knots of wind.
As it urned out they couldn't sail downwind for toffee and tactically they were nieve so they didn't win but it got us thinking.

So I bought a new fine entry jib and flatter main and still can't sheet inboard as much as they seemed to be able to without losing loads of boat speed.

How is it that they could sheet their jib so much more inboard than we can without exsessive backwinding the main given that the wind velocity entering the slot must be around the same.
Is there a critical slot width / apparrent wind speed, ratio?
I.e. at 10 knts wind velocity the slot must be 10" wide.

Schoonertack

04-17-2004, 12:15 PM

I am not there, but, the sail has two sides, entry and exit as you are thinking about it. So before you buy a new main to match the cut and CAMBER of your new jib, see if you can adjust your jib sheet angle to flatten the roach or leech of the jib and likewise when you are pointing that high flatten the back half of the main with outhaul and boom vang. let me know if that helps. As someone else said on this thread the big boats all sail with the main slightly backwinded by the jib/jenny/ etc.. This does not indicate air flow through the slot.

Javelin

04-17-2004, 08:12 PM

I see what you are driving at however what I am trying to define is how do you know when the slot is at optimum.
Using the tell tails helps set individual sails.
Using the leach tails on the Genoa and main helps get the twist and exhaust right.
Getting the slot right has largely been done by eye, dipping under the boom and adjusting the sheet lead to match the back of the main with the Genoa Leach.
The width of the slot is harder the set though.
What is the critical factor.
Do you narrow the distance between the Genoa leach and the main until the main just luffs or luffs a lot or until the lower Genoa leach tail starts going mad or etc etc.
Like I said tell tails are an easy indicator, what is the "easy" indicator for slot efficiency?
If we are getting drive because of the pressure differential then slowing the air flow down through the slot should increase the differential, however it is obvious that at some point you will go to far.
What is that point and how do you measure it?

Schoonertack

04-20-2004, 03:02 AM

Javelin, wish it was that easy. just like a sails camber depends on wind speed, the slot does too, and the only reliable way I have ever heard of dertermining all of this is match boat sailing and changing only one thing at a time. Which is the whole point of club sailing. The reason America's cup has become so expensive is that you really need two identical boats to see what combination is fastest on any given wind speed and heading. Years ago when I first started sailing I had a hard time understanding mainsail camber. the controlls are easy enough but why in light airs, little camber, increase for a little more breeze and then as wind speed picks up further continually flatten the leach? I have not heard of any reliable indicator of slot efficency, leech telltales flapping indicates you have seperated the so called boundary layer, and a soft spot in the main is supposed to indicate the same, but what is not said is that air flow even on laminar flow wigs is not completely laminar without boundary control devices, air bleeding, pumps etc. The flow does seperate from the airfoil does become turbulent, but that does not matter. You are looking for lift to drag ratios and in a boat lift drag/heeling forces. That still means time in a boat to find the right combinations. Have fun

Alixe

04-20-2004, 06:18 AM

But how come a mainsail is triangular, a jib has another different shape and a genoa too?

Alixe

redcoopers

04-20-2004, 10:43 AM

It's not necessarily...

Gaff rigs - a quadrilateral - were used (almost) exclusively until the development of the marconi - triangular - mainsail. While gaffs have more sail area than a marconi rig, it can be more difficult to obtain the required leech tension while sailing upwind. Furthermore, you have an extra "halyard" to pull up the gaff.

I'm unsure, but I think that currently, most racing rules discourage the use of the gaff. For a pure racing boat, it would be used only to gain extra downwind sail area.

Anyway, it's a lot more complex and can't go to wind as well as a marconi. However, it sure is an absolutely beautiful sail... My view, is that it's almost sinful to equip a classic boat such as a bristol channel cutter without a gaff ;)

-Jon

Javelin

04-20-2004, 11:23 AM

Ian

04-20-2004, 12:43 PM

Actually, gaff sails don't have to set with excessive twist. In recently designed gaff rigged boats the mast is usualy too short, so the peak halyard has a very unfavourable angle with respect to the gaff. Gaffers that carry topsails usualy have enough mast length, even discounting the top-mast, and the sail sets well. Acording to wind tunnell tests (marchaj) the gaff sail planform is superior off the wind. Marconi rigs are moving in the direction of getting rid of the useless pointy bit at the top of the sail. An eliptical or rectangular planform is superior. Of course, when a marconi-rigged boat is off the wind, it has a wide choice of large downwind and reaching sails to hang off that tall mast which much more than makes up for the inferiority of the main.

matt Livermore

05-13-2004, 11:43 AM

re: ...It makes the air flow separation on the leeward side of the main shift aft.

reply: Am I right in thinking that this shift in air flow separation would effectively stop the mainsail from stalling, allowing it to produce a propulsive force?

regards, ML

brian eiland

05-22-2004, 09:12 AM

From the archives of the multihull forum <http://www.steamradio.com/pipermail/multihulls/>
I retrived this posting by Tom Speer:
_________________________________
Tom wrote:
There's been discussion in the past here about the flow around sails and the wakes they shed. I just ran across this picture,

<http://www.ship.saic.com/pictures/acup-flow.jpg>

of the flow around an IACC yacht. Although it doesn't say exactly, I believw what you're looking at is the velocity on the sail surfaces and in a vertical plane behind the boat. The three lines at the bottom show traces of air particles approaching from near deck level.

What I found striking was the dark blue regions in the wake indicating the vortices shed by the rig. The upper one is more in line with the hounds than it is with the head of the main. There's also a concentrated, powerful vortex shed off of the foot of the rig, too, with a substantial wake extending some distance up from the boom.

We're looking at the windward side of the sails, and it's interesting to see yellow and green colors on the jib in the slot, indicating that the flow there is actually slower than freestream, not faster as is often believed. All in all, it's a cool pic
________________________
Brian writes:
This referenced site now leads to a short movie clip. I don't seem to remember this from the past, but rather just a couple of still photos, and in particular a close up of the 'hounds' area. Am I wrong Tom, or was it another photo I am thinking of that concentrated on the wake from the hound area??

At any rate, while I was reviewing a considerable number of papers, including A. O. Smith's "High-Lift Aerodynamics", and Marchaj's "Aero-Hydrodynamics of Sailing" to come up with a few other aerodynamics of sails forum submissions, I ran across this site again and though it might be of interest to some on these two forums.

PS. Its nice to have the archive section of the 'steamboat' multihull forum back up and running again. And the new format of the BoatDesign.net forum is very nice as well. The search feature in the BoatDesign forum is particularly nice!

brian eiland

05-22-2004, 09:58 AM

Opps, that movie site is at <http://www.ship.saic.com/overview_fans.html>

brian eiland

05-22-2004, 12:51 PM

Here is another interesting visual site:
<http://www.wb-sails.fi/>
and go to the "streamlines and swirls" under the article section.

Also click on the animation in the first, second, third simulation.
Did the 3rd dimension (vertical) of flow just force its way in on the 2 dimensional analysis under which we ordinarily consider such flow?? In other words we have in the past generally restricted our thinking to the 'plane' of flow parallel to the sea surface, choosing to ignore these 'minor'(?) vertical flows.
I am concerned with some of this vertical movement in relation to the narrowing sail 'slot' with increasing mast height while the apparent wind is ever increasing with this mast height. This fact has always concerned me with the traditional Bermuda rig, and is one I try to address with the parallel headstays on my mast aft rig.

Here's a 'full scale' wind tunnel:
<http://www.wb-sails.fi/news/470StreamAnim/V60.htm>
I know this one has been mentioned previously, but I also have another question about it. Does someone know the actual sail combinations being utilized by the V60's at this moment?? Was it there masthead Code 0's as I suspect??

dougfrolich

05-22-2004, 01:57 PM

very interesting thread! I reall do not have anything to add.

brian eiland

05-24-2004, 11:33 PM

I sail at a reasonably high level in my class and the opportunity to sail against others in the top 3 or 4 in Europe is limited to competition only!

Experience with two boat tuning was certainly beneficial when sailing mid fleet where the gains are much more apparent but gains now are far harder to come by.

If the Genoa is over or under sheeted by just 1 inch upwind we lose out. So sheets are marked and fairlead positions are all callibrated so we can find a fast setting. However fear of losing tends to stop us experimenting too much. If however we fall over some data that shows us that in theory at least X is worth trying I'll go for it.

Given a set camber, a set wind speed of say 5 knots, a set angle of incidence etc, you should be able to measure the lift and the corresponding airflow around the sail and therefore the differential in windspeed between the windward and leeward sides.

If this is true then surely it is possible to do the same with varing slot widths to identify at least the optimum width at x speed, x angle, x camber, x overlap etc.

If this were done over a range of conditions then it follows that data would be produced which could be used as a good starting point and an indicator of how far you are from the theoretical optimum.
_______________________
Brian replied:

I have noted your postings, and particularly your sailing at “a reasonable high level in my (your) class.” With that sort of a comparison available, I think that we all should make an attempt to solve your problem, answer your questions, and consider your observations about that other competitor’s ability to outsail you to windward. We might all lern something in the process, both practical and theoretical.

Would you happen to have a copy of, or access to, a copy of Tom Whidden’s book “The Art & Science of Sails” ?? On page 96 he presents an interesting dwg by Arvel Gentry that depicts several different examples of what happens to the wind flow in the sail slot, as well as shifts in the stagnation streamlines when the relative settings of the main & jib sails are changed. Whidden’s summary at this point, “Addressing the big picture, a correctly trimmed headsail slows the wind in the slot JUST the proper amount, so the air on the lee side of the main does not separate when it is trimmed at a tight angle. A correctly trimmed main places the jib in a lift, meaning the boat can be pointed closer to the wind without the jib luffing."
(I could attempt to add this dwg to this posting if I could figure out how to do it)

Am I correct to understand that you bought both a new jib and a new main?? Do you still have the old main?

Can you describe the shape of these sails?? Both old and new? I would be looking for the geometry at a minimum of three heights; 25,50, & 75%:
a) the camber (expressed as a % of the local sail chord)
b) the position of max camber (% of local sail chord)
c) the twist (expressed in degrees relative to sail foot chord)
d) the entry angle
e) the exit angle

I would also like to know:
a) how the jib sail is attached to the forestay, and any variations existing in the class?
b) the thickness and shape of the luff reinforcement (rope, etc)?
c) and any class variations allowed/existing there?

Did you notice anything different in the construction of the leading edge of your competitor’s jib sail?

I assume you have visited the ‘Quest for the Perfect Shape’ at <http://www.wb-sails.fi/> ? I don’t agree with all of their observations, just most of them.

Hope we all can participate in resolving your competitiveness

brian eiland

05-27-2004, 12:38 PM

Subject: Sail Aerodynamics, Sail Wake
_________________________________________________________________
Tom Speer wrote in the steamboat multihulls forum:

> Taking a second look at it
> <http://www.ship.saic.com/pictures/acup-flow.jpg>
I'm not sure what's shown on the back plane. Whatever is shown on that plane is probably different from what's shown on the sail - and I may have been mistaken in what I said earlier. You can see the boundary layer in the wind at the water surface.
It can't be static pressure, because static pressure would be essentially
constant from the water up, away from the boat . I don't think the colors
show the velocity magnitude because there are high velocities near the
trailing vortices, so the velocity should increase, then drop off at the very
center. Dynamic pressure drops off to zero at the water surface and there's a
total pressure deficit in the core of the trailing vortices, so dynamic
pressure is a possibility. So total pressure would be a good guess.
> The component of the velocity in the true wind direction is another
> possibility.
>
> The caption on the FANS page mentions pressure, so the sail colors could be showing static pressure. If that's the case, then there may be no
correspondence between the colors on the sail and the colors on the back
plane. And my remarks about the flow in the slot being slower than
> freestream could not be justified by the evidence in the picture.
______________
Brian responded:
Just in case there is some confusion over the still photo verses the movie I
would refer the viewer to either:
<http://www.ship.saic.com/overview_fans.html> and click on movie
<http://www.ship.saic.com/pictures/aone_small.mov>

What I am finding in my review of some of this computational methods of
investigating sail forces and flow analysis (CFA, CFD, vortex lattice models,
etc) is that generally there are so many assumptions made upfront in order to
simplify the equations so the computer can solve them, that the results get
skewed quite a bit from reality, ie, a quote from one of the annalist;

"CFD = Computer Fluid Dynamics. CFD is a great tool for visualizing and
explaining flow phenomena. While the latest flow software is very powerful and capable of calculating amazing things at astonishing accuracy, the old saying "garbage in, garbage out" is more true than ever. Besides of presenting the problem in a meaningful way, one needs lots of knowledge and experience to interpret the results correctly. Simulation through CFD is especially useful at giving qualitative information - when it comes to quantitative results or hard numbers, you have to be even more cautious when drawing conclusions about the merits of one design over another. Wind tunnel tests are needed to calibrate and validate the CFD code before reliable results are obtained."

"With the power of modern CFD at the desktop, it is too easy to produce
beautiful pictures with little connection to reality. Often these pictures are
produced by flow experts with little sail-specific knowledge, and then
interpreted by sail designers without sufficient understanding of the CFD
tool, and as a result you get just that - pretty pictures."

Brian continues:
And take a look at the LACK of a trailing vortex off of the upper tip of the
mainsail....I don't think this is reality.

I also find interesting the totally screwed-up flow lines on the portion of
the mainsail above the hounds....certainly makes one question the
effectiveness of this sail area, AND the negative effect the vortices off of
the fractional jib have on the mainsail's upper portion. I believe both Tom &
I have questioned the fractional rig verses mast head rig subject in previous
discussions.
________________________________________________________________
Tom Speer continues:

> However, I think the important point is the huge vortex coming off the boom. It shows that the sail rig does not act like a a wing with double the
geometric aspect ratio of the sail rig due to surface effects, and the
> optimum planform is not a semi-ellipse. The vortex is also strongly
> affecting the region of the mainsail where the chord is the greatest.
> People usually concentrate on the vortex at the top, but the vortex at the
foot may be more significant. It's worth considering how to shape the
> mainsail so as to reduce the strength of the vortex and to move some of the sail area away from its influence. One answer is to use a wishbone boom and round the clew so the planform of the whole sail rig looks more like a
sail-board rig.
______________________
Brian replied:
This is just example where the analogy between an aircraft wing and a sail rig
differs quite a bit, "it shows that the sail rig does not act like a wing with
double the geometric aspect ratio...." And this is another example of where
theory and reality differ, particularly when trying to analyze sail rigs with
aircraft wing technologies. I intend to point out some other examples soon,
and raise them for comment.

Per Tom's observation about the boom end of the rig, and substituting a
'sailboard' type bottom, I not so sure that the aerodynamic gain could be
traded for the neccessity to 'shape' the conventional mainsail (vangs,
outhaul, etc) to operate properly with the non-parallel, sometimes fractional
headsail.

Pardon my plug for my unconventional mast-aft design
<http://www.runningtideyachts.com/sail/>
one might note that the flows off of the bottom of my 'mainstaysail' (and my
headsail as well) should not create as much negative vortex action in this
region. And note, I did utilize a wishbone boom on my mizzen sail to
accomplish what Tom has suggested as an optional boom arrangement for the
conventional mainsail.

nibbit15

05-29-2004, 09:23 AM

I've seen the "slot effect" referred to as a "myth" recently but folk seem to sail fast by using it as a mental model... Is it a myth?

The slot effect is a myth. Don't get confused by all the aerodynamic models people try explain to you - most of them are degree level or beyond. If you want to sail fast then get a fast sailor to show you how to trim sails and if you really want to learn about aerodynamics or CFD then talk to students/graduates/lecturers from a university that does aeronautical engineering. Lastly if you want great advice on sail set up, talk to a competitive sail loft like North or Hyde.

Huw

brian eiland

05-29-2004, 10:06 AM

The slot effect is a myth. Don't get confused by all the aerodynamic models people try explain to you - most of them are degree level or beyond. If you want to sail fast then get a fast sailor to show you how to trim sails and if you really want to learn about aerodynamics or CFD then talk to students/graduates/lecturers from a university that does aeronautical engineering. Lastly if you want great advice on sail set up, talk to a competitive sail loft like North or Hyde.

Huw
_______________________________________________
Brian responded:
I might suggest you get yourself a copy of Tom Whidden's book, "The Art and Science of Sails". There is an interesting discussion of the slot effect herein by the knowledgable author. Mr Whidden was, and may still be the CEO of North Sails.

brian eiland

05-31-2004, 11:55 AM

SeaDrive

05-31-2004, 12:14 PM

Tom Whidden's book The Art and Science of Sails is pretty good about where the air actually flows. It illustrates the theory as well as the practice. I think it's out of print, though.

I think it's pretty well understood that, on a given hull, a plan with a big jib is faster upwind, maybe all around. The downsides are that it costs more in sails, equipment and crew. Perhaps you can get a faster boat with a longer hull and a cheaper sail plan.

brian eiland

06-13-2004, 04:14 PM

Subject: Sail Aerodynamics, Sail Wake
_________________________________________________________________
Tom Speer wrote in the steamboat multihulls forum:
excerpt....
> However, I think the important point is the huge vortex coming off the boom. It shows that the sail rig does not act like a a wing with double the geometric aspect ratio of the sail rig due to surface effects, and the optimum planform is not a semi-ellipse. The vortex is also strongly affecting the region of the mainsail where the chord is the greatest.
> People usually concentrate on the vortex at the top, but the vortex at the
foot may be more significant. It's worth considering how to shape the mainsail so as to reduce the strength of the vortex and to move some of the sail area away from its influence. One answer is to use a wishbone boom and round the clew so the planform of the whole sail rig looks more like a
sail-board rig.

Brian wrote:
Just found this rather interesting discussion by Steve Dashew commenting on the vortex at the bottom of our sails. Check out the very sizable increase in weatherliness he quotes!! I suspect this is far in excess of what a CFA would predict.

Hi Steve and Linda, Thanks for all of the excellent books and tapes on you adventures. They have been a great help. I have noticed the winglets on airplane wings over the last few years. Has any one tried making a "plate" at the top of the mast, maybe using carbon fiber as a frame covered with sail cloth, to form a device which would reduce the vortexes created by a headsail & main combination? If if would work with a plate on each side of the mast, to tending would be needed during tacking or gybing. Asked my sailmaker about it but he deals with racers more than cruisers, so he is not too interested in the idea. Since you seem to be interested in making cruisers go faster with less effort, thought this idea might be for you. Thanks for thinking about it. Crawford

-------------------------------------------------------------------

Hi Crawford: Interesting concept, and as a glider pilot, with some very long and exotically shaped "winglets" I can relate to what you are suggesting. However, in a sailboat situation there are a whole series of variable which make this idea impractical.

On the other hand, there is another approach which we've used over the years which does work in some cases. This is to "endplate" or seal off the bottom of the boomed sails. If you can achieve this for even half of the foot length, the increase in efficiency is dramatic.

On our 67' ketch, Sundeer, we were able to pick up five degrees in weatherliness--without losing boat speed, when we sealed the main and mizzen. We've just had seals made for Beowulf which we'll be testing in the near future, and will write up for SetSail.

The area added is down low, where it is in turbulent air flow and where the breeze is much lighter. However, the seal effect is very powerful, and if you can make it work with your rig and deck structure, will generate a huge improvement. Note--the less efficient your keel, the more this will help as it reduces induced drag--which hit cruising keels harder than those found on racing boats. Regards--Steve

Schoonertack

06-13-2004, 07:57 PM

Woodenboat did a few articles on Crab Claw sails, I forget the author, which implied a better efficiency. I think that if we follow this thread much further, some of us are going to have to take a look at the boom vortex,, anyone sitting in a cockpit of a sailboat knows well the airflow around the boom. At the risk of being redundant, vortexs indicate what I said in an earlier post' that the lift is caused by redirecting the airflow, a temporary condition, if you extend flow lines past the sail far enough you see the turbulence moves in the same direction as the "mean" air flow. The "Bermudan rigs" current favour could in all possability be to two factors, reduced chaff and the rule making bodies that be. I for one would like to spend some time with a "Sprit Rig".

brian eiland

06-13-2004, 08:49 PM

Woodenboat did a few articles on Crab Claw sails, I forget the author, which implied a better efficiency.

Here is a really nice Proa site with lots of other info including the rigs on these vessels:
http://www.schachtdesign.com/proafile/volume_3/rig_comparison_table.html

tspeer

06-13-2004, 09:30 PM

FYI, I've updated my article on wingmast aerodynamics (http://www.tspeer.com/Wingmasts/teardropPaper.htm).

Schoonertack

06-14-2004, 04:51 AM

I wonder if a new mast is more important than a new mainsail in class racing? Somewhere I have run across the pearl, that the first 20 % of an airfoil is of the most importance in determining reynolds number. Perhaps a flawlessly finished mast is of more than esthetic importance?? Thank you TSPEER and Brian

Chris 249

06-14-2004, 06:39 AM

The problem with dropping the boom to create an end-plate is that AFAIK there are some examples that go the other way, with great success. Even something as old as the Snipe has for decades only used the top of their range for the boom height. They'd rather have the rig high up (accelerated windspeed AFAIK) than try to seal it. Of course, maybe the boom even in the bottom position is too high to get a real end-plate effect.

But also look at 18' skiffs. They are technically sophisticated, yet they have very high booms for boat-handling reasons (ie you have to be a ble to run under the boom to tack ans gybe fast). Any improvement in having a lower boom must be less than the loss in tacks and gybes. In older boats the booms were lower, yet the sailors didn't hesistate to lift them up when hulls and techniques changed.

Another example is the International Canoes. They have very low tac ks on the mains, yet the booms poke up AND many skippers tack and gybe BEHIND the main so there is no reaosn that the boom has to be high for handling. Nor is the IC unsophisicated- current world champ is Steve Clark, owner of the wing-masted LAC holder Cogito.

In boards, there's a lot of talk about the end plate effect but how can this be separate from other effects that a low tack and rake create? Sure, there may be a theoretical advantqage, but there's a theoretical advantage in tilting the rig to windward but it's total B.S. in practise as proven by boards all the time - they go fastest with the rig almost vertical (sideways, not fore and aft).

So is the end plate that important??? It seems obvious from the many performacne boats that ignore it, that it can't be a huge improvement.

ErikG

06-14-2004, 09:12 AM

On the subject of end plate effect...

The air passing over the boat just above the hull has to be filled with som pretty mean vortices. So the airflow is severely disturbed in that area, and that can probably be the reason why the endplate effect does not make as much difference as it could in theory.
I think that boatspeed will be higher because of an efficient crew work area with a higher boom rather than trying to get a little end plate effect. As the slot between decklevel/cockpit sides and the boom is quite a distance aerodynamically.

So does the same happen at the foredeck...
When the boat heels 15-20 degrees the airflow at deck level is pretty banged up. So what gives the best effect? Discarding the eventual effect the endplate effect can have and hoist the foresail a little higher for more undisturbed airflow, or trust the endplate effect to do it's thing and make sure that the lower part of the sail is as close to the deck as possible?

I need to try to position som tellteles around the sail just above the deck to see if a lower or higher hoist makes any difference in the lower part of the sail. Also we have the thing with the airflow always being slower at the surface...

tspeer

06-14-2004, 09:28 PM

I agree. That's why I think there's a good arguement for wishbone booms and a raised clew. The area in the vicinity of a conventional clew can be better used elsewhere.

The problem as I see it is trying to combine wishbone boom, rotating wing mast and reefing. Two out of three is straightforward. All three is tough. I wish Team Phillips had lasted long enough to wring out her rig.

viper640 sailor

07-02-2004, 11:45 PM

A reduction in velocity of the flow in the slot will result in an increase of pressure on the WINDWARD side of the jib and on the LEEWARD side of the main .... This results in the jib's drive being improved, and the main's degraded.

Possible extreme illustration of this effect?

When sailing my light (725lbs) sportboat to windward in heavier air, we have taken to reducing heel by easing the jib before easing the main. it's a very effective technique on a boat that a 25kt wind will capsize with the main fully eased and the jib trimmed.

the jib's heeling moment (not necessarily a sign of efficiency) appears to increase with the increased pressure in the slot, and if I understand Frank Bethwaite's arguments in Performance Sailing, you will sail higher, flatter and faster in heavy air situations under main alone. (I haven't had enough experience to verify Bethwait's assertion for myself, but seems to make sense).

in a short-ish puff, often, with the crew easing the jib until it bubbles at the luff, I leave the main fully trimmed and feather the boat to windward for the duration of the puff. not positive whether there is a net gain or not, but we don't seem to lose speed and obviously we're sailing momentarily higher. on the other hand, feathering for an extended time definitely does not seem to pay.

reactions / explanations / suggestions for improvement?

brian eiland

02-10-2005, 09:38 AM

This question of the 'slot effect' has come up again on a subject thread entitled "Why does a cutter rig point higher & sail faster?" (http://www.boatdesign.net/forums/showthread.php?t=5596&page=6&pp=15) , with several persons referring to it as a myth and unexplainable. I thought we had disposed of that notion.

I thought I would bring up this thread discussion again so the contributors to both threads would be aware of each others previous discussions on this subject.

Skippy

02-10-2005, 05:00 PM

Tom mentioned five forms of sail or foil interaction from an article by A.M.O. Smith:
Smith, A. M. O., "High Lift Aerodynamics", AIAA Journal of Aircraft, Vol. 12, No. 6, June, 1975, pp. 501-530.

I hope nobody minds if I call the two elements the "jib" and the "main".
jib === the leading/leeward element
main === the trailing/windward element
"freestream" means the "external" apparent wind velocity that you would have without the sails, at the same boat velocity.

Could Tom and/or anybody else please tell me where any mistakes are here.

1. Slat effect: In the vicinity of the mast, the velocities due to circulation around the jib run counter to the velocities around the main, and so reduce pressure peaks on the main.
Since the jib is sheeted out more than the main, the air circulation around the jib resists the leeward airflow around the mast. The mast then slices the air more evenly on both sides, reducing both the lift and the drag of the main. I'm not sure "peak" pressures are important in and of themselves. The main benefit seems to be reduced flow around the high-drag mast.
The cause of this effect is the differing orientations of the sails.

2. Circulation effect: In turn, the main causes the trailing edge of the jib to be in a region of high velocity that is inclined to the mean line at the rear of the jib. Such flow inclination induces considerably greater circulation on the jib.
More air is passing leeward of the jib, so less air is going through the slot. To some degree, the two sails are acting together as though they were one big fat foil.

3. Dumping effect: Because the trailing edge of the jib is in a region of velocity appreciably higher than freestream, the boundary layer "dumps" at a high velocity. The higher discharge velocity relieves the pressure rise impressed on the boundary layer, thus alleviating separation problems or permitting increased lift.
The leading portion of the lee side of the main has a negative pressure gradient. This reduces the "pile-up" of air (adverse pressure gradient) on the weather side of the jib, and therefore keeps the air flowing more smoothly along it in light winds.
The benefit of this effect occurs in the slot, but is caused by an area farther downstream.

