Why my boat can't move forward in strong wind???

[redmapleleaf]

Hi all,

This weekend was my boat's maiden voyage. When I took it out the first day, the wind was light (about 5-6mph) and I was able to sail downwind as well as upwind without any problem. Then, on the second day when the wind really picked up (over 12mph), I thought it would have made the boat sail even faster than before. To my surprise, upwind sailing (sailing close haul) was terrible. The boat was not able to gain an inch on the upwind and instead just moving side way or being blown back by the wind (and wave).

Do you know if there is some fundamentally wrong with my boat design?

Please see the attached pictures for an idea of how the boat look like. Some info on the boat:

1. length: 16'
2. outrigger beam: 8'
3. sail area: 52 square feet
4. submerge area of centerboard: 1 square feet
5. submerge area of rudder: .5 square feet

Will it help if I add more sail area or centerboard area?

Thank you for your help.

[longliner45]

maybe your boat is at its limits ,,I dont see any kind of a keel,maybe im wrong,,,,,nice looking boat ,,but everything has a limit ,or zone of effitiancy,longliner

[the1much]

what part of boston?,,i swear ive seen that shoreline before,,,,,,,when your sitting in ya boat,, where does ya bow sit?,,or,, where at is your waterline.,,and before you get to much hope,,,im not REALLY gonna be of any help to ya ;)

[lazeyjack]

just kwikkly, one I dont think you have enough center board and two I think your sail is, well an awful; shape, send a pick on the beach with your sail in the close hauled position please

[marshmat]

Hi redmapleleaf,

I'm trying to figure out, firstly, just how your boat works. It looks almost as if it sails as an Atlantic proa (ama to leeward) on port tack, and a Pacific proa (ama to windward) on starboard tack, but still has a distinct bow and stern that do not switch places when tacking (which would make it more of an asymmetrical cat than a proa). Is this correct?

Regarding your problem with going to windward, it sounds like you're getting plenty of thrust from the sail (although the sail shape could definitely be a lot better), but the hull's not interpreting that force the way you want it to- it's sliding sideways, rather than forwards. The solution would seem to be more lateral resistance- what do you have now for a daggerboard / leeboard / centreboard?

[Meanz Beanz]

Sail looks crook, more board will help... first up I would play with rig balance and see what it does. Rake the mast back a few degrees at a time until you get some, but manageable, weather helm. See what that does for the uphill performance...

Cheers
bZ

[PAR]

1.5 square feet of appendages isn't enough area, though just speculation considering the limited amount of information available. A reasonable guess would be 4 to 5 sq. ft. as a starting point.

I've never seen a boat do well to windward without a fairly healthy amount of weather helm. This can be affected by several design elements, none of which we have information about.

I'll second the comments Matt has made concerning the set of the sail. In bigger wind strengths, you'll have much difficulty controlling the leach on that square topper.

Other issues could be entry shapes, lee bow wave formation, location and amount of "lead", effective lateral area, CG/CB relationship, frankly the list is quite long. Can you provide spec's for the design? Lines and sail plan?

[lazeyjack]

you could always add a small jib too

[the1much]

shouldnt the sail have more "puff" room,,haha,,,,like i mean not so tightly stretched?,, give the wind a place to "dish",,,< as you can see,, i jus built the boats,, didnt want nothing to do with rigging, and hated to deliver the things,,hehe ;)

[redmapleleaf]

Hi all,

I initially want to design a 'super stable' boat with two floaters, one on each size giving it a beam of about 14'. I guess you can call it a trimaran.

Well with two pantoons, I then found that it is a bit difficult to dock the boat with the pantoon on its side therefore I removed one of them. This make it an asymmetrical cat like Matt has noted. I was also hoping that this would make the boat a little lighter and move faster.

In my second picture you can see the centerboard sticking up for the shore landing. It shows how much centerboard the boat has. The sail is made out of white tarp cut to a triangular shape so it is not of very high quality. Since I saw a number of other DIY design used tarp before so I was hoping that it would work for me also.

My boat is in Halifax, NS and I am now back to Boston to work so I can't take any more picture of it to show you. I am going back there in two weeks. I'll increase the centerboard area and add a jib to it to see how it will perform. I'll let you know more about it then.

