sail aerodynamics

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.

 

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

Regards,
Colin
www.sailwave.com

 

Slot effect

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.

 

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.

 

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...

 

Wing lift

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.

 

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

 

Upwind sailing

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.

 

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".

 

Reynolds number

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.

 

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.

 

Mast Compression

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?

 

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

 

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!

 

Real World

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.