4. Off-the-surface pressure recovery: The boundary layer from the jib is dumped at velocities appreciably higher than freestream. The final deceleration to freestream velocity is done in an efficient manner. The deceleration of the wake occurs out of contact with a wall. Such a method is more effective than the best possible deceleration in contact with a wall.
This occurs downstream of the slot.

5. Fresh-boundary-layer effect: Each new element starts out with a fresh boundary layer at its leading edge. Thin boundary layers can withstand stronger adverse gradients than thick ones.
Multiple elements can be smaller than an equivalent single element, and therefore have a shorter chord, which is more efficient (lower Reynolds numbers).
This effect applies to each element individually. It's not a product of their aerodynamic interaction, via the slot or otherwise. It's allowed by the mechanical fact that the two elements are attached to the same vessel, both helping to propel it.

The "circulation" effect #2 seems to be most closely associated with the slot itself. As Tom pointed out, it's the opposite of the venturi-effect theory. Furthermore, its main effect seems to be to transfer lift from the main to the jib. I would like to know whether it really is advantageous in applications such as two foils, where the trailing/windward foil is just as efficient as the leading/leeward foil.

The "fresh-boundary-layer" effect #5, I would say has nothing to do with the slot at all, and would apply just as well, for example, to a parallel twin-rig catamaran, no matter how far apart the hulls and sails are.

tspeer

02-12-2005, 03:13 PM

1. The peak pressures are important because ultimately the flow outside the boundary layer has to slow down to near (actually a little below) the freestream speed at the trailing edge. The higher the peak velocity, the steeper the deceleration has to be, and if the deceleration is too great you get flow separation.

Bob Liebeck did a very interesting study of the tradeoffs involved with peak velocity and maximum lift (http://www.desktopaero.com/appliedaero/airfoils2/highclsections.html). The Stratford pressure distribution has a constant margin from separation all along its length so it is the shortest, steepest way to decelerate the flow from a given peak velocity. Therefore, the suction surface velocity profile that produces the maximum lift for a given peak velocity consists of a constant "rooftop" velocity distribution from the leading edge to the beginning of the recovery region and then a Stratford distribution from there to the trailing edge. He then looked at the tradeoff between a short, high rooftop and a long, low rooftop to find the maximum area under the curve for the greatest contribution of the suction side to maximum lift.

With regard to the mast, I don't think there's much reduction in the drag of the mast due to a change in dynamic pressure. But by reducing the peak velocity as the flow accelerates around the mast leading edge, the tendency toward separation on the back side of the peak is reduced.

2. Air passing to leeward of the jib is not what he meant. The trailing edge acts like a volume control knob for the lift on the rest of the surface. The circulation (lift = velocity * circulation) adjusts itself to match the conditions at the trailing edge. What's most important is the component of the freestream that is perpendicular to the trailing edge, because this is the component that is essentially cancelled out by the circulation. When you change the angle of attack of a surface, you are changing the perpendicular component at the trailing edge. When you deflect a flap or camber the surface, you are changing the flow component perpendicular to the trailing edge. And when you place another airfoil behind the trailing edge of the first, you change the flow component perpendicular to the trailing edge. In every case, the circulation adjusts itself to cancel out these changes at the trailing edge. As Smith shows in his article, even if you place a bluff body like a circular cylinder behind and below the trailing edge, you get an increase in lift on the surface due to the deflection of the flow at the trailing edge.

3. You've basically got it. The adverse pressure gradient is like driving on ice - if you slow down too fast, you'll break loose. From a given starting speed, you don't have to break as hard if you're only slowing down for a new speed limit, instead of coming to a complete stop. The dumping velocity is like slowing down to a less restrictive speed limit. For example, here's a Liebeck designed slotted section with its pressure distributions for a few angles of attack:

http://www.desktopaero.com/appliedaero/airfoils2/images/LiebeckFlap.gif

Notice now the forward section's trailing edge is at a pressure coefficient around -1.3 while that of the flap trailing edge is at a pressure coefficient of 0.6 or so. The flap boundary layer has a much lower dumping velocity than that of the main wing.

4. Yes, it not only occurs downstream of the slot, it can occur downstream of the whole airfoil.

5. Lower Reynolds numbers are not more efficient. Skin friction drag coefficients decrease with Reynolds number, so breaking the area up into smaller pieces is less efficient from this standpoint.

Breaking the chord up into searate pieces is an advantage because each piece can have more of a rooftop pressure distribution and a shorter pressure recovery region. The absolute maximum lift you could get from a given peak velocity level would be to have a rooftop pressure distribution that extended all the way to the trailing edge. Anything that cuts away at this rectangle is a loss of lift. With a multielement section, you chop off a number of corners to form the pressure recovery region for each element. But these are not as much as you have to cut away to form the pressure recovery for a single element section with the same peak velocity.

The main really does transfer lift to the jib. The combination of the two is more effective than the same area allocated to either one separately. This is true even if both of the elements are good performers in their own right.

For example, the NACA tested a 23012 section with a 23012 used as a flap (http://naca.larc.nasa.gov/reports/1938/naca-report-614/naca-report-614.pdf). This figure (http://naca.larc.nasa.gov/reports/1938/naca-report-614/index.cgi?page10.gif) shows the pressure distribution of the plain and flapped sections at the same lift coefficient, and at the same angle of attack. As the flap is deflected, you can see the circulation effect increasing the lift on the whole wing surface. The suction surface velocities are increased and the pressure surface velocities are decreased, showing the circulation effect. You can also see the increase in dumping velocity with increased flap deflection.

markdrela

02-12-2005, 06:55 PM

With regard to the mast, I don't think there's much reduction in the drag of the mast due to a change in dynamic pressure.
I dunno about this. I'd think the jib's effect on the mast drag would be pretty big. The contribution of a compact bluff body to the total drag scales as the cube of the velocity at the body. So if the jib induces a modest 10% reduction in airspeed at the mast, the drag of the mast goes down by a factor of 1-0.9^3 = 0.73 , or a 27% reduction. Seems quite significant.

brian eiland

02-13-2005, 11:20 AM

Interesting formulation of questions by Skippy, and as always, an interesting reply from Tom Speer. Regrettably I can't enter the discussion right now as I am leaving for the Miami Show. I hope to find a client that's willing to spend a little time on my predominately headsailed arrangement.

Skippy

02-13-2005, 04:42 PM

Thanks Brian. And thanks for the corrections Tom. It's nice to know I didn't get all of 'em wrong. :)

1. Slat effect: In the vicinity of the mast, the velocities due to circulation around the jib run counter to the velocities around the main
This is true only on the leeward side of the main. Are we talking about negative pressure peaks? That sounds like simple interference: The reduced net circulation is another way of saying the pressure is moderated in the slot. I can see that reducing separation off the leeward side of the mast. Wouldn't that reduce it's drag? I thought that would be a primary benefit. Otherwise, how do you take advantage of the reduced pressure peaks? Sail in higher winds?

2. Circulation effect.
I guess this sounds to me like an "upwash" effect. The upwash of the main increases the speed and improves the direction of the airflow experienced by the jib. It affects the trailing portion of the jib more than it does the forward areas, so it's kind of like lowering a trailing-edge flap on a foil.

5. Fresh-boundary-layer effect. Lower Reynolds numbers are not more efficient.
Oops. I think what I had in mind was that the shorter chord would dump the air sooner. How about this:

Multiple elements can be smaller than an equivalent single element, and therefore have a shorter chord. This dumps the air before or soon after the boundary layer turns turbulent.
This effect applies to each element individually. It's not a product of their aerodynamic interaction, via the slot or otherwise. It's allowed by the mechanical fact that the two elements are attached to the same vessel, both helping to propel it.

tspeer

02-13-2005, 11:11 PM

1. Slat effect: In the vicinity of the mast, the velocities due to circulation around the jib run counter to the velocities around the main
This is true only on the leeward side of the main. Are we talking about negative pressure peaks? That sounds like simple interference: The reduced net circulation is another way of saying the pressure is moderated in the slot.

Yes, negative pressure peaks. Yes, it is interference. The point is to use that interference in a way that is meaningful to improving the performance of the whole system.

I can see that reducing separation off the leeward side of the mast. Wouldn't that reduce it's drag? I thought that would be a primary benefit. Otherwise, how do you take advantage of the reduced pressure peaks? Sail in higher winds?

Yes, of course reduced separation means reduced drag. I suspect one of the benefits of overlapping headsails is to establish a favorable pressure gradient aft of the mast to reattach the flow separating from the mast.

One way to take advantage of reduced pressure peaks is to increase the angle of attack and go to higher lift coefficients in the same or lighter winds.

2. Circulation effect.
I guess this sounds to me like an "upwash" effect. The upwash of the main increases the speed and improves the direction of the airflow experienced by the jib. It affects the trailing portion of the jib more than it does the forward areas, so it's kind of like lowering a trailing-edge flap on a foil.

Exactly. The main does indeed make the jib act like it's got a flap deflected. This is why people find that jibs are more powerful than mainsails of the same area.

5. Fresh-boundary-layer effect. Lower Reynolds numbers are not more efficient.
Oops. I think what I had in mind was that the shorter chord would dump the air sooner. How about this:

Multiple elements can be smaller than an equivalent single element, and therefore have a shorter chord. This dumps the air before or soon after the boundary layer turns turbulent.
This effect applies to each element individually. It's not a product of their aerodynamic interaction, via the slot or otherwise. It's allowed by the mechanical fact that the two elements are attached to the same vessel, both helping to propel it.

Basically. Since each element starts off with laminar flow, it might be possible to achieve a greater proportion of laminar flow than for a single element.

However, what makes this difficult to realize in practice is the job of the after elements is to achieve the deceleration of the flow from the high velocity of the forward element to somewhat below freestream velocity at the final trailing edge. These elements tend to have adverse pressure gradients over nearly all their suction surface, and a turbulent boundary layer can tolerate a much more aggressive deceleration than a laminar boundary layer. The figure above is a good example. The forward element can maintain laminar flow over half the total chord, but the flap will see laminar separation almost immediately and transition to turbulent just behind the slot. So there's turbulent flow over pretty much all of the aft half of the total chord.

I think the more important aspect of the fresh boundary layer is it can stand a steeper initial deceleration. The Stratford distribution has a very sudden deceleration followed by a gradual tapering off. Each time you refresh the boundary layer, you get a new abrupt deceleration segment.

Skippy

02-14-2005, 03:26 PM

Thanks a lot Tom. I'll think I'll consider this my "final" answer for now. No response requested, unless you really want to. :)

0. Venturi-effect myth: Faster airflow through the slot has been said to increase the rig's power.

For the most part, this does not occur. A venturi is a closed passageway that forces air through a restriction. Air encountering a slot between two sails will also flow around the sails on either side of the slot. There is some variation in flow speed within the slot, but the average speed is slower than freestream.

[Edited per Tom's comment below.]

1. Slat effect: In the vicinity of the mast, the air velocities due to circulation around the jib run counter to the velocities around the main, and so reduce low-pressure peaks behind the mast.

Most aerodynamic elements (sails and wings) are susceptible to separation of the airflow from the leeward side just behind the leading edge. This is caused by strong low-pressure conditions there. With a standard masted mainsail, the flow almost always separates from the leeward side of the mast, drastically reducing power. Reducing the low-pressure condition there makes separation less likely for a wing, and with a masted sail, makes it occur later and helps get the air flowing smoothly again on the leeward side of the main. This allows the main to be sheeted in more and sailed harder.
This is interference between the two sails. It is a lack of circulatory airflow in the slot, rather than a presence of flow.

2. Circulation or "upwash" effect: In turn, the main causes the trailing edge of the jib to be in a region of high velocity that is inclined to the rear of the jib. Such flow inclination induces considerably greater circulation on the jib.

The upwash of the main increases the speed and improves the direction of the apparent wind experienced by the jib, significantly increasing the jib's power.
This requires that the jib be ahead of the main as well as leeward of it.

3. Dumping effect: Because the trailing edge of the jib is in a region of velocity appreciably higher than freestream, the boundary layer "dumps" at a high velocity. The higher discharge velocity relieves the pressure rise impressed on the boundary layer, thus alleviating separation problems or permitting increased lift.

As air passes along the leading portion of the lee side of of the main, it experiences a decrease in pressure (favorable gradient). This reduces the "pile-up" or unsmooth flow of air (boundary layer separation) due to an increase in pressure (adverse gradient) along the weather side of the jib, and therefore keeps the air flowing more smoothly along it in light winds.
The benefit of this effect occurs in the slot, but is caused by an area farther downstream.

4. Off-the-surface pressure recovery: The boundary layer from the jib is dumped at velocities appreciably higher than freestream. The final deceleration to freestream velocity is done in an efficient manner. The deceleration of the wake occurs out of contact with a sail. Such a method is more effective than the best possible deceleration in contact with a sail.

This occurs downstream of the slot, or even downstream of both sails.

5. Fresh-boundary-layer effect: Each sail starts out with a fresh boundary layer at its leading edge. Thin boundary layers can withstand stronger adverse gradients than thick ones.

Multiple sails can be smaller than an equivalent single sail, and therefore have a smaller width (chord). Ideally, this dumps the air before or soon after the boundary layer turns turbulent.
This effect applies to each element individually. It's not a product of their aerodynamic interaction, via the slot or otherwise. It's allowed by the mechanical fact that the two elements are attached to the same vessel, both helping to propel it.

tspeer

02-16-2005, 12:43 AM

0. The venturi is not 100% wrong in that the average flow speed does increase from the mouth of the slot to the exit of the slot. Once the massflow through the slot is determined, then it will be slower where there's more cross sectional area than at the narrow slot. You see this in particular near the pressure side of the forward element trailing edge.

But it doesn't mean that if you close down the slot the velocity at the exit of the slot will be faster than if you open the slot. This is because of the change in massflow through the slot. And there can be a significant difference in speed across the slot, too, as you can see in the slotted airfoil pressure distributions posted earlier.

You've pretty much got it for all the rest.

brian eiland

05-18-2005, 12:04 AM

The gentleman who most deserves credit for finally getting the explainations of sail aerodynamics corrected, and upon which several excellent books by Tom Whidden and C.A. Marchaj are based has recently updated his website to include many of the technical papers and magazine articles he wrote on the subject originally. I had mentioned his name, Arvel Gentry, in these postings previously, but I could not make a direct reference to his many documents as they were not posted on his site at that time.

From his site, "I got involved in the technical aspect of sailing because I started racing. Reading the sailing books and magazines, I began to realize that most of what was written about the aerodynamics of sails was wrong, or certainly very misleading."

"The explanations for how lift is generated were based on popular myths. The description of the interactions between a jib and mainsail, the "slot effect", did not make much aerodynamic sense."

"I was soon launched on a quest to discover how our sails really worked. Over the years this resulted in a number of technical sailing papers and magazine articles. The technical sailing papers are archived in this section of my web site. My magazine articles can be reached from my Home page."

"If you are interested in sailing aerodynamics and how your sails work, you have come to the right place. All of my sailing technical papers and magazine articles are archived on the Technical Papers and Magazine Articles pages.

http://www.arvelgentry.com/

brian eiland

07-18-2005, 04:16 PM

...from a recent issue of Scuttlebutt....
"As regards sails, the canted mast clearly reduces a little of the mainsail's surface area, but the genoa is shifted further back for the same height of rig, optimising aerodynamic efficiency. The difference is made downwind with the larger gennakers. The upwind performance is improved with the forestay shifted back, giving less deflection in the leeward shroud enabling one or two extra degrees to be made up in terms of heading."

Brian wrote:
I noticed this posting, but have not had time to evaluate it. Thought I would post it here until I could get back to it, or if someone else cares to comment.

The entire posting was;
Groupama-2 controls the fleet once again, taking its fourth Grand Prix victory since its launch in June 2004. In Vigo this weekend, Franck Cammas and his ten crew stole the show by winning five of the six legs in rather a light breeze... taking second just the once after a poor start.

There is no debate: the most modern trimaran designed by architects Vincent Lauriot-Prévost and Marc van Peteghem, Martin Fisher, Mike Kermarec and Groupama's shore crew, is unquestionably the fastest multihull. In the 33 legs raced since the Grand Prix of Corsica 2004, Groupama-2's first appearance on the racing circuit, the green trimaran has won 28 legs in the five Grand Prix ! The fine tuning didn't take long: apart from a crack on the hydraulic ram attachment (GP of Corsica 2004) and some delamination on the beam (IB Group Challenge 2005), the trimaran has been perfectly optimised on every level since coming out of the yard in June 2004.

As regards the platform, Groupama-2 has much finer floats, a sharper bow, a much narrower planform x- beam, a mast that cants 60cm more than the other trimarans, smaller profile beams and a weight saving of around 200 kg. As regards the appendages, the green trimaran also has a very thin daggerboard, with a small chord and a large trimtab (rear part of the daggerboard that has its direction altered by up to 8° and represents 28% of the lift surface), some high aspect ratio rudders and very curved foils (a 3 metre radius instead of a 4m one for the other trimarans). As regards sails, the canted mast clearly reduces a little of the mainsail's surface area, but the genoa is shifted further back for the same height of rig, optimising aerodynamic efficiency. The difference is made downwind with the larger gennakers. The upwind performance is improved with the forestay shifted back, giving less deflection in the leeward shroud enabling one or two extra degrees to be made up in terms of heading.

Lighter, Groupama-2 reacts faster to a gust and immediately transforms the acceleration, while the other trimarans take the pressure before transferring it into propulsion. It is also more reactive downwind and pulls away faster out of a manuvre (tack, gybe). With more canvas and lighter construction its power to weight ratio is a little better than the others. With the optimised aero and hydro-dynamics, Groupama-2 is also capable of sailing higher, which is crucial for getting a slight separation during the close contact phases of a start, and a lateral gain when rounding a mark...

All added up, each of these small advantages enable Franck Cammas to remove himself from situations which are beyond the control of any of the other boats and to snatch back a few tenths of a knot when he is sailing
in a stable breeze. He really has to be trapped during a start for Groupama-2 not to win, as has been the case only twice since the beginning of the season : in Marseille and Vigo where he was blocked by his competitors, it took him two laps to get back to the front of the fleet...
-- Laurence Dacoury (Kate Jennings for translation)

brian eiland

07-21-2005, 09:01 PM

Here is another affirmation of the slot effect....look at this new DynaRig proposed for the 285' Maltese Falcon:
<http://www.doylesails.com/newsletter-03-pg8.htm>

Note the sheeting angles for the three sails, and particularly that of the very forward sail. Now why is this?....look at the explantions of the slot effect.

And which sail do you think is providing the greatest drive forward, and which sail is providing the greatest leeway??

Interestingly, they are not trying to account for the wind gradient (twist) with this new design even though it is an extremely tall rig.

I had previously posted this back on thread #47. When I went to look at the referenced website it was no longer there. Meantime I did run across this CFD analsyis of Maltese's rig. http://syr.stanford.edu/HISWA_Tyler_2002.pdf
Have a look at the illustrations toward the end of the paper dealing with optimizing the sheeting angles for the 3 sails

jam007

10-21-2005, 01:12 PM

After reading this thread I did a run in JavaFoil (http://www.mh-aerotools.de/airfoils/javafoil.htm). (A 2D foil analyse program similar to XFOIL.)
See picture below.
The angle of attack (for the main) was 14 degrees.
Notice the low pressure near the leading edge of the foresail and the high pressure area just between the leading edges of main and foresail. The relatively high pressure on the lee side of the main and the upwash in font of the foresail and the downwash in front of the main.
Cl for this combination was 2.5. For a single foil at 14 degrees it was 0.9. One foil set at 4 degrees (the attack angle of the foresail) has a Cl of 1.1. The interaction adds an extra 25% of lift.
A single foil with a form approximating the two foils generates a Cl of 1.7 but Cd is much lower 0.03 compared to 0.1 for the two folis. This is in accordance with the supremacy of the sloop or single sail rigg going to windward where Lift/drag ratio is more important than lift alone. Of course real sails are 3D objects but this should enhance the differens rather than diminish it due to induced drag (higher lift is payed for by more induced drag).

brian eiland

10-22-2005, 11:01 PM

I’m having a little problem interpreting your interpretation of your JavaFoil analysis run.

The angle of attack (for the main) was 14 degrees......
Cl for this combination was 2.5. For a single foil at 14 degrees it was 0.9. One foil set at 4 degrees (the attack angle of the foresail) has a Cl of 1.1. The interaction adds an extra 25% of lift.
You included two sets of ‘single-foil’ conditions in addition to the ‘combination’ condition, and then called for an ‘interaction’ percentage increase. Are you really saying that the ‘interaction’ that would be experienced in the ‘combination’ condition adds an extra 25%? Does the 4 degree statement even belong here?

In other words in the case of 14 degrees angle of attack (for the main), the CL for this combination of two foils was 2.5, while the CL for single foil at this same angle of attack was only 0.9. That appears to be an almost threefold increase, not just 25%?

One foil set at 4 degrees (the attack angle of the foresail) has a Cl of 1.1
I don’t understand that this is the angle of attack of the foresail?

A single foil with a form approximating the two foils generates a Cl of 1.7 but Cd is much lower 0.03 compared to 0.1 for the two folis. This is in accordance with the supremacy of the sloop or single sail rigg going to windward where Lift/drag ratio is more important than lift alone.
The lift/drag ratios of aero-foils are not the only determining factor in the windward capability of a sailing vessel. One must also consider the vectored direction of the ‘lift forces’ verses the driving forces of the sail-foils. Higher ‘lift’ forces can result in considerable leeway forces and heeling forces. Looking at your attached diagram it appears as though the two foils you show are both practically identical. Yet, it is also quite apparent that the forward foil is significantly more effective at developing lift, and that were these sails on a vessel, this forward sail is much more effective at driving the vessel forward (its lift force is more forwardly directed). AND look at the capability that this forward foil has to heading up higher into the wind if momentarily needed.

Granted you might replace this two-foil combination with a “single foil with the form approximating the two foils” and thus reduce the drag forces per lift, BUT you will lose the higher pointing capability of the combination of two foils, and your lift forces of this single foil will act more to heel you and increase your leeway rather than drive you forward.

jam007

10-23-2005, 04:40 AM

I’m having a little problem interpreting your interpretation of your JavaFoil analysis run.
...

Yet, it is also quite apparent that the forward foil is significantly more effective at developing lift, and that were these sails on a vessel, this forward sail is much more effective at driving the vessel forward (its lift force is more forwardly directed). AND look at the capability that this forward foil has to heading up higher into the wind if momentarily needed.

Granted you might replace this two-foil combination with a “single foil with the form approximating the two foils” and thus reduce the drag forces per lift, BUT you will lose the higher pointing capability of the combination of two foils, and your lift forces of this single foil will act more to heel you and increase your leeway rather than drive you forward.

I will try to clarify my thoghts.
The two foils are identical. The leading foil (foresail) is set at an angle of 10 degrees from the aft (mainsail). I also did runs with one foil. I then got a Cl of 1.1 at 4 degrees angle of attack and Cl 0.9 at 14 degrees. So I then concluded that the interaction of the two foils added an extra 25 %. (Is this a resonable approximation or am I only exposing my ignorance?)

I totaly agree that the fore+main combination not only increases the Cl but in a considerable way the driving force compared to only main. The reason I mentioned sloops and single sail riggs was the earlier diskussion of riggs with many interacting sails. I think the sloop is the optimum for displacement boats and most multis for winward efficency (L/D ratio compared to force generation) due to the reasons you gave.

But I wonder. Is not Cl and Cd measured relative the free flow velocity of the air. This should result in that a large L/D is the same as a more forward directed force. An extreme situation would be with negative Cd if the net force is directed forward of perpendicular to the angel of atack.

What made me post the picture was the graphical illustration of the interaction that You, Tom Speer and other have described and I read about in Marshaj´s books and Gentry´s articles.

Anders M

tspeer

10-23-2005, 08:10 PM

...
But I wonder. Is not Cl and Cd measured relative the free flow velocity of the air. This should result in that a large L/D is the same as a more forward directed force. An extreme situation would be with negative Cd if the net force is directed forward of perpendicular to the angel of atack....

Yes, lift is defined as the component of the total force that is perpendicular to the relative wind and drag is defined as the component of the total force that is parallel to the wind. Once you know lift and drag, you know everything about the magnitude and direction of the force. There's no such thing as angling the lift vector forward, unless you change the relative wind direction.

But lift and drag aren't the only choices of coordinate system. Normal force and axial force are the components perpendicular and parallel to the chord. Normal force is always larger in magnitude than the lift because drag is always positive and contributes to the normal force. For example, at zero angle of attack, lift and normal force are the same. At 90 degrees angle of attack, drag and normal force are the same. Axial force can be positive (aft), or negative (forward) because the lift vector does point forward as angle of attack increases.

To say, for example, "its lift force is more forwardly directed," is to confuse normal force with lift. Instead, one should say, "its normal force is more forwardly directed." One can't incline the lift forward to increase the drive, one can only reduce the drag

The advantage of using lift and drag is you don't have to know anything about the orientation (angle of attack) of the surface. You can calculate the performance entirely in terms of lift and drag of the topsides and sails, and the lift and drag of the hull and foils. You don't need to know the orientation of the boat (leeway angle) or the orientation of the sails relative to the boat. But often it's useful to know the normal and axial forces, for example when calculating loads or hull trim. Another set of coordinates would be drive (forward) and side force (perpendicular to the boat's plane of symmetry). Given one set of components, you can transform the forces into any other set.

jam007

10-24-2005, 03:52 AM

Different coordinate systems can be confusing. Actually often aerofoils have the net force directed forward of the normal of the cord as it is the low pressure area around its forward part that creates the main part of the force.
Example: A NACA 6412 at 10 degrees angle of attack has the net force directed 2 degrees aft of the perpendicular of the flow. That is 8 degrees forward of the normal of the cord.
See picture below.
(Note the lines and vectors are drawn by hand and may not exactly be at the correct angles.)

Anders M

yokebutt

10-25-2005, 01:40 AM

Anders,

Is that also true of the same section without camber? Or is that why cambered foils have their low-drag bucket shifted? Please explain!

Jocke.

P.S. For the benefit of the non-scientists and non-native speakers of English here, the word "normal" as used to describe forces in this discussion means perpendicular to, wich in turn means "at right angle to" or "square to".

jam007

10-25-2005, 12:55 PM

Anders,

Is that also true of the same section without camber? Or is that why cambered foils have their low-drag bucket shifted? Please explain!

It is no clearcut answer so here goes...