[alan white]

More lateral plane needed, as mentioned. I'd double what you've got and you'll do fine to windward. I checked an old design of mine---- a proa---- and 16 ft at that. I'd calculated 2 sq ft for the free area of the leeboard. Rudder must have been at least 1 sq ft. Though I never built her, she was designed with those areas.
I'd imagine there's a formula for sq ft sail area to sq ft lateral plane in Gerr's 'Nature of Boats".
You'll also have problems going to windward in heavy air if your sail has too much draft, meaning you want the sail flatter when it pipes up, otherwise you'll induce more drag than lift and the boat will drag to leeward (and heel much more).
Also, try sailing full and by, meaning easing the boom a bit (away from centerline). You'll actually make better progress doing this in some boats. Put the boom at maybe 15 degrees off. Try a vang too, or rig something to pull the boom down on the leeward side.

Alan

[redmapleleaf]

Attached is a little more information on the boat dimension.

[tspeer]

Attached is a little more information on the boat dimension.

I would guess your board needs to be much deeper.

Going upwind is all about lift/drag ratio, and the best way to improve that is to reduce drag. The drag due to lift on the hull & foils goes down by the depth squared, so a little deeper can be very significant. For example, if you make the board 40% deeper, you cut this source of drag in half. If you double the depth of the board, you cut this source of drag by 75%!

You need to add a batten to your sail to hold out the corner at the head. A couple of battens farther down on the leech wouldn't hurt, either.

Technically, the proa aficionados would classify your boat as a "tacking single outrigger".

[lazeyjack]

Hello Tom Speer,
Its nice to see you post again!!
Well at least we are in agreement here
I well remember, as lad of 13, sailing the P class, which all NZ,ers cut teeth on, and from that boat came many world champ dinghy and round world sailer, I remember the PLATE, which was what we called the c board, and its importance
You should get your weight well forwards too, sailing this boat and as Tom said, battens, one about 2 feet down from head of sail, with soft spot third way from luff and tied in tight,
Ah those wonderful days of youth

[redmapleleaf]

Hello Tom,

You have raised an interesting point about the lift/drag force versus the length of the centerboard. It seems that you have some kind of formula for calculating this number. I am very much interested in learning about these formula as it would guide me to build a better boat.

I know that the aerodynamic of sailing is a very complicate topic but I have to believe that there must be some simple 'rules of thumb' to help us common designers to build a reasonable boat. Here are some of the things that I thought it would be helpful to know:

1. give a sail area, windspeed, angle of attack, how much lift force can we expect. Just maximum and minimum.
2. given a centerboard area, shape, how much side force can it sustain? Let say this is a really thin board so that one don't have to worry about the foil shape performance.

Knowing 1 and 2 then I guess one can calculate the resulting force vector to determine if the boat will move forward or not.

BTW, I read this page on the physics of sailing (http://www.physclips.unsw.edu.au/jw/sailing.html) and they indicated that the resulting force on the sail is due to 'change in the wind velocity' and the Bernoulli effect is only secondary. However, I read some other sites and they only mention of Bernoulli effect. So what is the main sailing force here???

[the1much]

wasnt there a thread on "rules of thumb",,or something like that?,,,my memory is like my credit score,,,,,short,,hehe ;)
try some searching on rules of thumb,, or something close to that,,, i know these guys have posted volumes on the subject..

[charmc]

Redmapleleaf,

Congrats to you for actually building a boat and trying to make it perform better!

Someone told me once it helps to define the terms of a discussion. So I'll ask: what do you mean by sailing upwind? Most multihulls will not point up nearly as well as most monohulls. 50 deg off the wind is often considered good upwind performance.

That being said, there are some good suggestions here. Lengthening your centerboard and adding a jib should aid performance in a significant way.

[Fanie]

Doesn't that sail look flat ? Also, you lost a hull on that tri ;)

The deeper and wider centreboard should make a big difference... I get the same problems with the little tri due to side drift being way too much as it sits right on the water, so it points upwind but goes sideways. Repeat after me... side drift :D 500mm wide and 700mm deep. Make a daggerboard...

[the1much]

im betting that a 10hp outboard would fix the upwind problem hehe :D
alright you guys,,,we all know i used to build em,,,but never gave a crap "how" they worked,,,,,,so from a "sailing" point,,,im newer then new hehe,,,,,,,soooo,,,,the center board will keep you from drifting sideways?

