# really basic question about foils

Discussion in 'Hydrodynamics and Aerodynamics' started by Anatol, Aug 15, 2015.

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### AnatolSenior Member

Foils (enterboards) are usually described as providing 'lift'. I see this in the case of the airplane wing, or the sail, where the sides have different shapes.
I do not see it in the case of a centerboard with a symmetrical foil. Clearly 'lift' is different from minimising leeway. On a tacking boat, why is an aerodynamic profile better than a thin flat blade? For a non tacking craft, ie shunting proa, can one capitalise on lift in foil design because the lee side of the foil is always the leeside? This rehearses the asymmetrical hull conversation, I know. And folks have moved away from the asymmetrical hull designs, I know. But is there an argument for a bidirectional lifting foil?
thx
Anatol

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### philSweetSenior Member

The shape (camber) of an airplane's foil is just a fiddle to manage some minor drag issues. A symmetrical foil can have the same performance wrt lift as a cambered foil. Acrobatic aircraft often still use symmetric foils. The cambered foil will just act at a different angle of attack. The angle of zero lift is different, but the behavior relative to this zero lift angle is about the same. (A cambered foil may have some other slightly different properties, like a different center of effort, and a different maximum lift before it stalls, and a different laminar to turbulent transition location)

The aerodynamic shape is better because the flow stays attached to the shape, adding very little to the pressure drag over a wide range of lift conditions. There are three main causes of drag on a keel - sheer on the surface (classic friction), induced drag due to finite span (this has the effect of rotating the lift force vector such that some of the lift becomes drag), and the momentum loss in the fluid caused by defects in flow attachment. The last one is primarily what is being addressed by streamlining. Many small dingy classes use flat plate boards. Properly detailing the leading and trailing edges of these can achieve most of the drag reduction of a proper foil section 3 times as thick.

Foils do, however, want to have a front and a back. Bi-directional foils work poorly. Theoretically - they don't work at all with rounded edges! The trailing edge needs to be sharp. The leading edge should be round.

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### DrawnOnwardJunior Member

Like Phil said, symmetrical foils develop lift with angle of attack. In addition, foil shape will typically target low drag in the neutral position and high Cl/Cd in attack. Some aircraft have symmetrical foils. The question remains re what the neutral position is. Upwind leeway provides attack for dagger foils and; weather helm: rudder attack. Offwind the attack angles are reduced if not zero.

Heel affects rudder attack angle. The Atlantic proa is a different beast to the weight to the windward proa. As weight shifts from balanced between hulls to the lee, the helm shifts relatively to the lee. The ww proa has a tendency to weather helm in light conditions. The Atlantic needs to be mindful of balance as heel changes.

Symmetrical rudders tend to have a low drag bucket at zero alpha. Lifting foils typically have lowest drag at a higher alpha and lift angle. Refer to the two attachments. Tom Speer's H105 lifting foil is an exception. It has low drag at zero lift. Apologies to Tom though. These graphs are drawn at 4M Reynold's. The H105 was developed for lower RE.

The big problem for a bi-directional foil is having a sufficiently blunt leading edge for wide angles of attack and Cpmin without having too blunt a trailing edge for vortex formation and high drag.

With all this in mind, you need to consider the in balance and turning attack angles the rudder will face in a variety of conditions. See the following link for a discussion re the desirability of weather helm: http://johnellsworth.com/writing/nautical/balance_helm/balance.html

I'm not so sure of this. I discussed this issue in some detail in this post: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/foil-performance-discussion-53742-2.html#post746105 So I wont go into much detail here. Theory hints at it working. At least Xfoil XFLR5 does. There is some limited empirical evidence for it doing so in the post as well. The U rudder and Y41 are bi-directional foils.

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### AnatolSenior Member

balancing a proa

Drawn onward said

" The Atlantic proa is a different beast to the weight to the windward proa. As weight shifts from balanced between hulls to the lee, the helm shifts relatively to the lee. The ww proa has a tendency to weather helm in light conditions. The Atlantic needs to be mindful of balance as heel changes."