Some important terms:
Angle of Attack: The angle between the airflow and the mean cord of the foil.
Lift: The force perpendicular to the flow of air
Drag: The force in the same direction of the airflow.
Cl: lift coefficient. A non dimensional number describing how much a lift a foil creates. To get the actual lift force you use:
Lift= cl*density*area*(velocity^2)/2
Cd: drag coefficient. same as lift coefficient but describes the drag.
Stall: When the flow no longer can follow the foil smootly. Instead a wake of turbulent air is created. (Like the suction area behind a lorry) At a certain angle of attack any foil starts to stall. When a foil stalls the lift decreases and the drag increases.

If there was no drag all foils would create a net force perpendicular to the airflow. The drag shifts the net force so it is angled slightly along the air flow
the angle can be calculated by:
drag angle=arctan(Cd/Cl)
See picture in my previous message
The Cl/Cd ratio are the non-dimensional version of the Lift/Dag ratio.

If a foil creates a forward net force as seen from the coordinate system of the foil depends on both angle of attack and dragangle. If drag angle is less than the angle of attack then the net force is forward.

For the NACA 0012 (The uncambered version of NACA 6412) the drag angle is 5 degrees (arctan(0.074/0.87)) at an angle of attack of 10 degrees. This means that this symetrical foil have a net force directed forward.

As understood from the above (hopefully) Cl and Cd and therefore the drag angle are not fixed numbers for a foil but depends on angle of attack. For every foil there is a certain angle of attack that gives the lowest drag angle.

A cambered foil is simply put a bent symetrical foil. A symetrical foil has its lowest drag (and lift=0) at 0 angle of attack. A cambered foil has a leading edge that will be straight towards the oncoming flow at a angle of attack different from 0 and have low drag around this angle. This shifts the bucket and gives a higher cl/cd ratio and therefore a lower minimum drag angle.

But remember that the foils I have shown are 2D foils. Real 3D foils have higher drag than their 2D counterpart (due to induced drag but thats another story). For a 2D foil the Cl/Cd ratio can easily be 50 or more, a 3D wing around 20, but a sail have a Lift/Drag ratio of around 6. A Lift/Drag ratio of 6 means a drag angle of 9 degrees. This drag angle is so high that the net force is more or less perpendicular to the mean cord of the sail.

Sails have a maximum Lift/Drag ratio at an angle of attack that is lower than the angle of attack of maximum lift, just before stall. Going to windward the lift/drag angle is important since it can be shown that this ratio directly affects maximum velocity made good.

C A Marchaj mentions in "Sail Performance" the interesting effect of pumping with the sheet to create "negative drag" by using unsteady flow effects.

I hope this gave some explanation. (And that it is in its essenssials correct. I´m not an aerodynamics expert just a physicist interested in sailing.)

Anders M

PS Had to check that normal is an english term and not a swedish after your reply. Hope I avoided any more strange terms, its so easy to write swenglish.

yokebutt

10-26-2005, 08:49 PM

Thanks Anders,

Bethwaite had an interesting example on the effect of pumping. The comparison was between two boats sailing along in 5 knots of wind, one is pumping and the other one isn't. In this idealized example the boat being pumped has a wind speed over the sail of 10 knots half the time, and zero the other half. And of course, according our old friend, the rather unpleasant Englishman, the pumped boat is pushed along faster.

Another source to peruse if you are interested in unsteady flows is Steven Vogel's book Life in moving fluids. Very good and also very funny, one of my favorite statements of his is to the effect that "many people have a fuzzy notion that the boundary layer is a discrete region, rather than a discrete notion that it is a fuzzy region"

Yoke.

mojounwin

11-04-2005, 10:36 PM

cyclops

11-04-2005, 10:45 PM

They like the old, Clippers, Tall Ships and the Wind Jammers, more than PR releases?

Skippy

11-05-2005, 11:50 AM

I think if you look at similar multi-foil airplane wings, the forward/upper foil is usually small. Aerodynamic support from the main is one of the things that make the jib so effective, so there has to be some kind of balance. Other than the disappearance of older racing rules that used to encourage more headsail, the large jib will have trouble from sagging in heavy weather. And for whatever it's worth, there's a stability issue with the headsail(s) grabbing more air rather than less as the boat starts to heel. Also, maybe that combined with the longer foot could result in a genoa getting dunked more often.

11-05-2005, 07:45 PM

Here is a thought to ponder. Aerodynamic pressure on an airfoil is generally centered about 25% chord. A complex wing that has lift enhanceing devices, flaps and slats, is not considered as three separate foils, but as a single unit, even though in most respects the high lift devices have separated from the wing. The new chord length is now measured from the new position of the leading and trailing edges, plus the added camber as a result of the downward deflection of said devices.

Is there a more direct correlation between wings and sails with this analogy rather than bi-plane theory. If the jib/main assembly is taken as a single unit with total lift placed at 25% chord, the center of lift will fall squarely on the jib. Hmmmmm.

Another ponderance. I think it was earlier in this thread that a discussion on fractional vs. mast head rigs took place. The common statement was that on similar boats, the fractional rig was just as effective as the masthead. On early Learjets, they had a problem with flow separation as airflow over the wing approached transonic speeds. Granted, we haven't quite acheived these speeds with sailing craft, but bear with me. To continue, this flow separation occured in the region of the ailerons rendering them ineffective. Look Ma, I'm going supersonic and I've lost control of my aircraft. Learjet's "fix" was to install votex generators on critical areas of the wing. These were "T" extrusions about an inch in length with the top of the "T" bonded to the upper surface of the wing with a slight misalignment to the airflow over the wing. Thus, creating a vortex, energizing the boundary layer and reestablishing boundary layer flow. Now the ailerons could change the amount of lift the wing generated in that portion of the wing.

Now here comes the stretch(analogy). Does the lift vortex that is attached to the head of the jib energize the flow over the main in the vicinity of the jibhead. Providing the main doesn't impose an endplate effect, the vortex that is being shed at the jibhead will be more highly energized because the flow velocity in the vortex will be greater than the local flow. Thank's to Bernoulli this supplies an additional local pressure drop that adds to the differential between the the weather and lee sides of the main. If the main does provide endplate effect, then the above is hogwash, but now drag is diminished, due to endplate effect, as opposed to lift being enhanced.

And one more ponderance . . . maybe. Is there really such a thing as laminar flow on a sail? In regard to aircraft, laminar flow wings are extremely picky about surface roughness. Bug juice will trip the boundary and cause turbulent flow. Not to get confused, turbulent flow is still attached to the airfoil. It's just a little more draggy. I find it difficult to believe that, with the preponderance of obstacles that support a set of sails, there is any laminar flow anywhare on a set of sails. When the luff starts luffing and the tell-tails start telling(sorry, poetic license), personally I think we are seeing flow separation and not a transition to turbulent flow. Laminar flow over the sails is tripped way before it ever sees the sails.

There, you have it now. I've set myself up for a royal lambaste.

Regards to everyone on the site. It's a great place to hang out.

Packeteer

11-06-2005, 06:04 AM

so when I build my wing sail I should look at aeroplane wings from bygone days?

ps. I haven't read the whole thread yet, my brain is mush after the first page

tspeer

11-06-2005, 08:37 PM

Jimmy Dolittle III (test pilot & grandson of THE Jimmy Doolittle) calls vortex generators the "horns of ignorance". They are used as a quick fix for premature separation arising from all kinds of causes, like transonic shock waves (the Lear example), interference effects (look at all the VG's on the underside and side of a B-1's tail), bends in an engine inlet duct, and flap deflection. The vortices shed by the VG's entrain higher energy air outside the boundary layer and bring it down close to the surface, and they sweep low-energy air from the boundary layer up and away from the surface. Like the transition from laminar to turbulent flow in the boundary layer, this adds drag but makes the boundary layer more resistant to separation in an adverse pressure gradient.

VG's are generally small vanes with an aspect ratio less than 1, and can be rectangular, triangular, or gothic (curved) in planform. When arranged in a herringbone pattern, alternate vortices rotate in opposite direction. When arranged in parallel, the vortices are co-rotating. There are also ramps and triangular ramps, and Y-shaped VG's like the Wheeler Wishbone. Micro-vortex generators that don't poke up outside the boundary layer still produce measurable effects and a small (1 - 2 db) quieting of the flow noise. NASA has funded research on on-demand VG's that are actuated by MEMS devices. These are flush when they aren't needed, but extended when necessary. The synthetic jets used in many active flow control applications are basically pneumatic vortex generators.

The vortex shed at the head of the jib might help energise the boundary layer on the main, but it's much larger in scale than the boundary layer. Instead, hardware at the hounds, forestay, even the wakes of halyards are more likely to act as turbulence generators for the boundary layer on the mainsail.

I've not been able to find much in the way of a good picture of the jib vortex. This one perhaps comes closest.
http://www.emiratesteamnz.com/images/ob/des_cfd-sailstreams1.jpg
You can just make out two "skeins" of twisted streamlines downstream of the leech, showing the vortex from the jib merging with the vortex from the head of the mainsail. If you look at how far down the lee side streamlines are lifting up, I think it illustrates how the "tip" vortices are not just shed from the tip, but are actually shed little-by-little along the span, with the greatest concentration being at the tip.

The vortex from the head of the jib has the effect of "lifting" the part of the mainsail above it and "heading" the mainsail below the vortex.

SuperPiper

11-07-2005, 04:39 AM

[QUOTE=Skippy]. . . And for whatever it's worth, there's a stability issue with the headsail(s) grabbing more air rather than less as the boat starts to heel. QUOTE]

This is a concept that I had never heard before. Can someone explain this for me?

sharpii2

11-07-2005, 07:22 AM

sharpii2

11-07-2005, 07:43 AM

I've been brought up to believe the headsail provides roughly 80% of the drive and the main was more for providing balance, but why is it that modern sportsboats seem to be going with smaller headsails and bigger mains?

Cheers
Mike

Because proper staying geometry and hull stiffness are harder to obtain on todays lighter (DLs of well under 100 on some) boats with ever greater working sail plans. A big jib needs a relatively heavy (DL of around 200) hull to hold the luff straight. A jib with a saggy luff is just about useless for anything except ballancing the main.

I know, I have used mine for that very purpose. Half roller furled, it was about as aerodynamic as a toilet paper roll, but it did hold the bow off, so the main could do its job.

I also know from experience that a 3/4 fractional rig can be sailed with the main only. Mine rutinely was. I have never seen a masthead sloop do that.

Bob

Skippy

11-07-2005, 10:26 AM

sharpii2: I was taught that the Center of Lift was aft the highest point on the airfoil section.

Is that relative to the foil's chord line or to the wind direction? I know "on the section" sounds like you mean the chordline, but most of the CFD pics I've seen appear to have a strong low-pressure peak just behind the mast or leading edge. That sounds more consistent with the other way -- highest relative to the wind, which looks to me like somewhere around 25% or so.

SP, just look at a jib that's sheeted out. It's canted INTO the wind. When the boat heels over a ways, the headsail stands upright.

brian eiland

11-07-2005, 12:36 PM

Looking at a couple of the recent postings, I thought it might be a good time to re-introduce a reference site I made back at posting #67, Avrel Gentry's Updated Web Site (http://www.boatdesign.net/forums/showpost.php?p=48508&postcount=67)

Or for a more illustrative view, have a look at Paul Bogataj's simple but effective article at http://onedesign.com/articles/article6-1.html . Notice in particular his location of the chords of the airfoils, particularly that of the mainsail.

jam007

11-07-2005, 01:52 PM

A simple experiment rearding position of maximum lift. Drop a rectangular pice of paper, lets say 15 cm x 3 cm with one of the long sides pointing towards the ground. It will not fall straight down but at an angle and start to rotate backwards as it falls. This is because the centre of lift is forward of the papers centre line.

Anders M

gggGuest

11-07-2005, 04:45 PM

Because proper staying geometry and hull stiffness are harder to obtain on todays lighter (DLs of well under 100 on some) boats with ever greater working sail plans.

Even supposing that modern structural engineering and materials doesn't produce stiffer boats than wood or aluminium boats the engineering of thirty years ago (which of course it does) the fact that no open rule dinghy or catamaran ever even considers a masthead rig should be enough to point out the flaw in that theory. The masthead rigs were all to do with rating rule benefits and the easiest way to get more rag on a small spar.

yokebutt

11-07-2005, 09:50 PM

Tom,

Does that increase the effective AR of the jib?

Yoke.

tspeer

11-12-2005, 03:59 AM

I think you ought to consider the planform area of the main and jib together when talking about aspect ratio. They really act more like one multi-element airfoil than separate foils.

Vortex generators won't change the effective aspect ratio. They "just" manipulate the boundary layer.

brian eiland

11-12-2005, 10:11 AM

...the fact that no open rule dinghy or catamaran ever even considers a masthead rig should be enough to point out the flaw in that theory. The masthead rigs were all to do with rating rule benefits and the easiest way to get more rag on a small spar.

Bullcrap. Masthead rigs were not "all to do with rating rules". There are other reasons that fractional rigs are chosen for vessels other than the aerodynamics of the situation.

Have a look at the 'discontinuites' between the interaction of fractional jibs and mainsails at this website http://www.wb-sails.fi/ (http://www.wb-sails.fi/) and click on "The Quest for the Perfect Shape" and page down to "Think One".
...an excerpt..
"There are "discontinuities" in the sailplan at the junctions of the three areas: a discontinuity in the chord and the twist where the mainsail starts (I & II), and also discontinuities in the "quarter chord line" (gray dashed line). The quarter chord line is an important aerodynamic factor, as it determines the sweep within each area.

At the hounds, there is a discontinuity (dent) in the leading edge : increasing rake and bending the topmast back smoothes out this discontinuity. Sails, looked as a wing, are complex creature: Very highly cambered, much twisted, with discontinuities, a sharp leading edge and a slot in the middle - much more complicated than a 747 wing with all its flaps and ailerons"

There are quite a few other real interesting wind tunnel illustrations on this site as well. A few in particular look at some of the negative interaction of the flow off the head of a fractional jib onto the main. And the neccessity to modify the top portions of mainsails that act in a uni-rigged fashion in a fractional setup.

Many multihulls have not utilized masthead rigs as they don't lend themselves to rotating mast so well. Thats not to deminish the masthead rig as unsuitable in itself. And how about those CODE Zero sails utilized very successfully on the RACE cats and the Volvo boats....and for going up-wing in light airs. I think they were masthead sails;)

There are some number of other interesting discussions at this website including a stress mapper for genoas, and as affected by sail cloth materials.
"StressMapper has proved to be an excellent help for the designer. Demonstrating how the cut of the sail affects the usable wind range of the sail is easy. The stretch is directly reflected in the sail shape and the customer can see how important correct the sailcloth and cut is."

"It is obvious how much more the traditionally cross-cut Dacron sail stretches. The Dyneema (Spectra) sail also weighs considerably less."
No wonder the older sail materials and cuts didn't lend themselves as readily to big headsails, nor roller-furling ones as well.

gggGuest

11-12-2005, 12:32 PM

And how about those CODE Zero sails utilized very successfully on the RACE cats and the Volvo boats....and for going up-wing in light airs. I think they were masthead sails;)

The code zeros were effective because they were a rating free way to get a lot more sail on a rule limited mast height. Rather my point.

RHough

11-12-2005, 01:20 PM

Is one of the advantages of a fractional rig (more accurately a rig that carries a large portion of it's area in the main) that the sailplan works better at points of sail other than beating to windward?

It seems to me that unboomed sails (Genoas) loose their shape quickly as you bear off the wind. The sheet lead needs to go outboard, so the AOA limit of a Genoa is limited by available beam at the clew.

Club footed jibs and Hoyt's (IIRC) wishbone jib allow proper trim at wider AWAs. Such sails cannot overlap the main however, so those sailplans may not (do not?) generate as high a CL as the same area with an overlapping headsail.

In light breeze (6knts) a typical masthead sloop flies it's Genoa up to BAW up to about 45 degrees, in 20 knts the Genoa or jib is up until BAW 70+ degrees.

It makes sense to me to have the mainsail larger than the headsail if the boat is to perform well on headings other than hard on the wind.

I haven't been able to find any hard numbers that compare distribution of total sail area and amount of headsail overlap. I have observed that in development classes where total sail area is limited, the designs end up being fractional rigs with large mains and small jibs.

I started the Area and Overlap thread in hopes that someone with more knowledge than I could shed some light on the subject.

yokebutt

11-15-2005, 10:05 PM

Essentially, the jib benefits from the mainsails upwash. When the main is large in proportion to the jib, the jib gets lots of upwash. When the main is small in proportion to the jib, the jib get less upwash.

Thus, large main/small jib= jib has more drive per unit area but less area, small main/large jib= jib has less drive per unit area but more area.

Pick your poison.

Yoke.

P.S. A look at a few development classes without sailplan restrictions might be a good place to start.

Inquisitor

11-18-2005, 07:48 AM

Wow… a bout of insomnia at 2:30AM… I thought I’d start this “Sail Aerodynamics” thread to knock me out again. It’s now 7:00AM and I am just now finishing reading it non stopped (no caffine). Very interesting material! I still have http://www.wb-sails.fi/ and Avrel Gentry’s web site to research. I will hit those tomorrow. I had some questions concerning sails (searched the threads this time) and thought it would be better to add them to this thread than creating new ones. I also need to add them here before I forget them tomorrow.

I am working on a rig design that is a little different. At this time, I don’t really want to show it because I’d rather shoot at it more before I embarrassing myself by letting someone else shoot it down with their first post.

The design necessitates a masthead rig. I noted the debate in this thread on this topic. Could someone explain or point me to a site that would compare and contrast this one aspect.

Hopefully I’m not oversimplifying this, but I see a balancing issue with sails. On one hand, the gods of high aspect ratio want a tall skinny sail – more efficient design, but higher moment arm. However healing moment wants a low squat sail. What method should be used to optimize this?

I’m looking at several boats for reference. Most sailboats of my limited knowledge run the forestay (and jib) as far forward as possible some out on past the hull. However, I see on several (Tornado and some Hobies) and I wonder why they don't run the forestay all the way forward and increase the size of the jib. It does not appear to be a structural decision. Is this a class rule thing or is there some efficiency aspect going on here?

I note most all jibs seem to slightly overlap the mast. If I go to a self-tacking jib (with its own boom), it could obviously not overlap the mast. Would I be destroying some vital slot/gap geometry?

Center of lift on chord – Looking at “Theory of Wing Sections”, it appear most all foils here would have a center of lift well beyond the 25% mentioned above. Is that a real-world number for a “typical” sail?

So that I keep the same boat balance… would it seem reasonable to determine the center of lift of a current design (say Torando) using the 25% chord and making sure my new design would have its center of lift placed in the same location.

sharpii2

11-19-2005, 08:21 AM

I’m looking at several boats for reference. Most sailboats of my limited knowledge run the forestay (and jib) as far forward as possible some out on past the hull. However, I see on several (Tornado and some Hobies) and I wonder why they don't run the forestay all the way forward and increase the size of the jib. It does not appear to be a structural decision. Is this a class rule thing or is there some efficiency aspect going on here?

[/LIST]

Catamarans like to keep their riggs as far aft as possible to:

1.) Prevent weather helm and
2.) Prevent pitch poling.

This is not theory but the result of hard won experience.

Also, Light multis often don't have the torsional rigidity to hold the luff of a large jib. The tension needed to hold a luff staight can be quite enormous.

Bob

RHough

11-19-2005, 08:32 AM

Catamarans like to keep their riggs as far aft as possible to:

1.) Prevent weather helm and

I always knew that catamarans where backwards and defy the laws of physics.

How far aft should the Rigg be? :D

http://www.ananova.com/images/web/29696.jpg

Inquisitor

11-19-2005, 09:34 AM

Thank you for your reply Bob.

Prevent pitch poling – Yeah, I’ve met that one up-close and personal like… cracked a couple of ribs. I’m working on a spreadsheet analysis for my rig’s healing (roll) and helming (yaw) moments. Looks like I need to add pitch. That’ll let me evaluate that.

Prevent weather helm - did you mean lee helm? My rudimentary sailing book indicates that some weather helm is a good thing and moving the center of effort further forward causes more lee helm. Since I’m doing a 20’, I running the comparison relative to a Tornado. I was running some trade studies with the spreadsheet and with the forward stay coming off the nose, I was having a great deal of trouble getting the center of action far enough back to get weather helm. So I was coming to your #1 in theory also.

New question: Assuming the pitch poling issue is solved by something else. If I move the dagger board forward (say even with the mast), it:

would get a little weather helm.
would be structurally easier and stronger. This is where the beam structure is already.
appears it would better balance the boat – eg there would be less change in lee/weather helming with dagger board up or down… just magnitude of slippage.

Am I looking at this wrong or is there some other negative ramifications of moving the dagger board forward?

12-05-2005, 08:31 PM

Is there a method for computing the CE for an elliptical sail? All of my sources use the traditional method for computing CE with no account for added roach of an elliptical sail.

I have a source that states that a fully battened sail will produce 15% more power that an unbattened sail. If that is the case, why don't we see more fully battened sails?

tspeer

12-06-2005, 01:14 AM

Is there a method for computing the CE for an elliptical sail? All of my sources use the traditional method for computing CE with no account for added roach of an elliptical sail.

This spreadsheet (http://www.tspeer.com/DesignTools/vortex95.xls) may help.

I have a source that states that a fully battened sail will produce 15% more power that an unbattened sail. If that is the case, why don't we see more fully battened sails?

It's just tradition and class rules. I've never seen a mulithull with anything else but full battens.

12-06-2005, 08:53 PM

OK, I'm going to reveal my ignorance.

In my mind, if a planform has a 15% increase in power for the same area, there would be no doubt as to the planform that I would use. What are the negatives with the eliptical form. Why don't we cut 15% off the top of a Bermudian rig and throw it on the roach of a shorter sail and increase our power output by 15%. Is the CE raised so that the heeling arm is increased beyond acceptable limits. Would the reduction in A/R reduce the efficiency of the sail? Again, in my own mind, since a certain percentage of the (main) sail is blanketed by the mast, why does the top portion of a Bermudian rig even exist. (Note: I've never seen a good explanation of a bermudian rig. My assumption is that it is just the main of a typical sloop rig. My discussion here revolves around an elliptical vs. a typical main in a sloop rig.)

This might be a stretch, but a gaff rig with a topsail (forgive me if my terminology is incorrect) starts to aproximate an eliptical sail. Think of it as your computer diplay on low resolution. Chapelle was getting close with this rig.

http://www.svensons.com/boat/?f=SailBoats/South/southwind.jpg

Catamarans, for the most part, carry elipticals. Is that because they have the form stability to carry more heeling moment? If that is not the sole reason, then can a monohull make efficient use of an elliptical planform.

If a person were to design a lightly ballasted, 30-40%, coastal cruiser, could there be advantage to multiple(2-3) elipticals on a shorter planform as opposed to a more traditional, taller sloop/bermudian rig. Granted, with three sails, you have three masts, but could three 20' masts have a lower lever arm(mass and CE) and inertial energy than a single 40'er? OK, I could have done the calculations, but I didn't, I'm lazy. I'll get back to you on that one.

Lots of questions. Just curious if anyone has some gut feeling/experience on the subject.

tspeer

12-07-2005, 03:05 AM

... Is the CE raised so that the heeling arm is increased beyond acceptable limits. Would the reduction in A/R reduce the efficiency of the sail? ...

Here's my take on answering these very questions. Let's say that you designed the planform to produce the minimum induced drag for a given hoist, taking into account the effect of the surface, but only considering an isolated sail rig (no hull). For reference, take the semi-elliptical sail planform sealed to the surface, and compare to that. Assume the wind is uniform.

This figure shows what the design tradeoffs are as you make the rig taller or raise it up and down, increasing the gap between the foot and the surface:
http://www.tspeer.com/Planforms/Fig07.gif
Each point on the grid represents a different design, optimized for that condition.

Now say that you wanted to reduce the center of effort to control the heeling moment, so you made the planform more tapered, but didn't go all the way to a triangular head. These planforms look a lot like sailboard rigs, with the clew about 30% up:
http://www.tspeer.com/Planforms/Fig08.gif
These rigs are also have the leech twisted off somewhat so that each section has the same lift coefficient (the rig is evenly loaded so the spanload looks like the planform shape)

The design tradeoffs for this approach look like this:
http://www.tspeer.com/Planforms/Fig14.gif

You can reduce the drag by making the rig higher, but that raises the center of effort. The tapered rig allows you to either lower the center of effort for the same drag, or to reduce the drag for the same center of effort by making the rig taller.

These charts represent the very best you can do, at least as predicted by simple lifting line theory. Your mileage may vary.

tspeer

12-07-2005, 03:19 AM

...could there be advantage to multiple(2-3) elipticals on a shorter planform as opposed to a more traditional(ha! ha!, he said, "traditional"), taller sloop/bermudian rig. ...

Along about 1920, a guy named Max Munk at the NACA showed that if you have multiple lifting surfaces lined up in the streamwise direction, the drag due to lift is the same as a single surface of the same span with its lift distributed the same as the total of all the multiple surfaces. It didn't matter how far they were separated in the streamwise direction or how the lift was distributed between them.

So your multiple elliptical rigs act like one elliptical rig of the same height. The effective aspect ratio is the longest luff length squared divided by the total area. If your goal is to lower the center of effort, there's no reason to divide up the sail area - you'll get much the same performance with a single mast of the same height. They might be easier to handle, though.

For going to weather in smooth water, the taller rig is probably going to be the better bet, even if the sail area is limited by the boat's stability. But for acceleration and reaching, a bigger sail on a shorter mast may have better performance.

RHough

12-07-2005, 04:46 AM

Tom,

Munk's theorem assumes that the total lift is the same? How do laterally offset foils (bi-planes) effect each other?, IIRC there is less induced drag for the pair than for the two wings separately (Prandtl Biplane Equation)?

In other words, if multiple foils are each trimmed to Cl max how do they interact?

In the case of a wing with a split flap the total CL seems to be higher than either foil can generate without the other. Is all Munk saying that the two can be considered as one at the same "impossible" CL?

But for acceleration and reaching, a bigger sail on a shorter mast may have better performance.

Think Schooner! :D

masrapido

12-07-2005, 05:57 AM

Along about 1920, a guy named Max Munk at the NACA showed that if you have multiple lifting surfaces lined up in the streamwise direction, the drag due to lift is the same as a single surface of the same span with its lift distributed the same as the total of all the multiple surfaces. It didn't matter how far they were separated in the streamwise direction or how the lift was distributed between them.

So your multiple elliptical rigs act like one elliptical rig of the same height. The effective aspect ratio is the longest luff length squared divided by the total area. If your goal is to lower the center of effort, there's no reason to divide up the sail area - you'll get much the same performance with a single mast of the same height. They might be easier to handle, though.

For going to weather in smooth water, the taller rig is probably going to be the better bet, even if the sail area is limited by the boat's stability. But for acceleration and reaching, a bigger sail on a shorter mast may have better performance.