[marshmat]

I'll take that as tongue-in-cheek, Jim... ;)

Redmapleleaf,
There's some good advice here from a few folks regarding a larger, deeper, higher-aspect-ratio board and modifications to the sail.
As to the aerodynamics of a sail, there are a number of ways to analyze this, all of which are beyond the scope of this thread. The UNSW article you found appears to be a reasonably accurate, yet still readable, summary. Marchaj's Aero-hydrodynamics of sailing is a frequently cited reference on the subject, although I have heard some people find its technical details a bit nasty. Any fluid dynamics textbook that covers potential flow theory would be a worthwhile read, though, if you want to understand this better. (For all practical purposes, we can understand a properly trimmed sail from potential flow theory without resorting to excessively nasty math; things do however start to get ugly when the flow separates from the low-pressure side of the sail.)

[tspeer]

...You have raised an interesting point about the lift/drag force versus the length of the centerboard. It seems that you have some kind of formula for calculating this number. I am very much interested in learning about these formula as it would guide me to build a better boat.

Estimating the drag of a boat is very much like bookkeeping - there are different categories into which one can put a given expense, and how one accounts for all of them depends on what is most convenient - what data you have or what methods you use to estimate each piece.

Physically, there are basically three reasons for a boat's drag. The first, and most visible, is the energy that is radiated away as waves on the water's surface. The second is due to the fact that the water that actually touches the boat's surface acts like it is glued to the surface and there's a shearing action between the boat's surface and the flow moving rapidly past the boat a little ways further out - this is known as the boundary layer. And finally, there is the drag that comes from deflecting the water sideways over a finite depth in order to create the side force that opposes the side force from the sail. These three physical phenomena interact with each other and it's often more convenient to bookkeep part of the influence of one phenomenon with the influence of another.

The most common bookkeeping system breaks things down like this:
- Lift is defined as the component of the total hydrodynamic force that is perpendicular to the direction of travel through the water.
- Drag is defined as the component of the total hydrodynamic force that is parallel to the direction of travel through the water. (But not necessarily parallel to the centerline of the boat.)

The drag is made up of:
- skin friction drag, which is due to shear and turbulence in the boundary layer. This drag pulls at the boat, parallel to the skin.
- wave drag, which is due to the radiation of energy at the water's surface. This drag pushes on the boat, as a pressure force perpendicular to the skin.
- form drag, which is a pressure drag like wave drag, but actually results from the way the boundary layer effectively changes the shape of the boat.
- induced drag, which is the drag due to sideways lift on the boat.
- excrescence drag, which comes from all the bits of things sticking out of the hull, such as prop shafts and struts, etc.
- trim drag, which is due to the deflection of the rudder
- spray drag, which comes from the energy spent splashing water around,
- and on and on and on.

It's quite common to lump everything but skin friction and induced drag into a big bucket and call it residuary drag, because the residual you have left after you take out skin friction and induced drag!

Because it can be confusing if everyone sets up their own bookkeeping system, especially if you run a test facility that has to deal with a lot of different projects,or have a project that uses a number of different facilities, the International Towing Tank Conference (http://ittc.sname.org/documents.htm) has set up some standards for drag accounting, including a list of symbols (http://ittc.sname.org/sybolslist2002.pdf).

The important thing is how these various sources of drag vary with speed, sail trim, etc. Except for induced drag, all these components of drag get bigger as you go faster. Since fluid mechanics is, "the science of the non-constant constant," the major influences are divided out to leave nondimensional constants that vary with the lesser influences on drag. Note that many of the choices, such as area, are really arbitrary. For example, for lift, you may choose to define the lift coefficient, CL, based on the planform area of the board, or the lateral plane area of the keel & hull, or the combined areas of the board and rudder, or just about any other area you want to name. The important thing is to be clear about your convention and stick with it. You'll get the same answer in the end.