This raises an issue I'm concerned about. - balancing the helm in an Atlantic Proa. Having had firsthand experience with a Pacific proa with appalling weather helm, I see the problem.

Its not clear to me how to predict the effects of different foils etc on helm. Coming up with a sailplan that puts the CE slightly aft of amidships seems desirable, hence the "standard" double ended Bermudan rig. I'd like to avoid the jib solution if I can, with cat-schooner rig - for ease of shunting. But not opposed to easy up/easy down pocket handkerchief jibs way out on the bows if absolutely necessary.

I am quite unclear about the (helm) effects of the leeside ama, under different conditions and point of sailing. What are the general rules?

"As weight shifts from balanced between hulls to the lee, the helm shifts relatively to the lee"

Does this mean you want to design in significant weather helm in light winds in order to balance the helm in heavier winds? Or are there ways of managing the balance at different speeds? Is light weather helm desirable on a Proa, or is this another case where the normal sailing rules simply do not apply?

What role does size and position of foils play in helm balance - in an Atlantic? As noted, I'm considering putting foils in the ama - is this a no no?

One criticism of Atlantics I've heard is that they'll 'round up' under bare poles in a storm. Any opinions? or better - experience?

thanks!

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### AnatolSenior Member

Rotating Foil in the Ama (Atlantic)

The bidirectional foil seems non-optimal.

I've been mulling this matter and a breakthrough came when I made the extrapolation from the adage that an atlantic is a trimaran minus windward ama - so putting the foil in the Ama is ok.

In my design, the (4) Akas are in a W shape, with two points on the Vaka. This means I have a very strong center point in the Ama, so I'm considering a single rotating foil hung from windward side of ama between central crossbeams (not unlike some of Rob Denney's designs).

This would flip automatically on shunt, but may need cinching with a line and camcleat either side.

This presents the interesting possibility of being able trimming the foil a few degrees for lift.

It would need a tilt capacity (with a dowel fuse) for collision safety.

I want to experiment with non-radical foiling. I like the idea of an ama-less Atlantic, but this is an easy/inexpensive build fast cruiser, not Sailrocket. The old school Bruce foil seems like it will lift the Ama slightly without being technologically over-the-top. What angle?

I'm pretty persuaded by tip-vortex reducing end-caps and want to exploit them where-ever possible. Since this foil is open at the top (no hull above) I'll put endcaps on both ends. How big? What shape?

I'd plan on a stainless steel tube rudder post (2") with large welded-on tangs, mounted to a ply center plane, with foam sides and glass over.

The end caps also mean I can use a non-elliptical bottom on the foil - a dead straight 'extrusion' should make it (much!) easier to build (one profile all the way). But what aspect ratio?

Foil balance - no skeg (obviously), so where should the shaft be relative to chord - commonsense would say at its widest point. But it has to flip and align without assistance on shunt - this would suggest moving shaft fwd. As close as possible to leading edge in this application?

Any suggestions on any of this - successful and failed examples - etc gratefully received !

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### AnatolSenior Member

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### tspeerSenior Member

There's no difference. The leeway angle will adjust itself until the side force from the board equals the side force from the sail rig.

For each foil there will be an angle of attack at which the lift is zero. For a symmetrical board, that occurs when the flow is aligned with the chord of the board. For asymmetrical sections, it occurs when the flow is aligned at a negative angle of attack relative to the chord (typically on the order of 2 - 4 degrees). If you measure leeway from the zero lift orientation of the board, the two work the same. Their response to a change in angle of attack is the same.

The difference between a symmetrical foil and an asymmetrical foil is the symmetrical foil has its minimum drag near zero lift, while the asymmetrical foil has its minimum drag at nonzero lift. This makes it easier to ensure that the operating lift occurs in the low-drag zone of the foil. Asymmetrical sections can also have a higher maximum lift before they stall than symmetrical sections.