Are you saying that two sails have the same lift(performance) as one of the same height?
Would that apply to wing sails too?

masrapido

12-07-2005, 06:00 AM

If Tom's statement is right, what about longitudinal configuration? one sail behind the other? Would that have better performance?

tspeer

12-09-2005, 01:37 AM

Munk's theorem assumes that the total lift is the same?

Yes. Just how that is accomplished isn't specified. For example, if you have two wings in tandem, widely separated, they may be producing the same lift. As you bring them closer while still maintaining their same attitudes, the lift on the forward wing will increase, the lift on the aft wing will decrease and the total lift may change due to their mutual interference. So you have to adjust them to continue to maintain the same lift. But given the lift, Munk's stagger theorem says the drag will be the same.

But this happens automatically with keels and rudders, because the total side force from the hull & foils has to equal the side force applied by the rig, and the leeway angle will adjust itself until this is true. Similarly for the sail rig, the boat's lateral stability will dictate the heeling moment that can be applied by the sail rig, and the crew will trim the sails accordingly. Or the designer will compare rig designs at the design angle of heel.

How do laterally offset foils (bi-planes) effect each other?, IIRC there is less induced drag for the pair than for the two wings separately (Prandtl Biplane Equation)?

If you compare the biplane to the monoplane n the basis of equal span, the biplane has less drag. The Prandtl biplane equation also says that the interference of one surface on the other is the same as the second has on the first. In effect, you've split the tip vortex at each end in half, and moved one pair of half-strength vortices farther away from the surface where it has less effect.

But almost nobody uses biplane rigs that way. Instead, they split the area between the two masts and the keep the geometric aspect ratio for each demi-rig the same. So the span gets cut by 30%. If you compare the biplane rig on that basis, then it has more drag than the single rig unless the two demi-rigs are very far apart. Because as they come together, they act like a single rig with 70% of the span of the baseline single rig, resulting in twice the induced drag.

So a biplane rig could have less drag for a given center of effort height.

In other words, if multiple foils are each trimmed to Cl max how do they interact?

In the case of a wing with a split flap the total CL seems to be higher than either foil can generate without the other. Is all Munk saying that the two can be considered as one at the same "impossible" CL?

CLmax and induced drag are two different things! Clmax is determined by the boundary layer on the surface, while induced drag is determined by the wake.

Multiple surfaces properly shaped and arranged can produce a higher maximum lift than any single surface of the same chord. For a more in-depth discussion, search this forum for A.M.O Smith's "High Lift Aerodynamics".

RHough

12-09-2005, 03:55 AM

CLmax and induced drag are two different things! Clmax is determined by the boundary layer on the surface, while induced drag is determined by the wake.

Multiple surfaces properly shaped and arranged can produce a higher maximum lift than any single surface of the same chord. For a more in-depth discussion, search this forum for A.M.O Smith's "High Lift Aerodynamics".

That CLmax and CDi are different leads me to conclude that for aircraft and the underwater foils of a boat Munk's applies directly, since both systems are automatically self adjusting to load.

Below the design wind speed there is "extra" RM on a boat. The CL needed so Heeling Moment = Righting Momentmax might be something like CL=9. In that case (all wind speeds below design point) the optimum sail plan is the one that can create the highest CL.

At and above the design wind, optimized for CDi makes sense.

Thanks for the reference to Smith's High Lift stuff.

jam007

12-09-2005, 07:47 AM

Below the design wind speed there is "extra" RM on a boat. The CL needed so Heeling Moment = Righting Momentmax might be something like CL=9. In that case (all wind speeds below design point) the optimum sail plan is the one that can create the highest CL.

At and above the design wind, optimized for CDi makes sense.

I don´t think so. RM is used when going to windward and to windward it is important to get the largest Cl/Cd ratio not the largest Cl at any Cd. It is the Cl/Cd ratio of the sails+rigg+superstructure that has to be minimized to maximize the speed going to winward. For reaching maximum Cl would be the optimum.

Anders M

RHough

12-09-2005, 12:14 PM

I don´t think so. RM is used when going to windward and to windward it is important to get the largest Cl/Cd ratio not the largest Cl at any Cd. It is the Cl/Cd ratio of the sails+rigg+superstructure that has to be minimized to maximize the speed going to winward. For reaching maximum Cl would be the optimum.

Anders M

This is how I look at it:

For a given RM of x, the sails heeling force that the boat can use is sf * arm.
where sf=Area*CL*.0012*V^2
arm = distance from lateral force to sail force

100 lb.ft. RM = 10 lb SF * 10 ft arm

thus RM/arm=sail force

Sail force changes with V^2 while RM/arm is constant

At CL=y
V=z
we can calculate the area needed to balance RM at CLx and velocity z

When V changes and the area does not, CL must change so that sf*arm=RM at the new V

For V higher than the design point CL or Area must be smaller.

For V lower than the design point CL or area must be higher.

Area is hard to change with every change in V, so we change CL to a new value.

For all values of V below the design point the CL must be higher
For all values of V above the design point the CL must be lower

The V range of the design is bounded by CLmax at low V and CLmin at high V.

V=5 will need CLdesign * 4 if the boat is to use all the available RM

Sails can't do CL=4 so after CL max is reached the boat is underpowered at V < design.

The ideal sail plan needs to have the highest CLmax value below design V and highest L/D at and above the design point until CLmin is reached and the sail area must be reduced.

High L/D at design point V looses races at V<design if the sail plan cannot produce high CLmax values.

Since multiple elements can produce higher CLmax, the multiple element sail plan should win below design V

Compare two sail plans:

Set span = 20
Set total area = 100
Set CLmax = 1.5
Set V = 10

Single sail:
Lift = 18
Drag = 2.86
L/D = 6.28
aoa = 24.11 degrees

Now split the area into 2 sails of 50:
Lift = 22.20
Drag = 2.18
L/D = 10.19
aoa = 24.11 degrees

More lift and less drag!

At the same aoa as the single sail each of the two sails would have CL=1.85

If we force them to have CL=1.5:

Lift = 18
Drag = 1.43
L/D = 12.56
aoa = 19.56

the total lift is the same as the single sail (area and speed are the same), but the aoa and L/D change to be even better than the single sail case.

Same lift, lower drag, higher pointing angle.

The split rig has higher L/D and higher lift. Unless they interact some way to reduce the performance of each sail.

Munk assumes Lift is equal for the multiple elements. On a sail boat this is true only when RMmax is reached.

Below the design speed we can use all the CL we can get, since boat speed depends on available force.

The comparison above is 3D lift line theory and does not include profile drag (which is assumed to be the same for each sail).

If we keep the same span, each time we split the rig the AR of each element goes up and induced drag goes down.

If profile drag is the same and the parasitic drag from the rig scales linearly with area, the more elements the better until the rig looks like a row of turbine fins.

RHough

12-09-2005, 01:17 PM

Just for fun I plotted the effect of multiple sails as I understand the theroy.

The sail area is constant
The CL is constant
The Span is constant
The wind speed is constant
The Profile and Parasite drag is constant (CDp = 0.1)
Since Lift is constant the induced drag is the same per Munk's theorem so total CD=CDi + CDp
Since Lift and Span are constant the RM needed is also constant if each sail has the same planform.

The Trim angle is calculated so the CL based on 3D lift line is 1.5

More elements should point higher?

jam007

12-10-2005, 02:17 PM

As the velocity is created by the sails and the relative windspeed increases with boatspeed any boat with sufficently low drag (mostly hyrodynamic) can reach a speed high enough to generate maximum sideforce for its optimum RM at any windspeed.
This shows that you can´t separate rigg construction and sailplane from boat design.
A high drag boat will benefit from generating a lot of lift even at the expence of extra drag. A low drag boat benefits from high L/D ratio at the expence of lift.
This can be seen in sailboat construction. As the drag of the boats decreases the rig gets more and more wing like trading max lift for L/D ratio.

Anders M

PS Cl/cd ratio has to be maximized, not minimized as I managed to write in my last post at one place. Sorry

tspeer

12-10-2005, 02:39 PM

...Below the design wind speed there is "extra" RM on a boat. The CL needed so Heeling Moment = Righting Momentmax might be something like CL=9. In that case (all wind speeds below design point) the optimum sail plan is the one that can create the highest CL.

That may be true if the height of the rig and planform shape are held fixed. But CDi is still important at low speeds. If you aren't pushing the hull's righting moment, your rig is too short. If you look at 18' skiffs, they have multiple rigs - the light air rig not only has more sail area, it's taller, too. This is an arguement for putting a very tall mast on a boat and reefing at comparatively low wind speeds.

I suspect one reason tall rigs perform well in light air is not just because the rigs can reach up into stronger winds, which is the typical explanation. But since induced drag for a given amount of lift is inversely proportional to velocity squared, it's even more important to minimize induced drag at low speeds than it is at high wind speeds.

Most boats have so much windage that the maximum L/D occurs above stall onset. So that's a big reason why you want to raise the maximum lift as high as possible.

RHough

12-10-2005, 08:35 PM

Good points made, maybe I'm not posing the question properly.

I'm only considering boats that sail upwind in displacement mode.

The design point for the boat is 12 knots VTW. The sail plan is designed for maximum L/D with enough area to use max RM. Say we managed to get L/D max at CL=.5 with a wing sail.

Now sail the boat in 6 knots, we need 4 times the area or 4 times the CL.

The wing can't grow, so it needs to make CL=2.0 to use the available RM.

If the CL max of the wing is 1.5 the boat will be underpowered, if we trim for max L/D CL = .5 the boat may not move at all.

The question is:

What combination of sails will give the highest CLmax of a given area without giving up a high L/D?

Historically, it's been a large main and a fractional height jib.

The only sail driven vehicles that have not ended up with this sail plan are ice boats, landsailers, A-Class cats, and windsurfers.

Skiffs, where boat speed is equal to or greater that VTW on all points of sail, use the large main, fractional jib upwind.

Why?

redcoopers

12-11-2005, 02:57 AM

I haven't been watching this post for a while, so please forgive me if my comments are not very applicable!

However, the design of sailforms is very much a practical nature. Almost every normal racer is designed to race in ~5kts to +50kts windspeed. This means that first and foremost, the sails must be easily handled.

If you've ever raced in <5kts, you know that you're praying for some smooth laminar winds near the top of the sail. Most of your crew are lying flat on the leeward side to try not to disturb any air at all. Hence, we want the tallest rig we can to both 1) develop a righting moment to lengthen our LWL and 2) catch any undisturbed air which is above the competition.

Many sailboats feel like they get in the "groove" in about 10-12 kts. At this point, the sails are trimmed in what we consider the optimum condition. However, nearing ~18kts, most boats need to start spilling air. The DELFT works point to this as well. In many VPPs, there are variables having the naming convention alike to "REEF". This has nothing to do with truly reefing the sails, but instead imply mostly to easing out the mainsail to reduce the righting moment.

In terms of sailing, the feel is that our jib supplies the driving power - and wind tunnel tests also show that it has a very large Cl compared to the main. The main on the other hand, is very responsible for the righting moment and general tuning of the boat. Skippers know that if they constantly communicate with their mainsail trimmers while sailing upwind they'll do well - the jib trimmers are mostly left to following their tales.

So ultimately, what I am saying is that for a sailboat, instead of thinking of a design point for a Cl, think instead of a large plateau in which you can spill enough wind out of your sails to maintain a stable platform. In 8kts of wind, you may be able to get 5kts of boatspeed. In 15kts of windspeed, you may need to start easing out the main to get 6kts of boat speed. What most designers want is a boat which quickly gets up to near designed boatspeed and then gradually increases afterwards.

So the reason many boats use a frac rig is because all sailing is a compromise. A nice roach on a main will supply power at very low wind speeds. In high winds, you can simply ease the traveller and let the jib with its lower CE do the work. Furthermore, when running / cracked-off, the extra mainsail area is very beneficial. Remember that all inshore is 0.5 offwind, and most offshore racing can be usually 0.7 offwind.

-Jon

brian eiland

01-15-2006, 06:04 PM

Something different...a gentleman wrote to me recently;

I have been interested in the idea of a forward raked mast with an
unusual sail layout for quite some time now, originally seeing the
idea at a student design show. The boat looked like the sailboat
equivalent to a future speculating auto show "Concept Car", with wild
ideas that gave little concession to practicality.
Kyle

Have a look at this futuristic design (http://boatdesign.net/forums/showthread.php?p=72778#post72778)

saildog

02-28-2006, 12:33 AM

So, do the same sail aerodynamics apply to square rigger sails? For example, as the foremast, main and mizzen are turned against the ratlines wouldn't the principal be one more of thrust than aerodynamics of high and low pressure areas? I'm building a model square rigger to test some of these theories....anyone with thoughts, ideas on this? I not an expert here but would love to hear thoughts and opinions...

RHough

02-28-2006, 12:51 AM

brian eiland

04-12-2006, 02:22 PM

Letters to the Editor
Yachting World magazine
Let me open this letter by complimenting your magazine on being my absolute favorite of all the yachting publications. It is extremely well balanced, and provides a wealth of information in each issue. The presentation is also extremely good.

With this mind I want to next express my real disappointment in a portion of an article that appeared in your May 06 issue on the “Parasailer, The Ideal Atlantic Sail”. The problem appears under ‘the history of the hole’ when attempting to explain the ‘slot effect’, the Venturi effect.

Quote: “On a yacht, adding a headsail speeds up the air through the slot and reduces the pressure on the lee side of the main(sail). The main then tries to equalize by moving forward into the low pressure, creating drive.”

This is entirely UNTRUE. This has so often been repeated in sailing books and publications that it has become accepted as gospel. As one knowledgeable sailor put it, like the Dracula legend, “there’s no way to finally put a stake through the heart of that old explanation—it just keeps coming back to life”

Adding a headsail to the rig actually decreases the flow in ‘the slot’ between the two sails, which ends up cutting down on the mainsail’s drive contribution and increasing the headsail’s contribution. The pressure on the lee side of the mainsail is increased as a result of the slowed slot air, not decreased as the article claims.

If a reader is so inclined to look for more discussions on this subject I would invite them to visit a number of forum discussions that have occurred in reference to this matter at the BoatDesign.net forums.
A few of them:
Sail Aerodynamics, http://boatdesign.net/forums/showthread.php?t=457
How Sails Work, the slot effect: http://boatdesign.net/forums/showpost.php?p=5685&postcount=3
Cutter Rig Pointing, http://boatdesign.net/forums/showthread.php?t=5596
The Slot Effect, http://boatdesign.net/forums/showpost.php?p=40247&postcount=108
http://boatdesign.net/forums/showpost.php?p=23390&postcount=13
http://boatdesign.net/forums/showthread.php?t=623

It’s really difficult to believe the number of accomplished sailors who still do not fully understand the ‘slot effect’.

Brian Eiland
RunningTideYachts.com

Guillermo

04-13-2006, 06:06 PM

Brian,
This is what I wrote to them on February the 18th:

"Dear Sirs,
In March 2006 number of your magazine, there is an article about the PARASAILOR chute (Page 44 to 47), where there is a wrong concept on the aerodynamics of sails. It is stated that air accelerates between main and a foresail, thus reducing pressure on the leeward side of the main and so increasing her lift. I'm afraid it doesn't happens like that. As matter of fact air flow slows in the slot, increasing pressure on the leeward side of main (This, in conjunction with downwash from foresail, is why a mainsail backwinds close to the mast). When a foresail is used, the mainsail experiments a detrimental interference from it (mainsail is sailing in the "bad air" of the foresail). The overall increase in sails force, when a foresail is used in conjunction with a mainsail, is due to the increase in the angle of attack of the air in the jib (upwash) induced by the presence of the main (jib is "sailing in the safe leeward position" relatively to the main), as well as more air is directed around the leeward side of the foresail, causing higher velocity (lower pressure) and thus more force. Venturi effect in the slot increasing the lift on the mainsail is an old "myth" stubborngly still wandering around, but long time discarded.
Regards,
Guillermo Gefaell
Pontevedra, Spain"

They did not published it yet.

Toby P

04-21-2006, 07:09 AM

Apologies if this has been covered previously, but I have a question.
Why is the positive effect of the main on the jib greater than the negative effect of the jib on the main? Why are the two effects not equal, thereby cancelling out? Or, why is it not dependant on the particular sail configuration (e.g. relative sizes of main and jib, overlap etc)? I could just about understand that there could be particular instances where the overall effect is positive, but i can't see why it is the general solution.
Can anyone explain?

jam007

04-21-2006, 08:36 AM

Why is the positive effect of the main on the jib greater than the negative effect of the jib on the main? Why are the two effects not equal, thereby cancelling out?

Reasoning about the effect from the main on the jib or the jib on the main might be missleading. It is better to see them as a multi foil system. The system depends on the parts in a comlex way.

See also earlier postings in this thread.

Anders

Toby P

04-21-2006, 09:54 AM

Thanks Jam.
As I understand it the flow between the jib and main, in 'the slot', is slowed down so that the air flow on the leeward side of the main and windward side of the jib is slower. This means that the pressure to leeward of the main is not as low as it would have been if the jib was not there, so the main is not producing as much vaccuum lift. However, this negative effect is more than offset by the fact that the jib produces more lift than if it were operating in isolation. How is this? I can see that the jibs lift is increased, but why is it increased more than the mains lift is decreased? Please explain :confused:

Rick Loheed

04-21-2006, 01:55 PM

go here, and study the images and text;

http://syr.stanford.edu/SAILFLOW.HTM

Another point I would like to make comes from the turbine, propeller, and windmill world. With a rotating disk, more blades= higher 'solidity'. Higher solidity means that without a duct, air would prefer to go around a windmill with many blades like an axial turbine compressor wheel, rather than through it. That is why windmill designers prefer 2 blades- or even one if the forces and balance issues can be dealt with. Turbines are OK with that because they are pulling it in- creating a 'sink' in the flow ahead known as the 'vena contracta', which also happens through an unshrouded propeller disk.

A set of sails in a sloop rig behave as one foil, with effectively sort of a "Hanley-Paige' slot between the jib and the main. The Hanley-Paige slot was developed with STOL aircraft in mind. I say 'sort of' because the configuration of the sloop rig is highly adjustable- we are only really speaking of the windward 'system' here. When it is wide open, it is a different animal.

One successful approach to racing I used to employ with great success is to load up the sails until I see the leeward sidestay telltale on my M-16 Scow point up into the slot, then come up just enough to see it trail back again. This effectively proved two things- that the flow can actually reverse in the slot which at the time I found fascinating, and that by barely bringing it back I could be assured I was 'stepping on the gas' as much as the sail set could take, so long as the telltales on the leach of the main agreed. Because the Scow is hard to sail fast in low winds, I spent considerable time on the leeward side of the sail inspecting my real life 'wind tunnel experiment'!

CT 249

04-21-2006, 09:50 PM

"A set of sails in a sloop rig behave as one foil".

Which leads me to ask - when we look at aspect ratio, is the important factor the AR of the individual sails, or the whole rig?????

I have a yacht with a low-aspect genoa and high-aspect main. I can see intuitively that this may mean that the low-aspect genoa "wants" to operate at a different angle of attack to the mainsail, but is this correct?

Tom? Rick? Buehler?

frosh

04-22-2006, 06:58 PM

Seems very correct to me?
The angle of incidence of apparent wind striking the luff of the low AR genoa will be significantly different to the angle of apparent wind striking the mast when considered in relation to a line drawn on a fore-aft direction on the hull.
When sheeting the two sails the luff area of genoa is generally more open than the luff area of the mainsail ( in the lower two-thirds ) which is tighter to the centre line. This lends support to the concept that AR of mainsail needs to be higher than the foresail as it necessarily must be more aerodynamically efficient to provide it's maximum potential drive.

Vega

04-22-2006, 07:53 PM

[QUOTE=Rick Loheed]go here, and study the images and text;

http://syr.stanford.edu/SAILFLOW.HTM
[QUOTE]

Thanks, great site.:)

tspeer

04-28-2006, 02:00 AM

"A set of sails in a sloop rig behave as one foil".

Which leads me to ask - when we look at aspect ratio, is the important factor the AR of the individual sails, or the whole rig?????

The whole rig. The best indicator of lift/drag ratio is the wetted aspect ratio - the span-squared divided by the total wetted area.

CT249

04-28-2006, 06:22 AM

Thanks.

Does that partly explain the success of assymetrics? They lower the aspect of the entire rig, perhaps down to the point where (according to Marchaj etc) there's a massive "spike" in the rig effectiveness at reaching angles. I'd imagine a skiff, fast cat etc may well go downwind at the 30-35 degrees apparent where Macharj's figures show the rig with an AR of 1 is extremely efficient (I've never bothered to really check the angles).

Does this massive spike (according to Marchaj) also partly explain the way assy boats really pick up pace at certain angles? Obviously attached flow and apparent wind are major factors, but the lobe for an AR of 1 at 35-45 apparent is incredibly distinct (in Marchaj) and it could perhaps account for a lot of the way that assys light up at certain angles. Symmetrical kites, with their generally shorter poles resulting in a higher AR (????) would have more even performance across the angles.

Is Marchaj correct re AR and angle of incidence? Much of his info seems correct to my gut feeling; I know high-aspect rigs (F16 cats, Canoes) really light up at tight angles and get quite ordinary at broad angles. It's always interesting to see a Laser (smaller rig, similar wsa, more beam, less LWL) hanging in with a Canoe on square runs and broad reaches in light winds.

However Marchaj's rigs are all so different (AR from 6 to 1/3) that they may be hiding trends, no?

Or is Marchaj old hat? Can he be slammed in the same way he dissed Manfred Curry?

Rick Loheed

04-28-2006, 02:59 PM

I'm not sure if it's apropo, but one thing a monohull sailor has to learn when sailing a performance cat is that you want to sheet in as hard as you can without stalling, not a little as you can without luffing.

When racing in any sport, pressing the gas as hard as possible is the right thing to do to win. It's trying to define 'as hard as possible' that is the trick. Monohull racers know this too, trust me....I have won six packs of beer off catamaran sailors who doubt it.

Skippy

04-28-2006, 05:32 PM

When racing in any sport, pressing the gas as hard as possible is the right thing to do to win. It's trying to define 'as hard as possible' that is the trick.
On a cat, it's probably defined as the point where the windward hull starts to fly. On a keelboat it's very different, because you have a direct tradeoff between lift and heel.

brian eiland

05-14-2006, 09:39 PM

interesting observation

....excerpt from Yachting World on America's Cup (http://www.ybw.com/auto/newsdesk/20060414193656ywamericascup07.html)...
But what was interesting was to see how tightly the new American boat can turn without losing pace. Just as impressive is the boat's ability to perform the America's Cup equivalent of a handbrake turn, carving to a halt before accelerating away once Dickson's foot is back on the gas.

Such nimble behaviour could well increase the speculation as to what lies beneath the waterline, but the clear waters off the Valencian coast are starting to reveal more and clarify part of the picture.
It seems more likely now that USA-87 has a 'conventional' rudder and keel configuration. The question now, is whether she has a trim tab on the back of the keel or perhaps a forward rudder.

Part of the reason for the widespread speculation is the position of the rig and the bow sprit that goes with it. But Shosholoza tactician Dee Smith believes that the reason for the difference is far simpler.

"BMW Oracle do not have anything strange under the water," he said. "All the rigs in the fleet are further forward because of the fat top mainsails. Also, the jibs are bigger, the boats are lighter and so the rigs have to go forwards.

"Last year we saw a lot of rigs moved forwards because nobody could change them in time for the sail development. We saw that the strongest boat in the fleet last year had the smallest mainsail, so what does that tell you about balance?"

That it continues to be the biggest key to Cup yacht design?

FuLLkEeLs

05-16-2006, 12:09 AM

With most of the emphasis being on the sails, it is important to give the keel its proper due when discussing what makes sails propel craft anywhere but before the wind. Although aircraft operate in 3 dimensions of space, they generally do not operate in two media of matter (air and water) like sailing craft and thus the keel becomes paramount in working with (or against) the sails to achieve optimum speed, balance and control.

saildog

05-19-2006, 02:27 PM

CT249

05-19-2006, 07:35 PM

RHough

05-19-2006, 10:12 PM

RHough

05-19-2006, 10:28 PM

Brian, while I understand the theoretical efficiency of the headsail due to its cleaner leading edge, I'm still puzzled.....if mainsails are so slow, then why do the most efficient boats of all (speed windsurfers, YPE, foiling Moths, C and A Class cats) all use cat rigs?

If jibs are more efficient, why have A Class and C Class stopped using them? Why is a guys like the world's fastest Canoe and C Class sailor so convinced of the speed of cat rigs that he's experimenting with them in Canoes?

Why do fractional rigs generally go faster than masthead rigs when they have been tried together on the same hull (apart from perhaps in light wind areas when the fractional carries a fractional kite?).

There is more control over shape with sails set on elastic spars. Better area distribution is also possible, you wont see many square top jibs. :)

Fractional rigs always win when total sail area is measured. Class after class has ended up with the fractional small jib and large main for course racing.

If the rig can be sized so that CL over about 1.6 is not needed (upwind), there is no reason for a multiple element sail plan. Windsurfer, foiling Moths and Cats all have very high righting moment to sail area ratios. There is little need to control the heeling moment with low aspect ratio rigs, or splitting the area between main and jib.

On single-handed dinghies the added workload of trimming two sails to work properly probably slows the boat more than any extra power from the rig gains. On boats that always sail with the apparent wind forward of the beam wing-sails are a logical choice.

One of the worst sail plan shapes is a triangle. The added efficiency of a clean luff does not make up for the losses from the tip shape.

I don't know that any one of these reasons works for all the boats you mentioned, but they all apply to some degree.

frosh

05-19-2006, 10:40 PM

Brian, while I understand the theoretical efficiency of the headsail due to its cleaner leading edge, I'm still puzzled.....if mainsails are so slow, then why do the most efficient boats of all (speed windsurfers, YPE, foiling Moths, C and A Class cats) all use cat rigs?

If jibs are more efficient, why have A Class and C Class stopped using them? Why is a guys like the world's fastest Canoe and C Class sailor so convinced of the speed of cat rigs that he's experimenting with them in Canoes?

Why do fractional rigs generally go faster than masthead rigs when they have been tried together on the same hull (apart from perhaps in light wind areas when the fractional carries a fractional kite?).