Here's a simplified way of expressing the total lift and drag:

qbar = 0.5 * density * speed^2
Lift = CL * board planform area * qbar
Drag = Cf * wetted surface area * qbar + CDw * immersed volume^(2/3) * qbar + CDp_hull * immersed volume^(2/3) * qbar + Lift^2 / (pi * e * depth^2 * qbar) + CDp_board * board planform area * qbar

qbar is the dynamic pressure - it's what you feel when you drag your hand in the water.
CL is the lift coefficient - it's a function of the board shape and the leeway angle
Cf is the skin friction coefficient - it's common practice to assume that the skin friction is the same as a flat plate with the same wetted area as the hull, and account for the fact this ain't true by lumping the difference into the form drag.
CDw is the wave drag coefficient. It starts off small, then grows in a series of humps and hollows as the speed increases.
CDp_hull is the form (pressure) drag due to the boundary layer and all the other stuff you don't know quite what to deal with, but whose drag varies as speed^2
e is the Oswald efficiency factor. It is a function of the board's planform shape. That whole term with Lift^2 in it is the induced drag.
CDp_board is the profile drag of the board - it's what you have when you slice the board through the water without any sideways lift.

The hard part is getting good values for all these coefficients. That's what grand prix design teams spend thousands and thousands of dollars on tank tests and wind tunnel tests and CFD calculations to find out. Mere mortals like us have to use sources like Hoerner's Fluid Dynamic Drag (an out of print book much prized by every fluid dynamicist - search for it on the web).

As a designer and builder, what you want to know is how to improve the performance of your boat, and you can see the trends in the terms of the equation without knowing the values. You can reduce skin friction by making the surface smoother (smaller Cf) and reducing wetted area (rounded sections instead of boxy sections). You can reduce wave drag by using slender hulls (smaller CDw) and keeping the boat light (reduced volume) - this is the whole point of multihulls. You reduce CDp_hull by making the boat fair, giving the buttocks a nice gradual run, keeping the transom out of the water, etc.

Lift is determined by the sail trim, not the design of the board. As you sheet in the sail, it drags the boat sideways and the leeway angle increases (increasing CL) until the lift on the board equals the load being applied by the sails. The bigger the sail, the harder it's sheeted, the more lift has to be supplied by the board.

But notice that the drag due to lift goes down with the square of the depth of the board. This is fundamental. There ain't no such thing as a truly efficient shoal draft keel. You can make up for it a little bit with the efficiency, e, which is what keel wings do. But there's no getting around the importance of depth. (And wings may work as much by their increasing the depth when heeled as anything else.)

Also note that qbar is in the denominator, so induced drag actually decreases with speed^2 if lift is held constant! (Which is pretty much the case when the sail trim is limited by the boat's heeling stability.) This is why you can sort of get away with using a strake or narrow hull instead of a board if you're fast. But such a boat still won't go upwind anything like it would if it had a nice, deep board.


I know that the aerodynamic of sailing is a very complicate topic but I have to believe that there must be some simple 'rules of thumb' to help us common designers to build a reasonable boat. Here are some of the things that I thought it would be helpful to know:

1. give a sail area, windspeed, angle of attack, how much lift force can we expect. Just maximum and minimum.

2. given a centerboard area, shape, how much side force can it sustain? Let say this is a really thin board so that one don't have to worry about the foil shape performance.

Knowing 1 and 2 then I guess one can calculate the resulting force vector to determine if the boat will move forward or not.


Yes, you can. That's called a velocity prediction program (VPP). Here are some fundamental sailing performance relationships at the heart of any VPP.

- Let gamma be defined as the angle between the true wind direction and the yacht's course through the water
- Let beta be defined as the angle between the apparent wind direction and the yacht's course through the water
- Vb is the speed through the water, Va is the apparent wind speed, and Vt is the true wind speed
- drag_hydro is the drag (force parallel to the direction of travel) of everything that's in the water
- lift_hydro is the total hydrodynamic force perpendicular to the direction of travel
- drag_aero is the drag (force parallel to the apparent wind) of everything that's in the air, including sail, rigging, hull topsides, crew, cooler sitting on the deck
- lift_aero is the total aerodynamic lift perpendicular to the apparent wind direction

Vb = Vt * sin(gamma - beta) / sin(beta)

Va = Vt * sin(gamma) / sin(beta)

beta = arctan(drag_hydro / lift_hydro) + arctan(drag_aero / lift_aero)

These are all interrelated, because as your speed changes, the lift and drag change, so the apparent wind angle (beta) changes, and the apparent wind speed changes, and the speed changes again. So you have to iterate to find a speed that makes everything come into balance. You also have to iterate on the sail trim to meet the constraints of heeling moment, etc.