If your operating range is biased in one direction, then you can tune an asymmetrical section so its low-drag zone encompasses the operating range.

For a proa, you need to decide if the board is going to have fore-aft symmetry or right-left symmetry. A board with fore-aft symmetry can be cambered to match the fact that the side force is always to the same side in a proa. But the ideal shape for a leading edge is quite different from the ideal shape for a trailing edge, so you have to make some compromises. It's probably better to give the foil sharp edges than rounded edges.

A board with right-left symmetry (conventional symmetric section) will need to be flipped around for each tack. The rounded leading edge will not be as susceptible to leading edge separation and ventilation as will be a sharp leading edge.

One way to have your cake and eat it too is to have two boards, and only use one at a time on any given tack. You could put both of them down while shunting, thus doubling your lateral area when at low speed, and then pull up the reversed board once you get going. Each board can then be optimized for its particular tack. This can also help solve the directional balance problem, because you can get the right lead for both tacks.

With regard to asymmetrical hulls, one misconception is an asymmetrical hull will provide "more" lift. But that's not true because the hydrodynamic side force always matches the aerodynamic side force. If you produce side force from the hull, then less side force will be produced by the board.

Another misconception is that an asymmetrical hull can provide the side force with less drag than a board. That's a false economy for two reasons. The first is an asymmetrical hull (ala Hobie 16) has more wetted area than a rounded symmetrical hull of the same length, and that eats into whatever wetted area reduction you make by eliminating the board.

The second reason is the lift-induced drag is inversely proportional to the depth. So when you create the lift using a shallow asymmetrical hull, there's a tremendous drag associated with that. It's well worth the extra wetted area to use a board to extend the span and get a huge reduction in the drag due to lift.

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### AnatolSenior Member

Tom
thankyou sincerely for taking the time to explain these details.

I see the advantage of the two board solution at low speed in shunt. But this is offset by the doubling of cassettes and/or flip mechanisms. One attraction of the single pivoting board is it will (ideally) swing as a result of boat change of direction and only need release and cinching. Of course, you do loose foil effect while its swinging. How much of an issue this would be will depend on lateral resistance of hull, etc. Any guesses?

So, as general rule, hull should approximate semicircular cross section, foils should be high aspect, and one should not do the job of the other.

On the subject of wetted surface - in the case of an atlantic Ama you want it as long as the Vaka, and you want maximum reserve bouyancy in the bows. So even with a semicircular cross section, you've got lots of wetted surface. This seems unavoidable.

Clearly, wetted surface is only part of the issue. If the sides are smooth and parallel to the direction of motion, resistance will be minimised. Hence rowing sculls and dragon boats. So why do I hear so much about wetted surface and less about fair curves. I could make an Ama out of linked spheres : minimum surface area, but no doubt much drag.

Maybe I should make a tripod proa with the 'Ama', divided in two, missing the mid section? It puts bouyancy where it is needed, but loses wetted surface amidships.

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### tspeerSenior Member

It's been done.

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### AnatolSenior Member

Damn clever Australians !

So if one was to break the Ama into two, this is like the bidirectional foil problem. What would be the optimum shape for a pair of bidirectional amas?
I'm thinking about 10' long each (tho 8 would be convenient , 2' draft. With enough bouyancy. ...?

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### tspeerSenior Member

I suspect the optimum shape is not to break it in two in the first place. it's hard to beat the low drag of a long, slender displacement hull.

You can use Michlet to calculate the drag of your candidates. At least under conditions of zero side force.

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### AnatolSenior Member

that makes sense. so what is the point of the Aussie tripod design?

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### tspeerSenior Member

They were one-way boats designed to set the world speed record. In shallow water next to a flat beach. The hulls were designed for planing only, because if the boat was traveling at displacement speeds it wasn't setting any records.

Probably not a design to emulate, unless you're planning on going >50 kt.

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### AnatolSenior Member

oddly enough, my uncle lives right there.

gotcha

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