The main inefficiency of the mainsail is the turbulence caused by the mast disturbing the laminar flow on the leeward side of the sail in the luff region. This is overcome by the pocket luffs of sailboards and most Moths and rigid wingsail of YPE and C class cats. With the A cat I believe that very high aspect mainsail (almost the identical planform to a high performance glider wing) overcomes any advantages of sloop rig due to sufficient RM to support the greater heeling force.
There is a point when the apparent wind moves behind the beam when the jib starts to become too full and twisted when it is eased (it has no boom). If at this point of sailing the area in the jib was transferred to mainsail the overall efficiency would be greater and result in a L/D increase of the rig as a whole.
The situation with fractional vs masthead rigs is more complex than it appears on the surface as usually many other variables are also present in the comparison. For starters only, most mastheaders have a simple triangular mainsail often with short battens and a non-flexing mast.

saildog

05-20-2006, 12:00 AM

Thank you for the reply and the help. You see, I have been pondering and thinking of the aerodynamic wind principles that the triangular cut sail is based on. High pressure areas, low pressure etc., but......, when the square rigger sails on a ship are set to run before the wind (basically a somewhat flat shape with the coming from the stern), isn't this more of a rearward force pushing on an object and moving it and the aerodynamic wing principles of somewhat null? Am I off course here?

Stanford University is working on a modern Clipper Ship Rig.

Here is a link to what they have published so far. They only went to AWA = 80, so that does not answer your DDW question.

Like any sail, when DDW the drive from the sail is CD x area x air density x velocity^2. For a flat plate of width = height x 2 (20 x 10), the CD is 1.18 or so.

Clipper Sails (http://syr.stanford.edu/HISWA_Tyler_2002.pdf)

Hope this helps. :)

CT 249

05-20-2006, 12:28 AM

brian eiland

05-22-2006, 11:23 AM

Frosh and Rhough, I agree with you and actually I'm up on that stuff - I just wonder what Brian, the man who is most involved with pushing the mast-aft rig, has to say about all the good reasons you have given.
Sorry CT, I've just not had any time to respond at this time. I would have to go back and find a number of other references to help substaniate my claims. I will eventually. Meantime a few others are taking up the argument.

Besides my real income, I'm working on a new RIB concept, and the water-jet propulsion, and rim-drive propulsion that goes along with it, and the fishing design. Guess you might say I'm in the 'power mode' at this time.

Real short reference on uni-rig capabilities....I would reference you back to #3 posting (http://www.boatdesign.net/forums/showpost.php?p=5685&postcount=3) in this subject thread:
More recently I ran across a news article in the Sept issue of Seahorse magazine which discusses the very interesting full scale prototyping work being carried out on a J-90 class boat by Eric Hall of Hall Spars. Eric is now on his third-generation, free standing ,carbon wing rotating mast, with a una-rig mainsail. His “ thought process (and maybe not entirely logical) was: If biplanes became monoplanes and monoplane wings shed wires, why not an unstayed una-rig upwind” Boy, you would surely think this was the ideal upwind rig. In responding to an inquiry on upwind performance, Eric responds, “ first, of course, the boat would be improved upwind with a No.1 jib. Generally, we could not point as high as the others here (Block Island) and therefore had difficulty holding lanes.”

RHough

05-22-2006, 11:43 AM

Thank you for the reply and the help. You see, I have been pondering and thinking of the aerodynamic wind principles that the triangular cut sail is based on. High pressure areas, low pressure etc., but......, when the square rigger sails on a ship are set to run before the wind (basically a somewhat flat shape with the coming from the stern), isn't this more of a rearward force pushing on an object and moving it and the aerodynamic wing principles of somewhat null? Am I off course here?

I think you have it. Dead Down Wind is dead slow for that reason. No amount of sail area will make a boat faster than the wind when sailing DDW.

Force = CD x density x area x velocity^2, as you sail faster the velocity drops and the force gets smaller. The aerodynamic forces that create lift (circulation and momentum) are not at work when the sail is square to the wind. On a square rigged vessel with more than one mast the forward sails are blanketed by the aft sails so V^2 is lower on the forward sails. Turning the boat up to unblanket the forward sails should allow more of the full sail area to work at full value of V.

As soon as the boat turns up a bit more so there is spanwise flow on the sails, lift is generated and CL replaces CD in the force equation.

saildog

05-23-2006, 09:46 AM

Perfect sir, thats what I needed to know. Greatly appreciated.

saildog

05-23-2006, 10:08 AM

I note that on this study the streamlines, when comparing 30degrees of sail angle up to 80degrees, the streamline shows a reach to be the most efficient point of sail, while DDW on the clipper ship the least. I am understanding this right?

Stanford University is working on a modern Clipper Ship Rig.

Here is a link to what they have published so far. They only went to AWA = 80, so that does not answer your DDW question.

Like any sail, when DDW the drive from the sail is CD x area x air density x velocity^2. For a flat plate of width = height x 2 (20 x 10), the CD is 1.18 or so.

Clipper Sails (http://syr.stanford.edu/HISWA_Tyler_2002.pdf)

Hope this helps. :)

RHough

05-23-2006, 10:50 PM

I note that on this study the streamlines, when comparing 30degrees of sail angle up to 80degrees, the streamline shows a reach to be the most efficient point of sail, while DDW on the clipper ship the least. I am understanding this right?

I think you are.

When there is spanwise flow across the sails, as is the case on a reach, aerodynamic lift creates a greater force than the pure drag when sailing DDW.

Rick Loheed

05-24-2006, 10:34 AM

I found a post that had a reference to JavaFoil (http://www.mh-aerotools.de/airfoils/javafoil.htm), Martin Hepperle's 'relatively simple' inviscid foil analysis program that will do multi-element airfoils. It is great for illustration purposes here.

Clearly it shows the affect of the whole system- actually, as a cascade of foils. Further Aft foils must have more incidence- but when incidence is added, they help increase circulation around the whole system, increasing the forward foils effectiveness by inducing more incidence and accelerating more mass about the whole mess.

Brian Eland will find this of great interest I am sure, it also shows clearly the slowing of flow in the slot and the resulting tendency to backwind the main. The middle sail does not suffer quite as badly. I tried to simulate an aft mast rig here by using a cutter rig and smaller chord (50%) 'main'. By opening it up to a reach, it is very similar to a square rigger on a reach as well, since this is only a 2D simulation, though Martin has included aspect ratio effects on the last tab, and they are shown to affect the lift coefficient!

Impressive program.....

Rick Loheed

CT 249

05-25-2006, 04:21 AM

Sorry CT, I've just not had any time to respond at this time. I would have to go back and find a number of other references to help substaniate my claims. I will eventually. Meantime a few others are taking up the argument.

Besides my real income, I'm working on a new RIB concept, and the water-jet propulsion, and rim-drive propulsion that goes along with it, and the fishing design. Guess you might say I'm in the 'power mode' at this time.

Real short reference on uni-rig capabilities....I would reference you back to #3 posting (http://www.boatdesign.net/forums/showpost.php?p=5685&postcount=3) in this subject thread:

Yes, there's one guy who reckons his boat would be improved with a jib - that's not surprising, since he didn't mention cutting down the mainsail we can assume he was going to add area.

But we still have one one-off J/90 where it is assumed a jib is faster, and on the other hand we have every competitive C Class, A Class to show us that years of development have demonstrated that cat rigs are faster.

I haven't got any problem with the idea that some - maybe the vast majority - of boats are faster with the added SA of a headsail. Headsails have lots of advantages, we all know that...seems to me they become more effective on heavier boats.

But it still remains proven by decades of experience that cat rigs are faster in A Class and C Class cats, and there's plenty of evidence that many of the times when similar monos have been rigged as frac or mastheader of equal SA, the big mainsail boats have been faster. Given all that, surely it's reasonable to say that mainsails cannot be all that slow.

Experiments with big genoas and small mains have been going on at least since Sherman Hoyt's 6m "Atrocia" of the '30s or so....big mains still tend to win most of the time AFAIK.

Isn't it possible that some testers have exxagerated the deleterious effects of a mast on a main? Bethwaite, for example, disagrees with Marchaj about RAF type sails and pocket luffs. I don't know who is right, but practical experience shows that "cleaning up the luff" via the use of wing masts is a distinct performance improvement, but NOT the sort of earth-shattering improvement that is worth giving up lots of things like gust response and handling qualities for.

Classic case comes in OD racing in wingmast boats. If you jam a Tasar wingmast the wrong way around, you'll drop back from the lead to back 1/3 of the fleet or worse in an interclub - that's a significant drop but only of the same order as (say) pulling on too much vang. The whole mainsail goes pear-shaped, the flow to lee is terrible, but the loss is only a fraction of a knot.

If you sail a Taipan 4.9, which has a very large wingmast, you'll find that some top sailors de-rotate in a breeze but others over-rotate more. Since one of those options would (one assumes) really mess up the airflow over the mainsail, yet both options actually perform similarly and both are faster than leaving the mast in the normal "clean" position, once again it would seem (to my inexpert mind) that the "cleanliness" of the flow over the luff is no more important than correct gust response, correct draft, correct twist etc.

In the light of all that, to go for a rig that tends to be less self-tending than a mainsail would be a hard choice to make.

sharpii2

05-25-2006, 05:40 AM

I'm studying square rigger sails and how they work. I assume the same theories of low pressure on the leeward side "pulling" the ship along are the same on a square rigger for the various points of sail as the yards are turned amidships (close hauled, close reach, beam reach, broad reach). However, what are the aerodynamics of the square rigger sails when the ship is running before the wind? Thanks beforehand for any help anyone can give.

On the old clippers the wind went over the yard and down the sail when the ship was sailing downwind. This gave two advantages:

1.) the low height to width aspect ratio now became the higher width to height ratio, and

2.) It also added a slight upward lift component which was very welcome by these hard driving clipper masters.

The clippers were intended for mostly down wind sailing anyway, so there was not a lot of incentive to improve thier upwind performance. Also, by the time most of them kissed the water, there were steam tugs available to drag them into and out of port.

Hence, especially on the later ones, you see very low height to width ratios on thier sails.

It is also interesting to note that the 'wind jammers' that followed tended to have higher height to width ratios on thier square sails. Maybe that was because lower operating cost was supposed their primary virtue. And tug fees just did not work out in that equation.

Bob

Rick Loheed

05-25-2006, 09:23 AM

I'll probably get in trouble here- aside from the fact that every case is a different problem in general, the fact that a sloop rig comes into it's own on heavier boats- whereas a single wing or well done cat rig works better for high speed- is no surprise to me. The game for lightweight high speed boats is more one of striving for max L/D simulteneously with max CL, whereas heavier boats tend to need Max CL. Speeds are lower, boat drag is higher, it's not as 'knife edged' of a design problem. Since the jib/main combination is widely adjustable, and high lift, it can sort of do both and you can usually change the headsail size.

For a nice wingmast discussion, and more on the aerodynamics of these things visit http://www.tspeer.com/ if you haven't already.

I don't have recent experience with the boats you mention, I sail a C&C 24, my sailing canoe, and an E-Scow, none have rotating masts but the sailing canoe sail is pocketed. The E-Scow of course is plenty fast.

However, I have had experience with rotating (but not wing) masts, both on an M-16 Scow as well as on small Catamarans. Overrotating allows the mast to bend more forward, tending to flatten the sail shape considerably in heavy air. Typically though, the M-16 carries more camber cut into the main sail (meaning there is more to remove) than a cat. Can these wingmast rigs bend laterally?

Here is a comparison using Javafoil of a simple 15% camber (pretty high lift) 40% max camber location Jib/main combination arbitrarily loaded to near Max CL, and a 20% Clark 'Y' wingmast shape based on methods from Tom Speer's wingmast paper. For an input Aspect ratio of 10 for each case, this simulation shows a Max Cl of 2.11 for the main jib combo readily achieved for the combination, whereas the wingmast gets to a fairly typical Cl of 1.2 max. before separation occurs. I did not look at the polars, I have to get to work- but the point here was about max CL. I need to read Martin Hepperle's web site to find out how he's implemented the aspect ratio corrections in a 2D section study, but for now I will assume it analyzes the center-span pressure distribution for a rectangular planform. It's the only quick multi-element 2D foil simulator I have...the VLM code would take longer and this has to happen over coffee this morning...

it should be a pretty fair apples to apples comparison and it's meant for illustration only.

tspeer

05-28-2006, 07:02 PM

Brian, while I understand the theoretical efficiency of the headsail due to its cleaner leading edge, I'm still puzzled....

The theoretical effectiveness of the headsail is not due to its cleaner leading edge. It's due to its interaction with the mainsail. The mainsail gives a boost to the head sail, and the headsail reduces the lift of the mainsail. It's exactly the same as lee-bowing your competitor - the jib gets a lift from the main, and the main gets a header from the jib. But the effectiveness of the whole combination is increased, so there's a net gain.

The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act. A mast with no separation zones would be a better leading edge than the thin leading edge of the jib - hence the effectiveness of wingmasts and pocket luffs.

There are two separated zones behind the mast - one on the lee side and one on the windward side. The jib reduces the effect of the leeward surface separated zone in a couple of ways. One is by reducing the wind speed at the mast, so the mast effectively sails in a lighter wind, producing less drag. The other way is by creating a favorable pressure gradient to the jib trailing edge that encourages the separated flow to reattach to the mainsail.

In high winds when the mainsail is operating at a lower angle of attack, the backwinding of the main by the jib, causing reverse curvature in the luff of the main, also reduces the windward separation zone. This is anagous to under-rotation of a wingmast.

So the jib reduces the drag of the mast over what you'd have with the same rig bare headed. This is in addition to the lee-bow effect.

Rick Loheed

05-30-2006, 10:02 AM

Yes, exactly what the JavaFoil results show for the main/jib interaction. Well put again, Tom Speer!

There is also a nice discussion of this in Tom Whidden's book, 'The Art and Science of Sails" presented in more layman's terms, which I read over the holiday. Interestingly, according to Tom W. the highest incidence is not at the highest L/D either, but may in fact still be the fastest depending on the other mitigating factors. Not a huge surprise- but interesting. I experimented with that some yesterday in my C&C using my GPS to verify speeds. That was very interesting- not extremely conclusive, but Loads of fun!!....

The book is a bit dated- published in the early '90's, the computers shown were not equal to what you are using to view this post today...but the principles are the same.

CT 249

06-05-2006, 08:00 AM

Very interesting, thanks.

Rick, some of those wingmasts do bend laterally. The point about max Cl seems good...a theoretical backing to the old "big jibs are powerful" rule of thumb.

Tom, I didn't realise the mast improved the main's efficiency, I was going off information like Marchaj's notes about things like "the adverse effect of the mast on the sail" and Bethwaite's words "if we take the mast away, and consider the performance of a sail set on a wire, such as a headsail, the situation becomes much better..."

Why has the view changed? Is it that the testing techniques have improved, or where some of the earlier theorists just using wrong principles? My Marchaj has him talking about air speeding up in the slot, which as you and Gentry point out, is the opposite of what happens. Or so I hope, as I feared (and I think Gentry says) aerodynamics is not an area for laymen.

As you say "the jib reduces the drag of the mast over what you'd have with the same rig bare headed. This is in addition to the lee-bow effect" why do A Class and C Class perform better without jibs? Were you just using the case of a mainsail of the same size and adding a jib, rather than redistributing a given area between the two sail? Is the drag reduction fairly minor? I assume it's actually fairly minor (although "minor" is indefinable) because otherwise boats with low foretriangles (and hence a smaller % of the mainsail with the advantage of the jib's effects) would be less effective than masthead rigs in some conditions, wouldn't they?

Aaaarrrrgghhh, it's all too complicated!

gggGuest

06-05-2006, 04:40 PM

My fairly ill informed opinion has it that a single sail rig can operate at lower angles of attack than a sloop rig, just because of the interaction. Where a craft is power limited in going upwind this is not an issue, where a boat is low drag (Icenpay, C, A) then the higher pointing from the single sail wins.

Another theory of mine, with only the very barest of empirical evidence from feel, based on recent IC experience is that I suspect that s sloop rig like the current ICis much easier to keep in "the groove" upwind than a single sail boat. That would suggest that for craft about the borderline between the two a sloop rig might be faster for shifty and variable comnditions and the less talented helm, and the single sail faster for more talented helms and steadier conditions. Stevbe C's opiniopn on this would be interesting.

Paul Scott

06-06-2006, 11:04 PM

Saildog-

My Grandad was the master of a few of the Guano Trade Clippers during the first part of the 20thC, and, unless the ship was loaded to sinking, they never went dead downwind, unless the conditions were huge and the waves were right, because being on any kind of a reach 'brought the wind forward', and all the sails were drawing, bigtime. Well, at least certain combinations of sail, depending on the conditions and the particular boat. Going dead downwind if I remember right was only done in big wind in certain seastates, pretty much with only the forward sails set, depending on the individual clipper. They really steered with the sails. He used to tell me about leaving a couple of the really high sails set so there would be some power while the ship was in a deep trough. The Southern Ocean. So cool.

Bob-

You are so right about the lift aspect. From what my Grandad told me, the forward sails were REALLY set up for lift.

Paul

saildog

06-06-2006, 11:55 PM

Lord Nelsons logs are actually available somewhere on the web, I wonder how much DDW they did and under what conditions.

MAINSTAY

06-10-2006, 08:22 PM

Tom Speer said:
"The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act. A mast with no separation zones would be a better leading edge than the thin leading edge of the jib."

I can hardly believe that a man of his reputed stature in this community, actually advocates that it would be better if the jib has a mast on its leading edge also. Has he never seen a slack luff in the wind shadow of a mast? Isn't this portion of the sail sufficiently forward facing? It seems this partion would produce more drive and less heel forces than any other portion of a sail.
Larry

RHough

06-10-2006, 08:43 PM

tspeer

06-11-2006, 12:06 AM

Take a look at Polhamus' leading edge suction analogy (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680022518_1968022518.pdf), expecially figure 2. Although the paper is concerned with planforms that have more sweep than a typical jib, the flow picture at a jib's leading edge is quite similar.

A sharp leading edge basically has no forward facing area except for the slope of the sail's camber. The low pressure peak at the leading edge gets pointed more to leeward than would be the case for a more rouded leading edge. Here's a physical way of looking at it. The air has to turn through a certain angle in order to follow the lee surface of the jib aft of the leading edge. If the air does most of the turning curving around the leading edge, that orients the suction more forward. But if the air flows past a sharp leading edge and does its turning around a vortex or leading edge separation bubble, then the low pressure of the turning is oriented at right angles to the sail's surface instead. This is still pointed forward relative to the boat, but you'd rather have it pulling forward and to windward instead of forward and to leeward.

This is the basic idea behind Polhamus' analogy. There has to be a low enough pressure over a large enough area to turn the air and produce a given amount of lift. You get the same lift in either case, but the sharp leading edge has a drag component that the rounded leading edge with attached flow doesn't.

The ideal shape would be thin everywhere except near the leading edge, like this Liebeck section:
http://www.desktopaero.com/appliedaero/airfoils2/images/HighCLSections2.gif

As a practical matter, jibs with head foils have proven to be every bit as competitive as jibs with wire luffs or hanks. They are an example of a jib with a thick leading edge.

brian eiland

06-11-2006, 11:34 AM

The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act. A mast with no separation zones would be a better leading edge than the thin leading edge of the jib - hence the effectiveness of wingmasts and pocket luffs......There are two separated zones behind the mast - one on the lee side and one on the windward side.
But of course a mast with no separation zones would not be possible

A sharp leading edge basically has no forward facing area except for the slope of the sail's camber. The low pressure peak at the leading edge gets pointed more to leeward than would be the case for a more rouded leading edge. Here's a physical way of looking at it. The air has to turn through a certain angle in order to follow the lee surface of the jib aft of the leading edge. If the air does most of the turning curving around the leading edge, that orients the suction more forward. But if the air flows past a sharp leading edge and does its turning around a vortex or leading edge separation bubble, then the low pressure of the turning is oriented at right angles to the sail's surface instead. This is still pointed forward relative to the boat, but you'd rather have it pulling forward and to windward instead of forward and to leeward.
You're right Tom, that 'very thin foil' leading edge can be a real problem in the practical world as well. In gusty, shifty conditions it would be real difficult to maintain an ideal flow over the headsails, particularly at this leading edge. Originally I had thought the big round furling foils of Profurl units were less efficient than the oval or foil shaped sections of some other manufacturers. But as I explored the situation further I became more inclined to utilize a round-shaped furler foil on my twin-headsailed rig. This round shaped foil rotates more evenly, and could be made more robust. And it would act to cut down some of the 'sensativity' associated with a very thin leading edge. Some 'bluntness' at this leading edge actually transmits an advanced signal to the incoming flow.

To suggest that a mast section at the leading edge of a sail is not that detrimental is not to my liking. Granted mast sections have steadily declined over the years either by material advances and/or creative rigging support. But most are still pretty big 'obstacles' at the leading edge of an airfoil. On most cruising boats with fixed mast I'd still be willing to wager that close to the first foot of sail area behind the mast does not contribute to driving the boat forward. Okay lets say this might be as little as only 6 inches in the very best cases. One half a foot along the entire luff of the conventional mainsail is a lot of lost sail area, and particularly towards the top of a fractional rig where there is no help from a headsail. If one doesn't utilize a fat-head mainsail you might as well write off the top 10-15 percent of the mainsail area as a contributor of forward drive to the boat.

Paul Scott

06-12-2006, 11:57 AM

I can't get the Polhamus link to come up, but this page does beg a few questions for me (if these questions have been covered in other threads, please point me to them)-

what size does the main have to be relative to the jib to provide enough upwash to put the jib into a effectively significant lifted condition (is this a ratio?), and what type of proximity does the main need to have to the jib (does it have something to do with Marchaj's Munk (biplane theory) derived 2.4 times chord?)? Is there a bright line of upwash past which the main/jib combination becomes desirable?

is there something magical about the jib (vertical lift? easier engineering?), or would a mast with a really thin section work just as well- for example, like the sections being used on some of the most recent class A cats (like a few Boyers I've seen in the last few weeks), which seem to have very little if any separation behind the mast?

Since the jib is in a lifted condition, and it seems, therefore, not having to deal with really small angles of attack, how much thickness for it's leading edge would be significantly negative, if the section was effectively shaped to reduce or eliminate separation, and if the section had a rounder leading edge as a result of the increased thickness, might that keep flow more attached with fluctuating angles of attack? (I'm thinking about the incredibly attached flow on some of the RAF's I've had on raceboards I've sailed that had better VMG to windward when sailed lower than you'd think prudent. And yes, I had about 20 telltales on some of them.)

Paul

brian eiland

06-13-2006, 09:30 AM

...a little diversion here...

How about a match race between two very different superyachts, one with a sloop rig, and the other with a 'modern' square rig.

"MV" vs "MF" (Mirabella V verses Maltese Falcon) unofficial challenge
http://www.yachtforums.com/forums/27531-post49.html

The World's Three Largest
http://www.yachtforums.com/forums/8815-post5.html

Square Rig Pointing (some aero questions here)
http://boatdesign.net/forums/showthread.php?t=5991
(regrettably this subject thread has been closed to additional postings)

This subject will likely generate considerable discussion on its own, so I thought it might deserve its own thread, HERE (http://boatdesign.net/forums/showthread.php?t=12172)

DanishBagger

06-13-2006, 05:34 PM

Square Rig Pointing (some aero questions here)
http://boatdesign.net/forums/showthread.php?t=5991
(regrettably this subject thread has been closed to additional postings)

Not anymore :)

CT 249

06-18-2006, 11:58 PM

Very interesting Tom, thanks. I remembered Stearns saying years ago that the headfoil actually increased the jib's efficiency. Obviously the Liebeck foil has a different leading edge to a conventional mast but I assume you reckon the same effect would still apply when there was a conventional mast at the leading edge?

On the proven superior performance (in some applications) of cat rigs, I only just woke up to the fact that a cat rig (in A and C Class cats etc) is higher aspect, and that Tom tells us that the critical factor is the AR of the whole rig.

Like Paul, I've noticed that sailboard rigs seem to like being oversheeted (although the fact that the leading edge moves to windward can make it look as though the rig is sheeted more tightly than is in fact the case). That's irrespective of the effects on heading and vertical lift, as far as I can see. What that means, I have no idea!

Paul Scott

06-19-2006, 11:27 AM

If you split a cat rig of AR 4 in two, turning it into a split rig of the same sail area that occupies the same 2 dimensional space as the cat rig (that is, the system planform is the same heighth, and the system chord is the same width as the single sail), is the whole sail/wing system AR still 4? If you then split the same (cat) rig again in two, but use an overlap of the 2 resulting wings/sails to put the same sail area into a 2 dimensional space that has the same (system) span, but is smaller in effective (system) chord, is the whole rig's AR higher? Does this result in better performance than the split, non overlapping, lower AR 4 system, everything else being equal?

The reason I ask is that Marchaj, in Sail Power, seems to indicate that separating the two elements would give more power. Which would seem, I think, to lower effective system AR. (This is apart from stagger, that is, moving the jib to leeward, at least upwind.) I don't think Frank Bethwaite would concur. So is overlap beneficial when it is counted as Sail Area? Any threads on this?

Or is this merely Gedanken Experiment?

Paul

gggGuest

06-19-2006, 03:59 PM

If you split a cat rig of AR 4 into a split rig that occupies the same 2 dimensional space, is the whole sail/wing system AR is still 4?

As I understand it the drag that aspect ratio relates to is induced drag. which is simply proportional to the span of the foil - ie length parallel to the airflow. So if you have two 10m3 sails, each of width 2m you end up with the same induced drag as one 20m3 sq ft sail width 4m. But if you have two 10m3 sails of width 4m you have double the induced drag,and if you have a 20m3 sail of width 2m you havw half the induced drag. Of course the lower rigs have less heeling mment, but no-one said this was easy.

brian eiland

06-21-2006, 12:01 PM

Aspect Ratio (AR) of rigs on cruising vessels

I was looking back thru “Principles of Yacht Design” by Larsson and Eliasson this morning and noted some of their observations on the subject of Aspect Ratio (I’ve underlined some portions where I sought to emphasize that passage):

At the top of the sail and at the boom the lift force goes to zero and vortices are shed, giving rise to an induced resistance. The larger the height of the sail, the smaller the effect of the vortices. As for the keel, the most important efficiency parameter for the sail is the aspect ratio. We define it here as the luff length (P or I) divided by half the foot length (E or J in the IOR notation) so neglecting the roach it corresponds to the definition of the previous chapter. It should be mentioned that in some sailing literature the foot length is not divided by two in the definition, so the aspect ratio is half as large.

Very interesting studies of different planforms have been carried out computationally by Professor J H Milgram at Massachusetts Institute of Technology (MIT). For a masthead rig he varied systematically the aspect ratios of the main and fore triangles by changing the foot lengths. The calculations were for the upwind condition, and the computed force was resolved into its driving component R and side force S. These forces are given in coefficient form: Cr and Cs respectively, in Figs 7.3 and 7.4 (attached) Note that the coefficients are obtained by dividing by a sail area, which is either the real one (thick line) or the measured one according to the IOR (thin line).