The details of making a reliable estimate for a given design are a bit tricky, but it can be done with a spreadsheet. Like I said, it's all bookkeeping. Some references you'll find very useful in creating a VPP are
Larsson & Eliasson, "Principles of Yacht Design"
Abbott & VonDoenoff, "Theory of Wing Sections"
Hoerner, "Fluid Dynamic Drag"
Hoerner, "Fluid Dynamic Lift"
Lazauskas, the Michlet hull drag code (http://www.cyberiad.net/michlet.htm)
Drela, the XFOIL (http://web.mit.edu/drela/Public/web/xfoil/) airfoil design and analysis code
Drela, the AVL (http://web.mit.edu/drela/Public/web/avl/) vortex lattice code

BTW, I read this page on the physics of sailing (http://www.physclips.unsw.edu.au/jw/sailing.html) and they indicated that the resulting force on the sail is due to 'change in the wind velocity' and the Bernoulli effect is only secondary. However, I read some other sites and they only mention of Bernoulli effect. So what is the main sailing force here???

All of fluid mechanics depends on three conservation laws: conservation of mass, conservation of momentum, and conservation of energy.

Conservation of mass means the whole flowfield forms one unified picture. If you push the wind aside with the sail, other air flows in behind the sail to make up for it. This is fundamentally the source of induced drag.

The conservation of momentum says that the forces on the boat are equal to the change in momentum of the fluids flowing around the boat. When the sail pushes the wind aside, that change in direction is a change in momentum, and lift is the reaction force from that change. The shear in the boundary layer robs the fluid of its linear momentum by spinning it up into little eddies. Skin friction is the resultant force.

Bernoulli's Law is a restricted form of the conservation of energy, which assumes the temperature is constant and there's no heat added or subtracted, nor work done on the fluid. Outside of the boundary layers and the wakes left by the sail & hull, the total energy of the flow is basically constant. There's kinetic energy (dynamic pressure, qbar) and potential energy (ambient pressure). These two forms of energy can be exchanged, while keeping the total energy constant (total pressure = ambient pressure + dynamic pressure).

It turns out that although the velocity varies inside the boundary layer, the ambient pressure outside the boundary layer is transmitted across the boundary layer to the surface unchanged. So if you know the total energy (total pressure), and you measure the pressure at the surface (easy to do), you can subtract the local pressure from the total pressure to get the dynamic pressure, and thus the velocity just outside the boundary layer. Bernoulli's Law doesn't apply inside the boundary layer or in the wake, because there is a loss in total pressure in these areas - the assumptions behind Bernoulli's Law are violated. But the fact that the pressure is transmitted across the boundary layer allows you to ignore it, up to a point.

Now all the conservation laws have to be consistent with each other. If you add up the pressures acting on each bit of area of the sail, that has to equal the total force at right angles to the sail. You're basically using the energy equation (pressure-velocity relationship) to figure out the result of the momentum equation (force due to bending the flow). I'll let you decide which is the chicken and which is the egg.

[redmapleleaf]

Man, if I could understand everything that Tom just wrote then someone ought to give me a PhD of some sorts :D.

I'll take next week off and see if I can try to understand it. I think this is the kind of information that is essential to boat building although I did hope that it could be a little bit less complicated (sigh). I thought that there are pure physicists and then there are engineers who could simplify things and put all these complex theories to practical usages. If there are I would definitely want to hear from these engineers.

[lewisboats]

i have a 13 ft flat bottomed canoe that I converted with outriggers. I use about a leeboard that is about 2.5 ft long (in the water)and 9" wide, 3/4" thick and I have had no problems with excessive side slip. Make your board longer and narrower and grind a bit of aerofoil shape into it and you should see improvement.

Steve

[powerabout]

Can you post another drawing showing the sail along with the centre board.
Maybe your centre of effort is wrong?
As you go from downwind to upwind what happens to the tiller load, weather or lee helm?
This will help you sort out the mast rake and or maybe you will need to move the mast.

[tspeer]

...Most multihulls will not point up nearly as well as most monohulls. 50 deg off the wind is often considered good upwind performance....

Excuse me! I regularly come up under the lee of well-sailed monohulls, sailing higher and faster than them!

This may have been true of the first generation of multihulls - the Pivers and such - but it's not true of modern multihulls. Less windage in the hulls, taller rigs, and most especially deep daggerboards instead of trying to get by without boards makes all the difference

[sharpii2]

Are you sitting near the stern of the boat?

If you are, that could explain the whole situation.

A narrow hulled boat, such as as yours, will squat significantly, if you sit too far aft.

The extra hull surface below the waterline, most of which is aft in this scenario, counts as lateral area. You may have unwittingly moved Center of Lateral Area aft.

If you are sitting near the center length of your boat, please disregard this post.

If you are sitting pretty fare aft, say about two thirds of the distance from the bow post to the stern post or more, the first thing you can try is too move forward, so your boat trims better fore and aft.

That alone could make a huge difference.

After that, try making the dagger board about half a foot (15.5 cm) deeper.

[Guillermo]

Estimating the drag.......
Tom,
My hat is off to you!
I tried to give you more rep points, but still not possible.

Cheers!

[redmapleleaf]

Sharpii2: when I sail alone I would be sitting at the stern of the boat controlling the tiller and the line. When I sail with another person then that person would sit closer to the centerboard of the boat. When I go out alone by myself again I will add a tiller extension so that I could stay near the middle of the boat like you suggested and see what change it would make.

Steve: I plan to make a longer centerboard (something like 3'x1'x3/4") the next time I go out. I will also add a jib to see if it will help.

Powerabout: I will try to get another picture of the sail and centerboard the next time I go sailing. I think it is possible that my sail's center of gravity is not well aft of the centerboard. I'll have another chance to go to gome and fix up the boat on July 26, I'll get some pictures of the boat then.

[Fanie]

Hi Redmapleleaf,

I actually spoke with my friend who has a small tri this morning specifically about the difference the addition of a jib to his rig made.

Apart from being faster, he said the centre hull buries less than without the jib, so there's a bit of lift there. His jib foot is wider than the gap to the mast, so it forms around his main sail a bit, and in my opinion enhances it very nicely. Without the jib the most speed he ever got was 16km/hr (gps)... with the jib I myself sailed it at 22km/hr (wet bike speedo) but it goes under water then... I guess I'm too fat :D

[lewisboats]

take it one step at a time...go with one or the other first before adding the second. You could get into overkill and go the opposite way.

Steve

[redmapleleaf]

Hi all, I have made the changes to the boat and now it is sailing quite nicely. Here is what I did
1. made a new centerboard that is 3 times bigger than before
2. made a new rudder that is 2 times bigger than before
3. add a jib

I took it out on a fairly windy day 12+mph and it sailed very well. I had to tie some heavy lumbers to the outrigger to prevent it from heeling and tipping over. Despite the heavy logs it still move well into the wind.

You can see some of the pictures of the boat at http://picasaweb.google.com/redmapleleaf/OutriggerSailboat

Thank you for all your help.

[Fanie]

I told you you have a hull missing. It adds nice space if you put a trampoline on, no more logs and lumber passengers and will be faster too.

Well done, glad it's coming together.

[the1much]

well THANK-YOU for updating us,,and for the pics,,and glad to see things are working better,,,,,just wait till your next 1 ;)

[Boatpride]

The Bernoulli effect drives your boat through translation of wind pressure

changes on both sides of your sail. Without going any further in to physics

on this, i would suggest that the sail generates too much potential power for

the boat at the higher wind speeds. So as the guys have suggested, you

need more resistance to the lateral effects of the higher wind. IE redesign

your keel and make it deeper offering more resistance so you can sail in

higher winds.

I have finished a page on the topic of how sails work. You can read here if

you like. It goes in to the Bernoulli effect and explains the effect of the wind

on sails. http://boatpride.com/how-sails-work.html

Best Wishes

[marshmat]

Hi Chris,

May I recommend you take a look through any textbook on aerodynamics, or anything at all by C.A. Marchaj.

The flow over a sail, or any foil for that matter, is much more complicated than can be explained by the Bernoulli effect ( P0 = p +1/2 ρ V2 + ρ g Z ) alone. There is much more to aerodynamics than "flow speeds up, therefore pressure drops". To estimate the actual lift and drag on a foil requires, at a minimum, the potential flow field and net circulation of the flow (look up the Kutta-Joukowski theorem as a starting point). Net forces are the result of integrating the pressure distribution over the entire foil.

Computing the amount of lift an aircraft wing, sailboat keel or other foil gets from the Bernoulli effect due to velocity differential between the high- and low-pressure faces, compared to the lift calculated from the net circulation of the flow, is a standard exam question in third-year fluid mechanics. The answer, by the way, is typically on the order of 2 to 3 percent of the total lift.