Although this rule is not much used today, it is interesting to see how the penalties imposed affect the efficiency. The coefficients may thus be considered representative of the force for a given area, either real or measured. The graphs for the mainsail have been obtained keeping the fore triangle aspect ratio constant (AR 6) and vice versa. In Fig 7.3 (attached)it can be seen that the driving force increases considerably with increasing AR. To a certain extent the advantage is offset by the penalties of the rule, but for the fore triangle there is still a great advantage in a high aspect ratio. The side forces of Fig 7.4 (attached)decrease greatly with aspect ratio if the IOR area is kept constant. This is because the real area is reduced due to the penalties. If the real area is used as a reference, the side force is relatively constant.
What I found rather interesting here is something I’ve commented on before, the influence of ‘rating rules’ on designing boats and interpreting sailing science, ie; “interesting to see how the penalties imposed, affect the efficiency.” We need to keep this in mind as we refer to observations (usually our reinforcing ones) based upon particular classes of racing sailboats.
______________________________________________________________

In another interesting series of calculations Professor Milligram varied the point of attachment of the forestay to the mast. Four rigs were computed, where the attachment point was at 3/4, 7/8, 15/16 and 1/1 of the full mast height, respectively. The results may be seen in Fig 7.5 (attached) There is a significant gain in driving force for the real sail, when the fore triangle height is increased, while the side force is almost constant. For the IOR sail the gain in driving force is not so large, but the side force is reduced
I found this observation interesting in several ways. I have been a proponent of the ‘masthead rig’ for a long time, and this work indicates a “significant gain in driving force for the real sail when the fore-triangle height is increased.”

I also believe this indicates a need to consider the AR’s of the individual sails separately rather than as one ‘combination of jib and main’.
Which leads me to ask - when we look at aspect ratio, is the important factor the AR of the individual sails, or the whole rig?????
The whole rig. The best indicator of lift/drag ratio is the wetted aspect ratio - the span-squared divided by the total wetted area
I believe the key word here is indicator. Sure the overall AR of the rig as a unit is an INDICATOR of the lift/drag aspects, but it does not explain the difference between a mastheaded sloop and a fractional sloop of the same height and AR.

I have a yacht with a low-aspect genoa and high-aspect main. I can see intuitively that this may mean that the low-aspect genoa "wants" to operate at a different angle of attack to the mainsail, but is this correct?
It not only wants to do so as a result of the aspect ratio, but also as a result of the difference in sweep angles for the two sails, and more importantly the upwash/downwash situations created by the interplay between the two sails
From Paul Bogotaj’s “How Do Sails Work" (http://www.pultneyvilleyachtclub.org/racing/How%20do%20sails%20work.pdf),
Flow Angles. Reviewing all of the affects so far reveals that both sails experience increasing flow angle with height. The foresail operates in the twisted flow of the apparent wind, with upwash induced by itself due to taper and sweep, and in the upwash field of the mainsail. The mainsail is operating in the same twisted apparent wind, with additional upwash caused by its taper, but somewhat lessened by its forward sweep. It is. also flying in the downwash field of the foresail, which is probably twisted because the foresail flies in a twisted fashion. This is particularly exaggerated with a fractional rig.
_______________________________________________________________

The results of the previous figures seem to indicate clearly that the aspect ratio of the sails should be as large as possible. This is not true under all circumstances, however. Considerations, which have to be made in a real case, include:
1) points of sailing other than upwind
2) the effect of the mast on the mainsail flow
3) the increase in heeling moment with aspect ratio.

The latter disadvantage is fairly obvious and its importance depends on the wind strength and the stability of the boat. We will not discuss this any further, but consider the other two points in some more detail.
C A Marchaj has reported wind-tunnel tests for sails of varying aspect ratios. All points of sailing were considered. Fig 7.6 shows the driving force and Fig 7.7 the side force for three aspect ratios: 6, 3, and 1. The latter is an almost square gaff sail. It can be seen that for small apparent wind angles, ie upwind, Milgram’s conclusions are confirmed. Around 30° the high aspect ratio sail develops more than twice the driving force of the square sail. However, at large wind angles the situation is different. Around 120° the square sail is superior, and develops 50% more thrust than the narrow sail. At 70° the thrusts are almost equal. The side force of Fig 7.7 increases somewhat with aspect ratio at 30°, but the opposite is true above 45°. The general conclusion is that the positive effect of high aspect ratio is reduced if all points of sailing are of interest.
In general we have three basic sailing directions we need to consider, upwind, reach, downwind. And for the cruising sailor the upwind 1/3 of the total is not even an equal partner (as many cruisers often chose not to fight upwind work). As Marchaj and many others have reported, high aspect ratio is principle beneficial for upwind work.
_______________________________________________________________

A disadvantage of the masthead rig which is often referred to for smaller boats, is the difficulty in trimming the mast properly
Or in other words, on many boats the capability to utilize the bendy characteristics of the mast to reshape the mainsail for a variety of conditions does not lend itself to the masthead configuration. Many racing classes depend upon this mast shaping feature and thus utilize fractional rigs. Or in the case of multihulls with rotating mast, the fractional jib is practically a necessity. But this should not be taken as an endoresment of the fractional rig nor a condemnation of the masthead rig from a purely aerodynamic standpoint. For a fixed-rig cruising vessel, I submit again that the masthead rig is likely superior.

_________________________________________________

The mast reduces the positive effect of a high aspect ratio mainsail even further. For a given sail area, the higher the aspect ratio the thicker the mast required, and the smaller the average chord length of the mainsail. Both effects tend to increase the proportion of the sail, which is ineffective due to the mast disturbance. Marchaj found in wind-tunnel measurements that a 6.0 aspect ratio sail was less effective, even upwind, than a 4.6 aspect ratio sail, and this was attributed to the mast disturbance. In these tests the mast diameter was 8% of the average chord length of the high aspect ratio sail. This seems to be a bit more than is used today, so the effect was probably somewhat exaggerated, but it shows that there is a limit for the positive effect of the aspect ratio of the mainsail.
Experiments at Southampton University with a mast/sail combination indicated large effects of mast disturbance. Thus, when a circular mast with a diameter of 7.5% of the sail chord was put in front of the sail the driving force upwind was reduced by about 20% compared to the case without a mast. A thicker mast of 12.5% was also tested and the driving force was almost halved. It was however, possible to regain almost half of the loss by turning the mast in such a way that the leeward side of the mast/sail junction became smooth….There is no doubt however that the top part of the sail will be significantly disturbed.

_______________________________________________________
_______________________________________________________

Pitch & Heave

Here is another item that might be kept in mind when considering tall high aspect ratio rigs; their contribution to pitch and heave motions. When a vessel moves in a seaway, the waves impose motions of all kinds on the hull(s). The most important ones, from a resistance point of view, are the heave and pitch motions, which are usually strongly coupled. When the hull heaves and pitches it generates its’ own wave system, which carries energy away in much the same way as the still water wave pattern, thereby creating a resistance force. The pitch and heave motion will be affected by the mass moment of inertia of the yacht and the encountered wave frequencies.

Every object on the vessel contributes to its mass moment of inertia not only by its mass, but also by the distance to the center of gravity squared. Objects positioned far away will have a large influence. It turns out that the rig itself is a very large contributor. In some cases it’s twice as important as the hull, and four to five times as important as the keel for the moments of inertia. Multihull vessels can be particularly affected as their overall lightness makes them more sensitive to pitching and heaving, as well as their long thin hull forms that are less adept to dampen out the those motions so readily. These motions can very quickly kill all drive from the sails regardless of AR or all other factors.

Paul Scott

06-21-2006, 12:56 PM

ggGuest & Brian

So, what do you think: assuming the same overall sail area & mast section & mast heighth & Mainsail area, in a rule free environment, does exploiting higher AR with an overlapping jib v. non overlapping jib (and let's say a masthead jib with a rigid mast, upwind, for grins) lead to performance gains?

Brian- as long as you mentioned IOR practice, any idea what the AR for mains was when the point of diminishing returns was reached in the IOR? Seems to me it was around 4ish, but that's a guess based on memory.

Paul

CT 249

06-22-2006, 01:17 AM

Brian, re "Or in the case of multihulls with rotating mast, the fractional jib is practically a necessity."

In that case why do A Class and C Class not use jibs?

The funny thing about the common complaint about rating rules is that they get blamed for so many things. For example I have many articles from the early days of the IOR which blame the IOR for encouraging massive genoas and tiny mains. The rule was NOT changed and yet a few years later, there was a swing to tiny genoas and massive mainsails.

For example, the IOR rule didn't rate overlap. Therefore, a large genoa had a lot more un-rated area than a small genoa.

gggGuest

06-22-2006, 03:34 AM

So, what do you think...lead to performance gains?

I think anyone who gives any answer other than "it depends" is hopelessly over simplifying things.

Paul Scott

06-22-2006, 10:51 AM

ggGuest-

So this inquiring mind has GOT to know! "it depends on....?"

Just for arguements sake, lets say a light planing dinghy, non bendy wingmast, masthead rig, 30% main, 70% jib (assume that the jib furls as wind increases), say 10 sq m of sail, AR 4.

Paul

gggGuest

06-22-2006, 04:09 PM

ggGuest-
lets say a light planing dinghy...

It depends...

There are still too many variables to call, not least being the average wind conditions where the craft in question sails. All you can do is take a look at what development boats there are out there that don't have rig height limits and see what they use.

To give an example of what does get used here are measurements for the last rig I had built for such a boat was a 12ft lightweight two hander being sailed at coastal events in the UK with max sail area of 12.5m2. Crew weight around 90kg on trapeze, Helm around 65kg sitting out, beam about 1.9m. This is a boat with a 15m2 asymettric kite to get it downhill. The same dimensions probably wouldn't be optimal for a boat without a kite. The key dimensions were as follows and I don't think I'd vary them if I were getting another rig for the same boat today. That doesn't mean I'm right: its awfully difficult to distinguish between two boats with equal racing results, one of which has a great rig and a mediocre crew, and the other a good crew and a lousy rig, but for whatever my opinion is worth I felt this rig was pretty good. The jib was fractional of course and almost completely non overlapping - (maybe just a bit of roach in the middle of the leech overlapping the main. I would never consider a masthead rig on a planing dinghy. The dynamics are all wrong. This rig would be regarded as being high aspect ratio by most folk and certainly has a higher AR than the vast majority of dinghies.

Mainsail
Luff Length 5.97
Foot Length 1.78
Leech Length 5.95
Total Mainsail Area 8.60

Jib
Luff Length 4.38
Foot Length 1.69
Leech Length 3.78
Total Jib Area 3.90

(all measurements in metres)

I reckon that's a 4.1 AR on the main, 3.7 on the jib and around 3 if you look at it as a whole (jib comes down lower than main).

Paul Scott

06-22-2006, 09:48 PM

gg

I have to admit here that what you've posited for your skiff is pretty much a smaller version of the rig on my 40' cruising sled (DL 96), which I based mostly on Redwing of Bembridge practice, although I have a tonne of Roach. And my mast setup, with 22 1/2 degree sweep of spreaders, and LOTS of prebend does not give. At ALL. I presume your rig is fairly bendy? I ask because I had three Bruders for my Finn (way back when), and the really really Stiff Mast was the fastest by far, although not comfortable or friendly in any way. Ditto for Windsurfer rigs-RAF's-, which is what got me thinking about a stiff masthead wing rig on a skiffish dinghy, (e.g. Brian's remarks, kind of). Which I realize violates modern practice in extremis, but hey- we're only abusing 1's and 0's here, no? If you can control the power???? And if there is more power??? (Big if.) It seems to me that sportsboat rigs are pretty stiff, and sportsboats are getting smaller and smaller. Crossover? Bongo?

Paul

brian eiland

06-24-2006, 09:27 PM

This aero discussion is sure getting lively. I'll try to catch up, but I'm not able to spend a lot of time at the computer at this time.

Brian, while I understand the theoretical efficiency of the headsail due to its cleaner leading edge, I'm still puzzled.....if mainsails are so slow, then why do the most efficient boats of all (speed windsurfers, YPE, foiling Moths, C and A Class cats) all use cat rigs?

Why do fractional rigs generally go faster than masthead rigs when they have been tried together on the same hull (apart from perhaps in light wind areas when the fractional carries a fractional kite?).
First I respectively ask for a little clarification of some of our terminology so we are on the same page as we discuss speed and efficiency.
Speed:
When we talk of speed, are we talking of ‘around the race course’ speed, or straight-line, head-to-head speed? From a purely aerodynamic point of view, I would prefer to compare the vessels with different sailing rigs on a head-to-head basis in each of the 3 primary sailing directions. Around the course speed includes so many other factors that might distort the purely aerodynamic factors. Admittedly around the course speed should also be considered in the equations for the ‘fastest boats’ on a race course, but lets look at those additional factors separately and distinctly.
Efficiency:
In a similar manner I believe we have to consider two forms of efficiency; 1)that efficiency from a purely aerodynamic point of view, ie, that forward drive we are seeking from the least amount of sail area, and 2)the overall efficiency of the boat itself to include the rig, and the hull, and the entirety of the boat.

So hopefully you can see where I have some problems with your statements such as “the most efficient boats of all use cat rigs”, and “fractional rigs generally go faster than masthead rigs”. If you are talking ‘around the race course’ your statements could well be considered correct in many cases, but on a head-to-head basis they might not hold up so well.
Even you’ve written:…but the lobe for an AR of 1 at 35-45 apparent is incredibly distinct (in Marchaj) and it could perhaps account for a lot of the way that assys light up at certain angles.
Is Marchaj correct re AR and angle of incidence? Much of his info seems correct to my gut feeling; I know high-aspect rigs (F16 cats, Canoes) really light up at tight angles and get quite ordinary at broad angles. It's always interesting to see a Laser (smaller rig, similar wsa, more beam, less LWL) hanging in with a Canoe on square runs and broad reaches in light winds.

___________________________________________________________
Frosh and Rhough, I agree with you and actually I'm up on that stuff - I just wonder what Brian, the man who is most involved with pushing the mast-aft rig, has to say about all the good reasons you have given.

If jibs are more efficient, why have A Class and C Class stopped using them? Why is a guy like the world's fastest Canoe and C Class sailor so convinced of the speed of cat rigs that he's experimenting with them in Canoes?

I think Rhoug answered this one very well: If the rig can be sized so that CL over about 1.6 is not needed (upwind), there is no reason for a multiple element sail plan. Windsurfer, foiling Moths and Cats all have very high righting moment to sail area ratios. There is little need to control the heeling moment with low aspect ratio rigs, or splitting the area between main and jib.
On single-handed dinghies the added workload of trimming two sails to work properly probably slows the boat more than any extra power from the rig gains. On boats that always sail with the apparent wind forward of the beam wing-sails are a logical choice.

With the A cat I believe that very high aspect mainsail (almost the identical planform to a high performance glider wing) overcomes any advantages of sloop rig due to sufficient RM to support the greater heeling force.

However, the design of sailforms is very much a practical nature.

In terms of sailing, the feel is that our jib supplies the driving power - and wind tunnel tests also show that it has a very large Cl compared to the main. The main on the other hand, is very responsible for the righting moment and general tuning of the boat. Skippers know that if they constantly communicate with their mainsail trimmers while sailing upwind they'll do well - the jib trimmers are mostly left to following their tales.

So ultimately, what I am saying is that for a sailboat, instead of thinking of a design point for a Cl, think instead of a large plateau in which you can spill enough wind out of your sails to maintain a stable platform. In 8kts of wind, you may be able to get 5kts of boatspeed. In 15kts of windspeed, you may need to start easing out the main to get 6kts of boat speed. What most designers want is a boat which quickly gets up to near designed boatspeed and then gradually increases afterwards.

So the reason many boats use a fractional rig is because all sailing is a compromise. A nice roach on a main will supply power at very low wind speeds. In high winds, you can simply ease the traveller and let the jib with its lower CE do the work

I’ve said before in another posting,
A disadvantage of the masthead rig which is often referred to for smaller boats, is the difficulty in trimming the mast properly
Or in other words, on many boats the capability to utilize the bendy characteristics of the mast to reshape the mainsail for a variety of conditions does not lend itself to the masthead configuration. Many racing classes depend upon this mast shaping feature and thus utilize fractional rigs. Or in the case of multihulls with rotating mast, the fractional jib is practically a necessity.

But this should not be taken as an across the spectrum endorsement of the fractional rig, nor a condemnation of the masthead rig from a purely aerodynamic standpoint.

CT 249

06-25-2006, 05:53 AM

Ok, thanks.

I agree with much of that. However, just as a suggestion unless I'm reading it wrong (which is of course possible) some of your promotion of the mast-aft rig can be seen as a little more one-eyed than the way you have put it here. Maybe that's a good way of promoting the rig, but it may also get people's backs up (as it did with me).

I still can't see how the jib is a necessity with rotating mast cats, since As, Cs, some fast old Bs, some F16s and T 4.9 1-ups don't have jibs and perform very well. But I won't worry you about the point further.

brian eiland

06-25-2006, 11:59 AM

I still can't see how the jib is a necessity with rotating mast cats, since As, Cs, some fast old Bs, some F16s and T 4.9 1-ups don't have jibs and perform very well.
I'm not saying jibs are necessary for a rotating mast rig, but rather that if a jib is to be used with a rotating mast, it needs to be a fractional one because of the staying arrangment.

Vega

07-01-2006, 10:43 AM

Excellent work guys, very good thread. I have been following it, but not posting here, simply because you guys seem to know more than I know about the issue:p

Thanks for the résumé Brian, very well done.

I have a question. There is a rig that can utilize only one sail and it is not the cat rig. It is an old one, the Latin sail. It is obvious that the rig is not practical for a number of reasons (reefing, tack, etc) but I have seen some Latin rigs in the Canary Islands (racing traditional boats) that look very sharp in what regards performance (even if they don't use high-tech materials).

I am curious about it. Can someone make an evaluation of those high developed Latin rigs?

gggGuest

07-01-2006, 05:16 PM

There are pros and cons to just about every possible rig that has been widely used.

For ultimate performance per square foot of cloth I wouldn't have a triangular planform, nor would I have a loose foot. I'd also have doubts about the dynamics, things like gust response.

On the other hand there are more knowldegeable folk than I who state that the angled leading edge has some neat advantages, notably the way that leading edge vortex creation gives high lift (but high drag) at large angles of attack without stalling. Bethwaite is a good reference on this.

MAINSTAY

07-04-2006, 09:58 PM

Brian Eiland,
The diagrams of Rick Loheed (#151) are good demonstrations of your theory. It shows very little contribution to the drive from the main.

If the diagrams were to show a non-rotating mast it would be even more in your favor.
Larry

brian eiland

07-04-2006, 11:22 PM

The diagrams of Rick Loheed (#151) are good demonstrations of your theory. It shows very little contribution to the drive from the main.

If the diagrams were to show a non-rotating mast it would be even more in your favor.
Thanks Larry,
I just haven't had the time to get back to this subject thread to the extent I would like.

sharpii2

07-05-2006, 05:56 AM

Hi:

I have been reading some of this thread and much of what has been said here is beyound me.

One thing I have noticed conspiciously absent in this thread is the contribution matterials science has made to rig design.

In the jolly olde days, before 1x19 steel cable was available, jibs really werent useful for much other than light air sails and ballance. The reason being was that the luff line would stretch and the sail would bag and sag. Even the cloth at the time was no great shakes.

For these reasons, low aspect rigs, that were not so dependent on foil shape, a concept that was not even understood at the time, ruled.

Once 1x19 rigging was available along with high strength cotton, it was possible to make really effective working jibs. Even then, they stayed small.

Thier size was then limited this time by the hulls inability to hold the now much stronger luff wire tight. The hull would just sag in the mddle under the tension and the luff would still go slack.

By then, the need for high lift wings for notoriously under powered airplanes became a major issue. The concept of air foil design was becoming common currency amongst the scientifically literate for the first time. amongst these people, of course, were sail boat designers.

My understanding of why a high aspect ratio wing or sail is more effective than a low aspect one is that, in physics, momentum is conserved, but kinetic energy is not. So moving a lot (of air molecules) a little is far better than moving a little a lot. If I have a sail that is 20 ft high and 10ft, wide it is going to redirect a swath of air nearly equal to its height and of indeterminate depth. Since changing the direction of anything involves acceleration, you are actually accelerating the wind molecules, giving them kinetic energy as well as momentum. The problem is that you only get to count the momentum, and the more you accelerate the molecules, the more energy you waste.

So if my proposed rig is now 10ft high and 20ft wide, I am now accelerating a much smaller swath of molecules to a much higher kinetic energy level. Because the formula for momentum is mass times velocity and the formula for kinetic energy is mass times (velocity squared), my new sail is going to to have half the 'lift/drag ratio' of the old sail. And that is not even counting vortises.

Now, if I am dealing with crappy sail matterials and inside ballast, I will probably have to go with a compromise between high aspect ratio efficiency and low aspect ratio lower center of area. I might even have to go with energy robbing sail over laps. The extra sail is no where near as effective as with no over lap, but it is much more effective than no extra sail at all.

A triangular sail, by the way, is a good cheap way to get a higher effective aspect ratio and still keep the virtical center of area within reason. One way of figuring aspect ratio is hieght squared divided by area. So a 1 to 1 trianangle is going to approach a 2 to 1 aspect ratio with nothing but a turning block at the top. Even if the top 25% of the luff is doing nothing but agitating the air.

A really interesting web sight to go to in regards to sail design is pdracer.com.
This crowd is the only one I know of in yachting that brags about the cheapness of thier boats. The hulls are cloely alike below the waterline but the sail rigs can be anything the skipper chooses to put up. Most of the sails are made of 'poly tarps' by inexpert sail makers. It is instructive to see how the trianguler sails do against the rectanguler ones in thier races.

I think they win every time.

Bob

frosh

07-05-2006, 03:29 PM

[Quote by Sharpii2)

A really interesting web sight to go to in regards to sail design is pdracer.com.
This crowd is the only one I know of in yachting that brags about the cheapness of thier boats. The hulls are cloely alike below the waterline but the sail rigs can be anything the skipper chooses to put up. Most of the sails are made of 'poly tarps' by inexpert sail makers. It is instructive to see how the trianguler sails do against the rectanguler ones in thier races.

I think they win every time.

Bob[/QUOTE]

Great web site, and excellent concept as well. It seems to have captured a lot of interest, and boats are being built. Interesting is the rule allowing freedom of sail area and rig design. It is too early in my opinion to predict what will eventually be the best allround. Sometimes extra sail area if it is easily controlled overcomes efficiency in rig sophistication especially in light and moderate air.

sharpii2

07-06-2006, 04:22 AM

Great web site, and excellent concept as well. It seems to have captured a lot of interest, and boats are being built. Interesting is the rule allowing freedom of sail area and rig design. It is too early in my opinion to predict what will eventually be the best allround. Sometimes extra sail area if it is easily controlled overcomes efficiency in rig sophistication especially in light and moderate air.

frosh:

glad you checked the sight out. It is interesting to watch the class evolve. It seems to be moving toward sail areas of 80 sf+. Triangular sails seem to have the advantage here. they have had it since the class started, and they still have it. The boats are so beamy and can be allowed to heel so little that it is possible to put rediculously long booms on them. an austrailian put an 85sft jib headed sail with a sprit boom on his and got it to move out at almost six kts! the boom was longer than the boat and the mast was over twice as long.

It would be interesting to design a 10ft x 10ft sprit sail rig and see how it does. But, most likely a very large setee or boomed lateen may be more effective.

Don't get me wrong, I have a real soft spot for rectangular sails. All my boat designs have them. But race results are race results. I have yet to hear of a rectangular rig beating out a triangular one in this fleet. But there is a first time for everything, eh.

One of my biggest frustrations with this class is that the sail area is unlimited.

My cc312 design, if it ever becomes a class (see "My own design" thread) will not have that idiosyncrasy. It will be limited to 4.7 sm or 50sft and no hiking will be allowed. Sail matterial will be restricted to 4mil visquine or poly tarp. No wire rigging will be allowed but polypropylene stays and shrouds will be ok. The hulls would all be pretty much the same and they will have mininmum weights.

Wouldn't this be an excellent labratory to test sail design theories?

Bob

frosh

07-06-2006, 05:45 AM

If owners ever decide to get really serious about racing and winning you would want about three different rigs like Aussie skiffs that allow various sail areas. Maybe 50 to 90 sq. ft. I personally like Wharrams "Tiki" style wingsail being efficient, aerodynamic (with the luff sleeve and no stays) and allows a lot of sail area for a moderate boom length. I am sure that on a short hull with almost no directional stability long booms are difficult to sail with once the breeze comes up. This was my experience on low volume short sailboards. High aspect sails for a given area were much more user friendly.
This link shows a photo of the wingsail; I would think that a boomed version would be better allround.
http://www.wharram.com/tiki_photos/tiki.shtml :)

brian eiland

07-06-2006, 12:56 PM

...This link shows a photo of the wingsail; I would think that a boomed version would be better allround.
http://www.wharram.com/tiki_photos/tiki.shtml :)
That photo sort of reminds one of the 'fat-headed mains' of today.

Appears to be a boomless sail as well, which I utilized on the Firefly trimaran, and the virtues of which have been discussed elsewhere in this forum

Paul Scott

07-06-2006, 01:31 PM

sharpii2

07-06-2006, 05:39 PM

Bob (aka sharpii2),

"...energy robbing sail overlaps. The extra sail is no where as effective as with no overlap..." Does this argue against using overlap to create higher AR for a sloop rig? Anywere I can find more info about overlap dynamics? (that is vs no overlap?) I've read Bethwaite (assertions only it seems) and Marchaj (???)

As long as I'm talking about overlap, anyone seen David Pugh's reply to Guillermo Gafaell about (dare I type the phrase?) "The Slot" in Letters to the Editor of the June 2006 issue of Yachting World? (Is DP on this forum?) If DP is right about the air accelerating (after stagnating at the entrance to the slot) "as it approaches the leach of the genoa, reducing pressure and re-establishing the suction that produces the mainsail's drive", does this have design repurcussions, like where the leach of the genoa ideally ends relative to the maximum chord of the main, and does it change where you would want the point of maximum camber on the main for that particular genoa?

Paul

Paul:

I wouldn't know.

I guess it has to do with the sectional shape of the mast. I suppose that if you have a 'dirty', square section mast, the overlap may actually help the situation by forcing the wind around the mast and thereby suppressing the inevidible eddies.

With a 'cleaner' mast section, by the same logic, the same overlap may actually detract performance in comparison to the same sail area with no over lap, because the air molecules are experiencing friction on both sides of 'the slot'.

Many of the arguements I have read in the thread remind me of arguements I have heard in political discourse. The principles and arguements by themselves may be very sound. As long as the implicit assumptions are in place.

The trouble is, in the real world, they often aren't.