[markdrela]

Computing the amount of lift an aircraft wing, sailboat keel or other foil gets from the Bernoulli effect due to velocity differential between the high- and low-pressure faces, compared to the lift calculated from the net circulation of the flow, is a standard exam question in third-year fluid mechanics. The answer, by the way, is typically on the order of 2 to 3 percent of the total lift. That's not correct. You can calculate the lift by integrating the surface pressure obtained from the surface velocity via the Bernoulli equation, OR you can calculate the lift from the circulation, OR you can calculate the lift from the Kelvin impulse in the trailing vortex wake.

These are alternative lift-calculation methods --- they are not additive. Each one gives 100% of the lift all by itself.

[ancient kayaker]

Nice thread with a nice conclusion; the boat goes faster and better now. You've done a lot to improve the situation, but there's more to be had. A sailing canoe can be one of the fastest boats of its size around.

I had personal experience with a canoe rigged for sailing similar to your one, but a bit smaller sail. I sailed it sitting on the gunnel to keep the ama down and most of the time it plodded along in a pedestrian manner as an assymmetrical catamaran. Then one day things went very quiet; assuming I had slowed down for some reason I looked over the side, the water was ripping by and the ama was a foot above the waves. I'm embarassed to admit that I panicked, shifted my weight to bring the ama down and immediately lost half the speed. At that point I realised where I had been going wrong.

From your photos I see the ama is pressed into the water. With its displacement hull it would have a low hull speed because it is short so you get extra drag. A skimming style ama would plane at speed with less drag. I preferred two floats mounted high which are for insurance only, body weight is used to balance the boat so both are out of the water. Get it up man; live fast and dangerously! If you start to encounter a wet underwear condition you can always let out the sheet.

I would have thought that 50 sq ft was plenty of sail: most canoes seem to be over-canvassed; I think this is inherited from purpose-built sailing canoes of the early 1900's which were usually rich man's toys, lead ballasted with bronze centerboards and no ama. If you need more area I'm not sure I would add a jib; the classic rig from a canoe seems to be ketch or yawl, which is very easy to control and balance. Once balanced the weather helm vanishes, the rudder quits grumbling and the speed increases. But I haven't tried it. Yet.

[marshmat]

Mark,

You are correct that integrating the surface pressure distribution gives the total lift and drag. How you obtain this surface pressure distribution is up to you.

The description on the website Chris references is inaccurate in describing the difference in pressure as being due purely to the flow over the convex side having to go faster than the flow over the concave side. It makes no attempt to explain why this is true and tends to reinforce the pervasive, incorrect definitions of lift we all recall from grade school. This is why I suggested he research it more thoroughly.

When you take the difference in velocity between the upper and lower faces of a wing, assuming the (incorrect) equal-transit-time approach often taught in grade school, and calculate the 1/2 ρ V^2 term for each, you find that the lift due to this effect is a tiny fraction of the total. And when you go to measure the flow on an actual wing, you find that the actual velocities are nothing like that- the flow over the convex side is much, much faster than equal transit times would suggest. Applying Bernoulli's equation to the actual velocity distribution does work and does yield a reasonable approximation to the actual lift.

When you do the same for a sail, you find that a sail has negligible thickness- the convex and concave sides have the same path length. The Bernoulli effect doesn't contribute, in the sense of the (incorrect) "equal transit time" concept. But the Bernoulli equation is still valid, if you look at the true velocity profile of the flow.

Net circulation is not the only method of calculating lift. As Mark points out, the trailing vortex can also be used, among other methods. I reference the net circulation method frequently because, in my experience, it is the one that is misinterpreted least often.

Bernoulli's equation works. You just need to be careful how you apply it. The "Bernoulli effect" means that a flow speeds up, therefore the pressure decreases. Attempting to explain lift in this manner without also explaining the origins of the true velocity distribution (not the equal-transit-time thing) is at best incomplete and at worst tends to perpetuate misunderstanding.

Does this make more sense? I apologize if any of you had trouble following my logic in the earlier post.

[Boatpride]

Hi Marshmat,

yes i will read up on those books by C.A. Marchaj. I am aware that an eighteenth century concept is a bit leaky in this day and age! 2 or 3% is very low - wow!