Yes, I agree that a very high aspect ratio eliptical planform single sail behind a perfectly clean mast may be the best sail for a given sail area. But you would never see one on my boat. Why? because, in order to have one, I need expensive, high tech matterials, for the mast, and a very deep, snaggy, bulb keel, or even a canting keel, to hold it up. This implies, not only expensive building costs, but high berthing and upkeep costs as well.

There may be a time in my life when such things may not be an issue. But I don't anticipate that happening any time soon.

Bob

brian eiland

07-06-2006, 10:26 PM

Anywere I can find more info about overlap dynamics? (that is vs no overlap?)
If DP is right about the air accelerating (after stagnating at the entrance to the slot) "as it approaches the leach of the genoa, reducing pressure and re-establishing the suction that produces the mainsail's drive", does this have design repurcussions, like where the leach of the genoa ideally ends relative to the maximum chord of the main, and does it change where you would want the point of maximum camber on the main for that particular genoa?
Overlap dynamics: Have a look at back thru Postings # 63,64,65,66 of this subject thread.
http://boatdesign.net/forums/showpost.php?p=40634&postcount=63
http://boatdesign.net/forums/showpost.php?p=40634&postcount=64
http://boatdesign.net/forums/showpost.php?p=40634&postcount=65
http://boatdesign.net/forums/showpost.php?p=40634&postcount=66

And then an excerpt from here (http://boatdesign.net/forums/showpost.php?p=23390&postcount=13), "We know that the restriction presented by the ‘slot’ tends to divert more air around the two sides of the slot, i.e. the windward side of the main and the leeward side of the genoa. This higher flow rate on the lee side of the headsail increases its effectiveness. Now if we also overlap the mainsail with the trailing edge of the headsail, we further increase the effectiveness of the headsail, as it is able to carry this increased flow rate much further aft along its span than if it was to have to dump its flow at free stream velocities up at the leading edge of the mainsail. This overlap is important."

Paul Scott

07-07-2006, 12:51 PM

Brian

Thanks. The direction is greatly appreciated.

As I strive to become a dedicated 'aero amateur', I realize more and more that my father was right when he said "work is the bane of the leisure class."

Paul

brian eiland

07-07-2006, 07:51 PM

Not a bad looking twin headsail deployment.

(BTW, the catamaran Orange II just completed a record run across the Atlantic during which time it also set two new 24hr records as well)

globaldude

07-19-2006, 06:42 AM

Hi Brian, a couple of quick questions --- forgive me if this is not the correct place to post these Q's, but it's all about sailing isn't it !? -----
where is the centre of effort of the sails, in relation to the lateral centre of resistance of under water sections ?.
I was told it should be ahead of , but wouldn't that cause the yacht to fall off if hit by a big gust ? , & then it'd get knocked flat. If not, why not.

I love your rig [ aft mast] and am planning to build a bipod version -- make that a "wishbone" as you called it, as I want the top section to telescope in to stop forward overhang when down to traverse rivers & the like --for my 50' yacht [ hull & decks right now ] I'm sorry Brian, I'd love to be able to pay you to design it for me, only I'm talking real budget here !!,so I'll just do my best [ as a general engineer & rigger ] as it is, most of my land lubber mates still think I'm dreaming. Well they see the hull, see I've built the 7x20m x6.5high workshop in the last 3 months by myself , but will only "believe it when I see it !" regarding the rig , as it's unheard of round here.
I've been " dreaming" on paper of late drawing how I'll get the rig to raise & lower and drawing different keel options & or foil shaped assemetrical lee boards.
She's shoal draft, 1m, but I'm not happy with the huge designed centre board taking up , or should I say , dividing the sallon into two smaller cabins.
So I've been looking at lee boards or ----
reducing the centreboard case by 2 thirds and useing a daggerboard type retractable keel with a small wing & balasting it [ the original design called for all balast to be in the shoal bilge & the centreboard had no balast ]

I have asked the question re the placement of lee boards and Mike Johns rightfully said it'd be crutial [ damm, must download that spellchecker one day] that the placement be right or I'd have an expensive disaster on my hands .

Anyway I said most of above so you could see why I wanted to know where the centre of effort of the sails should be in relation to the underwater profile ,
I'm thinking I may have to move my "new" keel aft somewhat.
I'd really like to post some lines / drawings of my boat -- I'lll try to, & if it works, I'll get my drawings scaned so I can post them as I'd really like input from all you experienced lot.

brian eiland

08-03-2006, 09:16 AM

Brian…. where is the centre of effort of the sails, in relation to the lateral centre of resistance of under water sections ?. I was told it should be ahead of , but wouldn't that cause the yacht to fall off if hit by a big gust ? , & then it'd get knocked flat. If not, why not.
There is normally a certain ‘lead’ distance factor to be incorporated in the design in order to balance out the turning moments produced by the sideways forces of the sailing rig verses the leeway resistance forces of the underwater shapes (centerboards, dagger boards, whatever). This lead distance has to be more substantial on a monohull as there is heeling involved that further displaces these force centers from one another.

The force centers on the sails are usually taken to be at the geometric center of the sail, even though this is not real accurate. And the underwater centers are similarly taken at the geometric centers as well, so one error cancels the other.

In light airs the ‘balance’ is not as critical. The lead distance is usually based on the ‘substantial’ heel angle of 20 degrees that might be experienced under full sail, under heavier conditions, or even gusty conditions; all where more balance is desirable.

Non-heeling cruising catamarans might require no lead distances, while smaller tris might average out at 5 degrees.

…. planning to build a bipod version -- make that a "wishbone" as you called it, as I want the top section to telescope in to stop forward overhang….
I would strongly recommend NO telescoping sections.

…..drawing different keel options & or foil shaped assemetrical lee boards.
She's shoal draft, 1m, but I'm not happy with the huge designed centre board taking up, or should I say , dividing the saloon into two smaller cabins.
So I've been looking at lee boards or ----reducing the centerboard case by 2 thirds and using a daggerboard type retractable keel with a small wing & ballasting it…
Why not simplify and use twin fixed keels (http://boatdesign.net/forums/showthread.php?t=5315). But 1m in a 50 foot boat….wishful thinking.

globaldude

08-04-2006, 03:46 AM

Thanks Brian, I understand now. To put it another way [ how I see/understand it ] as the mono hull heels it puts it's " shoulder" to the sea, that is, the forward , roundy section, which would, of course turn the hull [ round up ] windward naturally. So to balance that tendency, the centre of effort needs must be forward of the CLR , right !?.
Regarding the 1m draft being " wishfull thinking" , sounded a bit harsh mate !!
But hey , you might well have given me a good name for said vessel !!! as like most people I want my cake & eat it too - meaning shoal draft, preformance, etc etc.
Could you elaborate on why definately NO to a telescopeing top section of a wishbone rig !? --- or is it that you think/feel it unatainable due to ??.

I won't assume you think I'm an idiot who couldn't engineer one sufficiently, or that it's just wishfull thinking ,but rather that it's too difficult to try to explain why not .
I've read every post you've put up regarding your aft mast rig as I think it exelent[ I have printed of a copy of your rig & it's stuck on my office wall !], and seen how, like me, you've been studying to identify the loads/ stresses on on rigs, so as to be able, one imagines, to build such a rig .
There are several advantages,[ perhaps only for me !?] I believe, for being able to safely lower one's rig aspect/ height while afloat.Beter stability, less windage, a better "fit" when main rig lowered, still able to be sailed while getting under some obstacles --- the scows who dropped their rigs & " shot " the bridges on the thames !!, wow, that's a classy act . [ engineless of coarse ] and probably more I can't think of [ yep, no doubt lots of dis advantages also ]

RHough

08-04-2006, 06:44 AM

Could you elaborate on why definately NO to a telescopeing top section of a wishbone rig !? --- or is it that you think/feel it unatainable due to ??.

Take a 50ft boat, figure what the RM is. Then figure what the loading on the join in the mast will be. Once the loads are known, see how heavy the telescoping mast would have to be. Try to design a system that would allow the mast to telescope reliably and figure out how much that will weigh.

How are you going to deal with halyards? the electronics at the masthead?

I'll wager that a telescoping mast will be 3-4 times the weight of a standard mast.

If you want to be able to reduce the rig height easily, build a gaff rig. :)

globaldude

08-05-2006, 03:37 AM

Hi Retro dude, " RM ", ??, rolling moment !?.
I'd love to be able to draw a whee picture for you, only I don't have the means so I'll try to "draw " one with words.
The Top mast section would needs weigh no more than a standard mast, as it's stayed in the same manner .When extended it would have a reasonable central "bury" inside the A frame - Bipod - [ whatever you'd like to call it] triangle section of the wishbone mast .
This bury would be necissary to initially support the extended top section, with a somewhat "sloppy " fit - for & aft only as the shrouds support sideways - untill the forstay and triatically stayed back stay were tuned.

Viewing the rig from aft, the lower section of the mast/s are a rectangle with a triangle on top. [ all foil sections].
The rectangle, hinged at the gunnels, is naturally braced diagonally, corner to corner and there is a small horizontal cross member about a meter from the top of the triangle section.
Across the very top of the triangle there is a crossmember equal to the width [ though not necisarilly ] of the rectangle section . That "cross member" is the hinged spreaders supporting the extended top section .
It appears to look like a cross member only when the top shrouds have "picked them up" and they in turn " pick up two shrouds that connect to the two " corners" of the rectangle .
The 4 shrouds are all a given leangth so must return to the same position every time.

Retracted, the most likley circular alloy mast section [ could be foil ] would be flush with the top of the triangle so as to be flush with the bow when the rig is lowered. The spreaders, with no tention on the shrouds to hold them up, will now be flush with the two downward chords of the triangle.

Extended ,the top sections "bury" starts from that small cross member near the top of the triangle.

Halyards & electronics ?
I am a rigger, the type that works with cranes and have owned several.
Most cranes, if the boom is extended and you don't winch down the hook, the hook will "travel" all the way to the top, as the boom gets longer but winch wire remains the same leangth. [ it's called double blocking and will pop the hook off - and anything hooked to it !!]
To combat this, there is a way to reave the rope internally [ in the boom] so that it remains static - in relation to the extending boom .
The same could be done for both halyard/s and electroincs. remembering one wouldn't raise & lower the top section much at all, it'd be simpler to just give the halyards some slack .
Gaffers are great, but I'm my own worst enemy in that I like to think & build outside the square and the aft mast really appeals to me as it seems to make logical sence to me and have a number of advantages specific to our needs.
Yeah yeah , everythings a compramise darn it !!.
Hope you can envisage it.
Globaldude

AleX`G

08-06-2006, 11:55 AM

I havn't read all of this thread yet. Just upto page 7.
If the distance between the trailing edge of the jib and the leading edge of the mainsail is large can you get extra performance for a set sail area. If you measure the chord as the whole sailplan. Kind of like extending flaps on a plane?

Im sorry im a bit of a noob

Thanks Alex

brian eiland

03-13-2007, 11:05 PM

Does anyone have any references that address the question of where is the most likely true Center of Effort of a rectangular sail as used on a square-rigged vessel.??

Take for instance the sails on Maltese Falcon (http://www.boatdesign.net/forums/showthread.php?t=12459), best guess, where is the CEO of each individual sail

percyff

03-21-2007, 10:35 AM

Brian
You may find this site of interest:

www.weatherlysquareriggers.com

where the square rigger that points is considered, and the fact that the spacing between the masts makes a considerable difference!

brian eiland

03-21-2007, 11:41 AM

Thanks Percyff,
Yes I am aware of that site...in fact I recommended it to a few friiends. Just recently I wrote the author Philip Goode about several questions I had on his 'horizontal upper stays'...I did not understand his descriptions at first. I subsequently found them running to the forward mast.

And yes, I did note his interesting finding about the mast spacing.

I found his findings about the 'leading' headsails of interest as well. I've always maintained that the headsails of a rig are extremely important and productive. Took note of his 'sky link assembly' to provide forward staying unaided by the addition of a forward mast. Since the whole mast of the DynaRig rotates there would have to be some modification of this idea, but I think its possible. I'm looking to fly a headsail on my uni-rigged Dynarig motorsailer (http://www.runningtideyachts.com/dynarig/)...I've got a few ideas myself, but I'm willing to except other ideas from anyone who wants to contribute:idea::idea:

RHough

03-22-2007, 01:30 AM

Thanks Percyff,
I found his findings about the 'leading' headsails of interest as well. I've always maintained that the headsails of a rig are extremely important and productive. Took note of his 'sky link assembly' to provide forward staying unaided by the addition of a forward mast. Since the whole mast of the DynaRig rotates there would have to be some modification of this idea, but I think its possible. I'm looking to fly a headsail on my uni-rigged Dynarig motorsailer (http://www.runningtideyachts.com/dynarig/)...I've got a few ideas myself, but I'm willing to except other ideas from anyone who wants to contribute:idea::idea:

Very interesting design Brian. For what will be in effect a single foil, I would think that the C/4 point at the mean aerodynamic centre should give you a very close approximation of the CE when sailing upwind and the flow is chordwise.

It will be interesting to see if the rig is dynamically stable. Have you looked at the moments? With the pivot/support point at C/2 I would expect the rig to try to twist the leading edge to leeward, creating wash-in at the Royals. Kind of exactly opposite to what you want for wind shear and gust response.

I understand your reasoning behind the Gallant/Course areas. Have you thought about bigger Courses and smaller Gallants? Striking the Courses and using the Gallants for higher winds and sea-states means that your working sail is less effected by calms in the troughs and they should see less variation (although higher average) wind speed. Using the Gallants for working sail also increases visibility and reduces the chance of waves hitting the sail.

As far as head sails go, I'm not sure I see much benefit in adding a fore and aft sail to a DynaRig for upwind work. Stanford Yacht Research did a workup of MF's rig. IIRC, the pressure distributions were just about what you would expect from three wings more or less in line. The trim angles on each mast were lower on the centre and aft mast, much like the trim angle difference between the jib, main and mizzen of a ketch or schooner. Each leading element is in the upwash ahead of the element behind it. Adding a triangular sail to the rig will lower it's AR and may reduce windward performance due to the increase in induced drag. If you make the rig big enough to have good light air upwind performance and count on not flying the Royal in a breeze, you might have a real winner.

Off the wind, there is probably a good case to be made for a "Code 0" type flying sail for close reaching in light air and other flying sails for off-wind work.

I like what you've done with the concept. I find it much more attractive than the mast-aft designs. I see the mast-aft rig as a riggers nightmare for loading and the DynaRig as simple and elegant. Very, very well done. :D

brian eiland

03-22-2007, 11:26 AM

Very interesting design Brian. For what will be in effect a single foil, I would think that the C/4 point at the mean aerodynamic centre should give you a very close approximation of the CE when sailing upwind and the flow is chordwise.

It will be interesting to see if the rig is dynamically stable. Have you looked at the moments? With the pivot/support point at C/2 I would expect the rig to try to twist the leading edge to leeward, creating wash-in at the Royals. Kind of exactly opposite to what you want for wind shear and gust response.
I understand what you are getting at...the actual CE being located approx 25% from the leading edge. So far I am hearing 25-35% range. But you know this is true for the Burmuda rig sails, and water foils (CLR) as well, yet we by convention first seek out the geometric centers to determine 'balance', and somehow that seems to work, even to my amazement

You are correct, the rig will try to twist, and this was pointed out as one of the reasons that the Dynarig built with older materials was going to be not so successful. It is only the recent carbon fiber materials that can resist the twist, that makes the DynaRig feasible.

I've adressed that 'wind gradient' question on several of the Maltese Falcon discussion threads. It was decided that to account for the gradient would add too much complication to the rig. Gust response might be the same.

I understand your reasoning behind the Gallant/Course areas. Have you thought about bigger Courses and smaller Gallants? Striking the Courses and using the Gallants for higher winds and sea-states means that your working sail is less effected by calms in the troughs and they should see less variation (although higher average) wind speed. Using the Gallants for working sail also increases visibility and reduces the chance of waves hitting the sail.
Two items were brought to my attention. First I orginally mislabeled the Gallants as Topsails and vice-versa. The Gallants flew over the Topsails which flew over the Courses...sorry about that. I corrected it on my website dwg. now.

This striking of the Courses in big seas was brought to my attention by the author, Philip Goode, of www.weatherlysquareriggers.com. Thanks for bringing to my attention though, in case I had not heard from Philip.

As far as head sails go, I'm not sure I see much benefit in adding a fore and aft sail to a DynaRig for upwind work. Stanford Yacht Research did a workup of MF's rig. IIRC, the pressure distributions were just about what you would expect from three wings more or less in line. The trim angles on each mast were lower on the centre and aft mast, much like the trim angle difference between the jib, main and mizzen of a ketch or schooner. Each leading element is in the upwash ahead of the element behind it. Adding a triangular sail to the rig will lower it's AR and may reduce windward performance due to the increase in induced drag. If you make the rig big enough to have good light air upwind performance and count on not flying the Royal in a breeze, you might have a real winner.
I guess I'm partial to the effectiveness of headsails, having preeched it so long. I definitely think they help the uni-rig point higher. Maltese Falcon (3 mast) and the 2 mast version proposed by Perini Navi have the benefit of 'interaction' between the fore-aft sails. The uni-rig does not.

Besides I really want that 'storm support forestay'

In reading 'between the lines', Philip's weatherly squarerigger, I definitely get the sense from his model studies that he appreciated the headsails in addition to the forward mast to a great extent

One quote from his paper that really caught my eye, "Close hauled at 45 degrees to the wind the effect of increasing the traditional mast spacing by 22% was astonishing, with the sailing force going up by 84%"

I like what you've done with the concept. I find it much more attractive than the mast-aft designs. I see the mast-aft rig as a riggers nightmare for loading and the DynaRig as simple and elegant. Very, very well done. :D
Thanks very much. I don't find it 'more attractive' in the visual sense, but it has its own appeal. I think this would be an exciting project that could build further upon the pioneering work done by the Maltese crew

yipster

04-21-2007, 01:26 PM

america cup alinghi likely to sail Inflatable sail battens which if successful are likely to be seen in the main stream of sailing in the future. http://www.alinghi.com/en/32ndac/rules/

Omeron

04-26-2007, 08:48 AM

visualise an aeroplane landing, with flaps fully extended. ın modern commercial planes you would normally see two sets of flaps with a considerable gap between each of them.
I believe those slots in between are there not because the designers could not make them a one continous curve touching each other.
The curve of the entire wing in that configuration is just too great for the
air flow to remain attached to the upper wing surface without seperation and stalling.
The slots in between help the flow to remain attached to the adjacent wing section, creating a series of deflected flow patterns rather than just one big surface unable to cope with the total demand.
Perhaps it is not too far fetched to carry this to the mainsail and jib configuration. the jib being the static wing and the mainsail acting like a flap, with a slot in between.

tspeer

04-27-2007, 02:15 AM

...The curve of the entire wing in that configuration is just too great for the
air flow to remain attached to the upper wing surface without seperation and stalling.
The slots in between help the flow to remain attached to the adjacent wing section, creating a series of deflected flow patterns rather than just one big surface unable to cope with the total demand.
Perhaps it is not too far fetched to carry this to the mainsail and jib configuration. the jib being the static wing and the mainsail acting like a flap, with a slot in between.

You have it basically correct. A.M.O Smith's Wright Brother's Lecture, High-Lift Aerodynamics (http://www.boatdesign.net/forums/attachment.php?attachmentid=12710&d=1177653204), fills in the technical details.

Paul Scott

04-27-2007, 01:12 PM

I'm trying to print out the A.M.O. Smith Lecture, but while my computer tries to do that, to carry Omeron's point a bit further, I'm looking at fig 134 (pgs 228-9) of The Theory of Wing Sections, which has to do with slot combinations, and it seems you get more lift with the major portion of the divided section in front of the slot, which would seem to imply that the old IOR style big jib/ small main combo may be better than the blade jib/ big main combo we see now. And which I have on my 40er. That, and the Gentry fig 17 Streamlines around jib and main sail pic I have in front of me on my bulletin board, which are for a 165% jib, which is about the same area as the main. ?????

Paul

PI Design

05-15-2007, 05:14 AM

Changing tack slightly...

For anyone interested in understanding how wings, sails and foils work, I have found a couple of interesting and simple(ish) to follow websites. They seem to disagree about the precise role Bernoulli plays in the whole affair, but otherwise both sites are pretty thorough.
1. http://www.regenpress.com/
2. http://www.av8n.com/how/htm/airfoils.html

It led me to thinking though, which is always dangerous. Wings and sails may use the same principles, but they have quite different requirements.The forces on an aerofoil can be resolved into lift and drag components. On a glider's wing, the lift acts mostly upwards, with a forward component at the front and backwards component at the back. The magnitude of the forward component is greater than the backwards bit, so the glider moves forward as well as staying up in the air. However, when applied to a sail, the component of lift that is upward on the glider (a Good Thing), is causing leeway and heeling on a boat (a Bad Thing). Only the forward component of lift at the front of a sail is actually beneficial. I guess what I'm saying is that lift, whilst a good thing for gliders, is a mixed blessing for boats.

To look at it in a slightly different way, presumably a glider is designed to be able to maximise the downwash angle at the trailing edge, whilst a boats sails should minimise the downwash angle. That seems a fairly fundamental difference between wings and sails, and one that that is not often talked about in most books, which usually claim that sails are just wings on the side. And that's before you even get to the more obvious differences like (range of) angle of attack, twist, mast interference, sail interaction etc etc that makes a sail so different from a wing.

So, am I right in thinking there would be merit to a sail that looked a bit like the low pressure side of a NACA section, but chopped of at the maximum camber point? It seems to me that aft of that point, the sail is only producing forces that heel and drive the boat backwards, so you may as well do with out that bit.

Does this also explain why wing masts can work so well? A normal mast disturbs the airflow at the front of the sail, which is the very bit you need. By the time the flow re-attaches, the force vectors are pointing in the wrong direction.

Paul Scott

05-15-2007, 11:12 AM

PI, given the general direction of your last post, you might like this;

http://www.modelresearchlabs.com/hand_launch_glider_airfoils.htm

I've been looking at this one for a while now, and while I'm not quite sure what to do with it as far as sailboats, it is thought provoking....

Paul

Omeron

05-16-2007, 03:11 AM

Perhaps the leeward curve of the jib and mainsail combined should look like
the low pressure side of a NACA wind section, starting from the forestay and all the way to the leech of the mainsail.
This also makes you think what a different size of airfoils we get. Near the top of mast, the chord length is perhaps a metre or less, whereas close to the deck, we may be talking about 10,20,+ meters.
This also requires substantially different section thicknesses, if you want the entire sail to function. I think that is just not possible, and here the sails and wings go their own way.

PI Design

05-16-2007, 05:24 AM

Interesting site Paul. I'm not sure that that most of the lift on a sail is generated in the windward side though, so don't know how relevant to boats that would be, but worth considering.
Hi Omeron, I agree you ought to consider all sails combined. I was sticking with a single sail for now, to make it easier. You're right, for most boats with triangular sails the chord varies massively. However, if you look at something like an A Class cat the sails are nearly rectangular, so the chord stays constant for most of the span.
If minimising downwash is the aim of the game, then flat sails (low camber) must help. Perhaps this is another area where over-rotating wing masts help, their sails tend to be much flatter (may be 8% camber v. 13% on a 'normal' rig).

Paul Scott

05-16-2007, 12:05 PM

PI, The site makes me wonder what he might say about the Kutta condition?

But: if he's right even a little, so this little gedanken experiment with breakfast goes, with a 165% jib, as pilots might say, is the main more Newton, and the jib more Bernoulli? With a 95%?

Paul

Matth

05-16-2007, 05:57 PM

PI: I recently went through some painful 'thinking' about lift and drag, but came to very different conclusions. These are the simple examples I 'thought' my way through.
First some definitions: Drag is a force parallel to the apparent wind direction. Lift is a force perpendicular to the apparent wind. Note that the angle of attack, geometry of the aerofoil etc do not enter into this definition. When I first started investigating aerodynamics, I was confused until I picked up this definition, and thinking in these terms really helped to simplify things.
All real objects produce drag. Aerofoils also produce lift. Which is most useful to a sailboat?
Some examples:
Craft A is a raft with a crude hut built on the deck--no sail, no keel. Its 'airfoil' (hut) produces only drag, and its 'hydrofoil' (raft) produces only drag. The 'sailboat', powered by the wind, drifts straight downwind (assuming no current).
Now imagine adding a sail which can produce lift (perpendicular to the wind). Now this Craft B can be made to sail in any downwind direction. If the aero lift were equal to the aero drag, the boat could sail at 45 deg off the wind direction. If there were pure aero lift with no aero drag, in theory it could sail at 90 deg to the wind (very slowly).
For Craft C, take away the sail, but add a (steerable) keel, and you have the same performance as Craft B. Still can't go upwind, but it can sail in any downwind direction, depending on the Lift/Drag ratio of the keel.
For Craft D, you have both a sail and a keel, both capable of creating lift. Now you can sail upwind, and the better the L/D ratios of your sail and your keel/hull, the steeper you can go upwind.
Things I learned from this are:
- Lift is good, drag is bad -- even if you want to go straight downwind. A high drag (spinnaker) skiff can never reach windspeed, but a high L/D craft can go downwind faster than windspeed by moving at an angle to downwind. I've got a spreadsheet that demonstrates this, which I will try to post in the next few days.
- The keel/hull is just as important as the sail.

Hope this helps, and doesn't contradict any laws of nature.
Best,
Matt

PI Design

05-17-2007, 11:28 AM

Hi Matt,

Lift and drag are really just a convenient means of breaking the force into component parts. It is equally valid to chose another set of axes, which run parallel and perpendicular to the desired direction of motion. The drawing attached attempts to show this, and also clarifies, pictorially, the points I was trying to explain earlier. Drag and (predominantly) lift, both contribute to the sway force, which causes leeway and, when coupled with a keel/centreboard, produces a heeling moment. In this respect large amounts of lift are bad when sailing upwind.
In the diagram, the red zone shows the part of the sail that is wholly detrimental to upwind performance - the force is backwards and sideways, when all you really want is forward. The green zone covers the part of the sail that provides the majority of the forward surge, with little sway force (heeling moment). Note, this is not the area of maximum lift, but it is the most useful part. The amber zone contributes to forward motion, but also contributes significantly to the heeling moment, so whether it is a Good or a Bad Thing depends on the available righting moment of the boat.
What a high lft/drag ratio does is allow the force vectors to point as far forward as possible. Minimising drag is definitely the goal, but for the part of the sail in the red zone (half the sail!) lift should also be minimised. Seems to me the easiest way to minimise both lift and drag is to do without that part of the sail altogether.
The other point to note is that the air flowing round a conventional mast is seperated for most of the green zone, so presumably relies more on the amber zone for its drive, which means a much larger heeling moment for the same forward surge. Wing masts prevent thi seperation so can take advantage of the green zone.

Shoot away if I have it wrong. I don't know the answers, just putting this here for discussion.

Matth

05-17-2007, 12:51 PM

Pi: Nice diagram. I think I see what you're getting at.

You're right about any set of axes being valid--I've just found that referring it to app. wind direction is the simplest when looking at sail forces. After all, the sail doesn't know which way the boat is moving. You can work out the sail force knowing only the apparent wind and the angle of attack. (well, except for interactions between main/jib changing with boat heading, etc... I'm trying to keep it simple)

The problem I think is that you can't just cut off the 'bad' parts of the sail without affecting the 'good' parts. The leading edge section which is actually pulling the sail upwind is only possible with the draggy trailing section.

Looking at the whole sail, rather than just parts of it, the net sail force will always be somewhat downwind. The theoretical ideal would be a sail that is all lift and no drag. (I added a page 2 to your diagram and attached) Even this idealized sail does not give a net upwind force.

As the boat points higher and higher upwind, the lift has less and less of a forward component and more of a heeling component. Nonetheless, drag is worse, since it has a heeling plus a backwards component, for a net loss.

That's a good point about the mast causing separation in a critical area. I have often wondered if having a giant luff sleeve could help this, or would it assume the wrong shape? Maybe some balance between aerodynamic forces (suction pulling the luff sleeve forward and leeward -good) and sail tension (pulling the luff sleeve back -bad). Has anyone tried/studied this?

Matt

PI Design

05-18-2007, 03:40 AM

Pi: Nice diagram. I think I see what you're getting at.

You're right about any set of axes being valid--I've just found that referring it to app. wind direction is the simplest when looking at sail forces. After all, the sail doesn't know which way the boat is moving.

...

That's a good point about the mast causing separation in a critical area. I have often wondered if having a giant luff sleeve could help this, or would it assume the wrong shape? Maybe some balance between aerodynamic forces (suction pulling the luff sleeve forward and leeward -good) and sail tension (pulling the luff sleeve back -bad). Has anyone tried/studied this?

Matt

Cheers Matt. The direction that the boat is moving is critical. For example, if the boat is travelling perpendicular to the app wind, then lift should be maximised (all over the sail), where as, as explained above, when travelling upwind there is different criteria. So whilst the sail still generates forces in exactly the same way irrespective of the direction of travel, the requirements of the sail differ.

Marchaj has done some research into luff sleeves, but concluded they didn't work due to the blunt leading edge of the round masts. However, he seemed to use pretty big diameter masts, and of course windsurfers use luff sleeves and sail quicker than most dinghies.

Omeron

05-18-2007, 04:19 AM

Hi everybody., I haven't been able to read all posts on this subject, but now that you are discussing luff sleeves, i would like to throw in another idea.(apologies if already mentioned/discussed)
I guess our sails have somewhat more in common with STOL aircaft (short takeoff and landing) than any other aircraft due to relatively slow flow of air over the foils.
This type of craft, and indeed all modern passenger planes have leading edge slats, which are deployed where maximum leading edge (lift) performance is required, which is during take off.
We know that we have a problem with our masts, so why dont we use slats attached/ hinged to the mast, which can also be tacked from one position to another and regulate the inflow of air entering the equation, thereby making the mast itself less significant.
I am not intending to produce design objectives here, but just an idea of introducing a control surface before the mast to enable a better controlled entry into the mainsail.

Hansen Aerosprt

05-18-2007, 12:45 PM

Marchaj has done some research into luff sleeves, but concluded they didn't work due to the blunt leading edge of the round masts. However, he seemed to use pretty big diameter masts, and of course windsurfers use luff sleeves and sail quicker than most dinghies.Actually, a modern state-of-the-art boardsail has a cambered sleeve with a max thickness exceeding the mast diameter. The wider the sleeve chordwise, the greater the effect. Here is an illustration:

frosh

05-18-2007, 04:17 PM

I have got to agree 100% with Hansen having owned and sailed every sailboard sail type ever made since 1980. The mast in most sailboard designs, even going back 20 years, was not really an impediment to performance of the sail. At one stage super skinny masts were introduced to the market and sails were made to fit these skinny masts. To my best knowledge these rigs were NOT noticably faster than conventional diameter mast rigs for sailboards.
Maybe Marchaj was plain wrong, and was unable to foresee future mast and rig develpments.

tspeer

05-26-2007, 08:08 PM

Lift and drag are really just a convenient means of breaking the force into component parts. It is equally valid to chose another set of axes, which run parallel and perpendicular to the desired direction of motion.
Perfectly true. However, lift and drag are very useful for calculating performance because you don't need to know the orientation of the sail or the boat. All you need to know is the point of sail, lift/drag ratio of everything in the air, and the lift/drag ratio of everything in the water to calculate the boat-speed/wind-speed ratio.

BTW, the name typically given to the force component that is parallel to the chord is axial force, and the component that is at right angles to the chord is normal force.

The drawing attached attempts to show this, and also clarifies, pictorially, the points I was trying to explain earlier. Drag and (predominantly) lift, both contribute to the sway force, which causes leeway and, when coupled with a keel/centreboard, produces a heeling moment.
The problem with the drawing (assuming it depicts attached flow) is that if you add up all the vectors shown, the drag is near zero (exactly zero if you use inviscid flow assumptions to calculate the vectors). The aerodynamic drag of a sail comes mostly from aerodynamic forces that are tangential to the sail (skin friction), and from the fact that the local apparent wind direction is not the same as the difference between the boat's velocity and the true wind direction (the free-stream apparent wind direction). The skin friction isn't shown at all on the diagram.

The boat sails in a header of its own making, tilting the aerodynamic force vector aft, and the component parallel to the free-stream apparent wind that results from this tilt is generally accounted for as part of the drag. This induced drag is a major source of drag, and it's not shown directly on the diagram, either.

So the diagram is somewhat misleading because it implies that the shape of the camber has a major impact on the drag or thrust because of the way that the pressures are oriented along the chord of the sail. I turns out this isn't really the case at all. Although camber affects the lift at a given angle of attack, the angle of attack can, in principle, be adjusted to restore any given amount of lift.

The amount and shape of the camber influences the drag, but does so indirectly through its effect on the boundary layer by way of the shaping of the pressure distribution and the effect of the pressure distribution on the boundary layer development. If it weren't for the boundary layer, it wouldn't matter at all what shape the pressure distribution was, and thus what the camber shape was.

...
The other point to note is that the air flowing round a conventional mast is seperated for most of the green zone, so presumably relies more on the amber zone for its drive, which means a much larger heeling moment for the same forward surge. Wing masts prevent thi seperation so can take advantage of the green zone.

The chord-wise component of the pressure at the leading edge is known as leading edge thrust. Thin, sharp leading edges tend to promote a separation bubble on the lee side of the leading edge that can produce low pressures that result in comparable lift to a thick leading edge, but are not oriented as much in the forward direction, reducing leading edge thrust and increasing drag. In inviscid flow theory, the thin leading edge produces infinite velocity around zero area, and the combination just balances all the other aft-directed pressure components. In practical flows, this isn't the case, and the improvement in leading edge thrust is, indeed, the major advantage of the wingmast.

Wilkinson (http://www.eng.usf.edu/~wilkinso/) uses a schematic diagram similar to yours, but broken up into nine zones, shown below (1., 2.). The flat regions (II, V, VII) are separated flow. Zone I provides the leading edge thrust. A wingmast makes zone II much smaller and allows the pressure distribution (http://www.tspeer.com/Wingmasts/teardropPaper_files/image002.gif) in zone I to be much higher.

The second figure below of a wingmast pressure distribution has the features of Wilkinson's illustration. Zone II is very small - it just bridges the crease in the lee-side mast/sail junction. There's an additional separation bubble on the mast itself, due to laminar separation and transition to a turbulent boundary layer. This particular example is taken below the onset of stall, so there's no zone V.

1. Wilkinson, Stuart, "Partially Separated Flows Around 2D Masts and Sails", PhD thesis, Ship Science Department, University of Southampton, 1984)

2. Wilkinson, Stuart, "Static Pressure Distributions over 2D Mast/Sail Geometries", Marine Technology, Vol. 26, No. 4, Oct., 1989, pp. 333 - 337.

PI Design

05-29-2007, 05:52 AM

Hi Tom,

Thanks for your input - much appreciated as always.

The advantages of using lift and drag for aeroplane wings is clear, but I think axial/normal is more useful for boats, precisely because the orienation of the boat to the wind is so important. Assume that (upwind) the sail is sheeted to the centreline so that the boat and sail point (and travel) in the same direction. The aim is (obviously) to maximise VMG to windward, which is entirely dependent on the boat's heading as this affects both the angle of attack of the sail and the cosine of the angle to dead up wind.

"The boat sails in a header of its own making, tilting the aerodynamic force vector aft, and the component parallel to the free-stream apparent wind that results from this tilt is generally accounted for as part of the drag. This induced drag is a major source of drag, and it's not shown directly on the diagram, either."

My understanding is that induced drag is a function of AR and planform, not section shape, therefore I was simplifying to 2d. Presumably induced drag is constant across the chord length and can simply be superimposed with the effect of tilting the pressure vectors aft?

Also, I’m assuming that skin friction is a function of surface roughness and surface area. Provided the same material is used, surface roughness is constant across different section shapes. Surface area increases with camber (for constant chord, not constant force), so a flatter sail is better in this respect, although I guess skin friction is a fairly small total of overall drag so can be ignored for comparative purposes.

"So the diagram is somewhat misleading because it implies that the shape of the camber has a major impact on the drag or thrust because of the way that the pressures are oriented along the chord of the sail. I turns out this isn't really the case at all. Although camber affects the lift at a given angle of attack, the angle of attack can, in principle, be adjusted to restore any given amount of lift."

This is exactly the point I was trying to make, but I draw a different conclusion. If you have to increase the angle of attack by bearing away, this is a bad thing because you are not sailing as close to the wind, hence VMG is worse. The shape of the sail camber DOES have an affect on the thrust and drag.

At least we agree about the positive impact of wing masts!

Thanks for the references. The problem I always have with pressures illustrated this way, is that you have no idea of the axial/normal components, which (IMHO) is the vital info for boats.

sigurd

06-04-2007, 04:51 PM

My understanding is that induced drag is a function of AR and planform, not section shape
Not entirely right, since for the same span and lift and induced drag, you can trade chord for CL.

Pericles

07-18-2007, 04:29 AM

Brian Eiland suggested this might interest you all.

From Duckworks, details of a Wing sail from Israel.
http://www.omerwingsail.com/

Pericles

Pericles

07-18-2007, 04:08 PM

Ilan Gonen has replied.

http://www.boatdesign.net/forums/showthread.php?p=152300&highlight=pericles#post152300

Pericles

Erwan

08-08-2007, 07:20 AM

Hi Everybody,
Reading your thread, I feel like a MIT student. many things to learn for me. In order to bring my little contribution, I would like to mention the following concept, I didn't see yet, it is:
" Non Linear Aerodynamic consideration" or "higher order effects on vortex drag", as rocket scientists call it.
Or it is also observed on eagle wing tip feathers.

I recommend to read page 19 of the PDF attached, where a split tip wing vortex is modelized. Conclusion is a significant reduction in induced drag and/or increase in L/D.

Trying to understand this issue, I guess that a vortex is a 3 dimensions "item", one dimension is related to apparent windspeed, but the other dimensions are likely to be related to pressure difference between the 2 sides of the sail/wing section at the tip, combined with the chord's lenght at the tip, which is likely to drive vortex diameter
So if you halve the chord at the tip, (everything else constant, ...)each vortex is likely to be twice smaller in diameter, compared to the basic wingtip vortex (reference vortex), and therefore the volume of air involved is proportional to Diameter squared, so each small vortex generates 4 times less induced drag than the reference vortex.

Separation of vortex wakes, using some diehdral for the split tip might also help to reduce drag ?

May be we could find here a simple and cost effective way to improve sails'performance.

For a catamaran I imagine that it could be possible to use a split foot sail, combined with trampoline effect, to improve the low component of induced drag.
For the top of the sail, it seems more challenging as you are supposed to have the fore part of the split-tip bending on the leeside while the aft part of the split is supposed to come windward, in order to avoid vortex interferences.

Sorry, my English is far from perfect, I hope it remains understandable.

tspeer

08-11-2007, 01:26 AM

...
Separation of vortex wakes, using some diehdral for the split tip might also help to reduce drag ?

May be we could find here a simple and cost effective way to improve sails'performance. ...

I think one of the most important papers on the subject is Max Munk's "Minimum Induced drag of Aerofoils (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091456_1993091456.pdf)," NACA-TR-121, 1923. Ignore all of the sextuple integrals and read the words - especially the ones in italics. His theory applies to nonplanar as well as planar lifting surfaces.

To minimize the induced drag, the induced velocity at right angles to the surface ("downwash") shoud be uniform across the span and proportional to the cosine of the dihedral angle. And for multiple lifting surfaces, the induced velocity should be the same for all the surfaces. All of the nonplanar surfaces in Ilan Kroo's paper could have been analyzed, and their loadings optimized, using these principles.

Probably the most important predictor of lift/drag ratio is the wetted aspect ratio: span^2/(total wetted area). For example, the induced drag of a ring is equivalent to that of a planar surface whose span is sqrt(2) times the diameter of the ring - so the ring has much less drag for the same span. But the circumfrence of the ring is pi times the diameter. So the ring will have more than double the wetted area of a planar wing with the same induced drag. This is typically the case with nonplanar surfaces. The induced drag of the nonplanar surface is lower for the same span, but the same result could have been obtained with less surface area by extending the span of the planar surface. So nonplanar surfaces are most useful if there is some limitation on the span.

Sail rigs are limited by the stability of the hull when the wind gets above a certain speed. So a key criterion for a sail rig is not minimum induced drag, but the minimum induced drag for a given heeling moment. There's not a unique solution to this problem. R. T. Jones showed the minimum drag for a given moment is obtained with a linear variation in the induced velocity across the span. (Jones, Robert T., "The Spanwise Distribution Of Lift For Minimum Induced Drag Of Wings Having A Given Lift And A Given Bending Moment", NACA-TN-2249 (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082889_1993082889.pdf), 1950) You can get any moment you desire, including zero, if you wish. However, there are practical limits for sail rigs, such as zero lift at the head instead of negative lift.

If you combine the results of Munk and Jones (http://www.tspeer.com/Planforms/Planar.htm), you can take into account factors like the gap between the foot and the water's surface, heel angle, and heeling moment. This spreadsheet (http://www.tspeer.com/DesignTools/vortex95.xls) implements a lifting line analysis that allows you to define an induced velocity profile in the Trefftz plane, and then calculate the spanwise loading and planar planform to implement it. The spreadsheet could easily be extended to nonplanar surfaces.

This would allow you to optimize a nonplanar rig for minimum drag given a constrained heeling moment. This will not necessarily result in a symmetrical geometry, however. For example, say you were optimizing a "T" shape, like a keel with wings. The optimum loading of the wings for a given tack may not result in the same planform shape for the windward and leeward wings. But it may be possible to use Excel's solver to find the optimum shape with practical geometric constraints.

tspeer

08-11-2007, 01:31 AM

...For a catamaran I imagine that it could be possible to use a split foot sail, combined with trampoline effect, to improve the low component of induced drag....

Here are data (http://www.tspeer.com/landyachts/twin/dragstudy.htm) for a similar concept. The range of planform shapes investigated is depicted in this illustration:
http://www.tspeer.com/landyachts/twin/yacht.jpg

TTS

08-18-2007, 01:16 PM

The other issue with the a-class and jibs are the class rules. Attached is the copy of the rules. It seems to state that it is for a main sail only, or at least the measurement form is laid out in that manner. Maybe only the sail area is dictated and a-class sailors have gone for a single high aspect main sail, rather than a smaller main with small jib.

Tom

ISAF A-Class Catamaran Rules

IACA Championship Rules (Draft)

Measurement Forms (in Microsoft Excel format)

The A Class Catamaran is controlled by the rules set down by the ISAF under the "Rules for the Divisions of Catamarans and The International C Class" (Last issued May 1985) which are as follows:

1. A catamaran is defined as a two-hulled sailing boat with essentially duplicate or mirror image hulls, fixed in parallel positions.

2. Sail area shall not be more than 13.94 square metres (150 square feet) Sail area to be measured in accordance with the" IYRU Measurement & Calculation of Sail Area Instructions" (Last issued May 1985)

3 The overall length of the catamaran shall not be more than 5.49 metres (18 feet)

The length shall be measured between perpendiculars to the extremities of the hulls with the catamaran in her normal trim. The measurement shall be taken parallel to the centre line of the craft and shall exclude rudder hangings, but if the athwartships width of a rudder within 153mm (6 inches) of the bottom of the hull is more than 76mm (3 inches), the length shall be taken to the aftermost point of the rudder.

4. The extreme beam shall not be more than: 2.3 metres (7 ft 6½ inches)

The beam shall be measured at right angles to the centre line of the craft at the widest point and including all fixed or adjustable apparatus with the exception of a normally accepted trapeze or retractable seat.

5. The crew in Division "A" shall be one person (helmsperson)

6. Unballasted retractable seat or trapeze shall be allowed for the helmsperson. When in use the helmperson at all times shall have at least one foot in contact with the boat.

7. The A Division emblem shall be carried on the mainsail and shall consist of the letter A over two parallel horizontal lines over national letters and sail numbers,

Sail numbers shall be allotted by the National Authority or Class Association appointed by the National Authority. The class emblem, national letters and distinguishing numbers shall be placed as prescribed in the Yacht Racing Rules.

( RRS 77 & RRS Appendix H )

8. Hydrofoils are permitted.

The following rule was passed by ballot 28th February1998 and will take effect from 1st April 1998:

9. Minimum weight in full sailing trim shall be not less than 75 kilograms.

The following rule was passed by ballot August 15, 2001:

10. Hydrofoils shall not be permitted in ‘A’ Division. (Approved 2001)

Advertising; for all National, Continental and World Championships and events which are not restricted by the organising authority to Category A advertising status, a boat may choose to display unrestricted Category C advertising as defined by ISAF Regulation 20.

Measurement Form

All A Division Catamarans shall have a valid measurement form and for all yachts measured after 1st January 1998 it shall be on the latest style form dated 4/97. This form is composed of four separate areas that are largely self-explanatory. The following notes should assist in their understanding.

Mast & Boom Measurement Form

The purpose of this measurement is to find half the total area of the mast and any mast base fitting attached. On a straight section (ie not tapered) it is simply the length L x half the mast girth. The Measurement and Calculation of Sail Area instructions defines girth as follows:

"The girth measurement shall be taken as the distance from the centreline round the surface of the spar to the same point on the centreline. The resultant dimension shall be divided by two to give the half girth measurement."

Should mast be tapered extra measurement U1 & T need to be taken and the formulae on measurement certificate utilised. Black band measurements L1 & L2 play no part in the mast measurement at this stage.

Boom measurement is only utilized if the profile height of the boom is more than 1.5 of the width.

Hull Measurement

This area is self explanatory with only two measurements needed ie width & length.

Things to look for with overall length are:

If the width of a rudder within 153mm (6 inches) of the bottom of the hull is more than 76mm (3 inches) the length measurement needs to go to the aftermost point of the rudder.

With the width, measurement is at the widest point of the hulls, this may be at some point down the sides of the hulls, especially if hulls are angled. It may also be possible that the maximum width is at bottom of centreboards when fully down.

Sail Measurement Form

When undertaking the sail measurement the following points should be noted.

Sail to be measured on a flat surface and laid out in terms of IYRU Measurement & Calculation of Sail Area Instructions

ie. "With battens set in their pockets the sail shall be pegged out on a flat surface with just sufficient tension to remove waves or wrinkles from the edge rounds and to spread the sail, as far as possible, substantially flat. Once the sail has been pegged out in this way all the required measurements shall be taken and no alterations to the tensions shall be made."

Luff length A is the maximum distance from the head to the tack of the sail. It is taken on the inside of the boltrope, which is not included in any measurement.

Base length P is a measurement from the clew to a point at 90 degrees to A.

Measurements M, F, K, D & H are all made at 90 degrees to their respective lines. All are to be the maximum distance that can be taken.

General Calculation Form

This form brings all the measurements together and all that is required is to transpose measurements from the other forms. On this page it allows you to calculate the theoretical Black Band distance that will allow a maximum sail area of 13.94 square metres for each sail that the boat ma have. It should be noted that black bands should be on the mast and form part of the measurement requirements.

The weight of the boat and any correcting weights are also listed on this page.

The weight of the boat consists of all items associated with the boat in full sailing trim. It does not include such things as shackle keys, water bottles, spare rope etc.

The boat must be weighed in a dry condition and any weights attached to bring the boat to a minimum weight of 75 kilograms must be permanently affixed and their weight duly noted on this page.

CT 249

08-19-2007, 03:21 AM

RE "The other issue with the a-class and jibs are the class rules. Attached is the copy of the rules. It seems to state that it is for a main sail only, or at least the measurement form is laid out in that manner."

There were sloop-rigged As in New Zealand in the '60s and maybe early '70s. They soon found out that they were not competitive with cat rigged cats, just like the Bs did (before the development class died) and the Cs and Ds have proven.

PI Design

08-20-2007, 03:44 AM

The NS14s also have very free rules regarding the rig (total sail area 100sqft, mast height 18ft, any split of sails allowed). But the norm in this class is for a high aspect mainsail and small jib (approx 75/25 split), on a wing mast, rather than a main only. Whether this is due to the relatively low righting moment needing a lower centre of effort, or whether the mast height restriction would result in too low an aspect ratio if all the area was in the main I don't know. However, Andrew Landenberger (Tornado silver medallist, Moth World Champ, good A Class sailor) has been considering a cat rigged NS14 but isn't really involved with the class much so may not get round to realising his idea.

dimitarp

11-08-2007, 05:57 AM

You can find more for sail aerodynamics at:
http://www.partenovcfd.com

PI Design

11-08-2007, 06:26 AM

There doesn't seem to be anything there?

CT 249

11-09-2007, 06:27 AM

BOATMIK

11-18-2007, 11:47 AM

Howdy,

Regarding cat rigs vs sloop rigs, my gut feeling is that it relates to span.

For classes with restricted sail area, the classes that have spanwise freedom (to make their masts as tall as they like) seem to favour cat rigs.

But when span is heavily restricted - either by righting moment or an absolute measurement (eg an NS14 has a limit of 18ft from the deck, however that has now been metricised) then multi sail rigs seem to be preferred.

Classes from the NS14 through to the Bembridge Redwing seem to find the jibs diminish in size but will never quite disappear.

Part too might be the advantage of two sails on a reach (if I remember Gentry's original articles on circulation properly). An A or C class cat spend much more time going upwind which has to favour a higher aspect sail, and compared to an NS they have stability to burn.

Maybe a good experiment would be to restrict NS14s to a max mast length of 10ft and see if the fastest boats are schooners, ketches or yawls.

Best wishes
Michael

PI Design

11-19-2007, 03:51 AM

Landy's thoughts on the cat rigged NS14 after its first sail:
"It is very fast downwind but upwind is still unknown. It felt fast but height was an issue. I had some problems with the setup so I hope I have solved that for next weekend."

Second outing:
"I did sail a regatta on the weekend with the big main. It is a struggle upwind against the top boats but I was very fast downwind. I managed to win one heat in lighter air but generally the sloop config was clearly faster. I will try again with a flexible round mast eventually."

astevo

11-24-2007, 06:51 AM

should point out that sometime boatdesign member 'phils' won an a-class "worlds" using a sloop rig in the 70's. 'Harmony' was the boats name. multi chine ply hull, precursor to the 'rhapsody' hullshape, which was very tornado like.

ive seen the cat rig ns14 a few years ago, my memory was that it was never faster, but suffered especially upwind. theory says it might be quicker off the breeze but i couldnt tell you based on th at experimental rig/boat.

brian eiland

03-07-2008, 06:32 PM

I've been away from this subject thread too long. I see some things I need to review.

Meanwhile just to throw some monkey wrenches into the works, consider these unusual findings from Mother Nature:

DURHAM, N.C. -- Wind tunnel tests of scale-model humpback whale flippers have revealed that the scalloped, bumpy flipper is a more efficient wing design than is currently used by the aeronautics industry on airplanes. The tests show that bump-ridged flippers do not stall as quickly and produce more lift and less drag than comparably sized sleek flippers.

The tests were reported by biomechanicist Frank Fish of West Chester University, Pa., fluid dynamics engineer Laurens Howle of the Pratt School of Engineering at Duke University and David Miklosovic and Mark Murray at the U.S. Naval Academy. They reported their findings in the May 2004 issue of Physics of Fluids, published in advance online on March 15, 2004.

In their study, the team first created two approximately 22-inch-tall scale models of humpback pectoral flippers -- one with the characteristic bumps, called tubercles, and one without. The models were machined from thick, clear polycarbonate at Duke University. Testing was conducted in a low speed closed-circuit wind tunnel at the U.S. Naval Academy in Annapolis, Md.

The sleek flipper performance was similar to a typical airplane wing. But the tubercle flipper exhibited nearly 8 percent better lift properties, and withstood stall at a 40 percent steeper wind angle. The team was particularly surprised to discover that the flipper with tubercles produced as much as 32 percent lower drag than the sleek flipper.

“The simultaneous achievement of increased lift and reduced drag results in an increase in aerodynamic efficiency,” Howle explains.

This new understanding of humpback whale flipper aerodynamics has implications for airplane wing and underwater vehicle design. Increased lift (the upward force on an airplane wing) at higher wind angles affects how easily airplanes take off, and helps pilots slow down during landing.

Improved resistance to stall would add a new margin of safety to aircraft flight and also make planes more maneuverable. Drag -- the rearward force on an airplane wing -- affects how much fuel the airplane must consume during flight. Stall occurs when the air no longer flows smoothly over the top of the wing but separates from the top of the wing before reaching the trailing edge. When an airplane wing stalls, it dramatically loses lift while incurring an increase in drag.

As whales move through the water, the tubercles disrupt the line of pressure against the leading edge of the flippers. The row of tubercles sheer the flow of water and redirect it into the scalloped valley between each tubercle, causing swirling vortices that roll up and over the flipper to actually enhance lift properties.

“The swirling vortices inject momentum into the flow,” said Howle. “This injection of momentum keeps the flow attached to the upper surface of the wing and delays stall to higher wind angles.”

“This discovery has potential applications not only to airplane wings but also on the tips of helicopter rotors, airplane propellers and ship rudders,” said Howle.

The purpose of the tubercles on the leading edge of humpback whale flippers has been the source of speculation for some time, said Fish. “The idea they improved flipper aerodynamics was so counter to our current doctrine of fluid dynamics, no one had ever analyzed them,” he said.

http://www.pratt.duke.edu/news/?id=101