Trifoiler-style foils on IMOCAs?

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Robert Biegler
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Trifoiler-style foils on IMOCAs?

Postby Robert Biegler » Mon Sep 10, 2018 7:44 pm

Is there anything to prevent IMOCAs from using Trifoiler-style foils on either side of the hull, perhaps attached to the outriggers that provide a broad base for the standing rigging? One constraint is that there should be only one degree of freedom to how the foils move, but that condition would be met. Or would such foils be inferior to the Dali foils for any reason?

John Perry
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Re: Trifoiler-style foils on IMOCAs?

Postby John Perry » Tue Nov 06, 2018 2:56 pm

By Trifoiler style foils do you mean fully immersed inverted Tee foils that have flaps linked (as with the Moth main foil) to a 'wand' that senses height above water surface? If that is what you mean, then I am not sure it would be allowed under the current IMOCA class rules (2018).

Rule C.7.1(e) says: Trim tabs as well as deformable surfaces are prohibited for hull appendages, except for fixed keels, which can have a trim tab

Would the flap on a trifoiler foil be a trim tab? - if so it would not be allowed. And the alternative option of making the whole Tee foil rotate to change foil angle of attack would probably fall foul of rule C.7.1(h)

Would a foil control wand be an 'appendage'? The rules allow no more than 5 hull appendages so a trifoiler stye arrangement would use up 4 of those, then you do need a rudder or two and a keel.

The rules require foils to be retractable, an extra complication with Trifoiler style foils.

But the above is based on only a quick look at the rules, I have not studied them in detail.

As for wether Trifoiler style foils would be better than the current crop of shallow Vee foils I would have thoght that they would be more complicated and more prone to damage. If you look at the Utube videos of these boats sailing in heavy weather you can imagine that is a big factor to consider. Have foil control wands ever been used under severe ocean conditions? But possibly they could be more efficient in terms of L/D since the lifting surface could be more deeply immersed. The foils on IMOCA60 and also on trimaran floats seem to be operating so close to the surface that I suspect that much of the lift for much of the time comes from the underside (HP side) of the foil, which limits efficiency.

Robert Biegler
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Re: Trifoiler-style foils on IMOCAs?

Postby Robert Biegler » Thu Nov 08, 2018 9:26 pm

John Perry wrote:By Trifoiler style foils do you mean fully immersed inverted Tee foils that have flaps linked (as with the Moth main foil)

No. The Trifoiler uses L-foils linked to sensor surfaces well ahead of the foil. See https://www.youtube.com/watch?v=zXSgZCDVWOM

John Perry wrote:The rules require foils to be retractable, an extra complication with Trifoiler style foils.

Could be done by rotating around the same horizontal(ish) axis as used for adjusting angle of attack.

John Perry wrote:Would a foil control wand be an 'appendage'? The rules allow no more than 5 hull appendages so a trifoiler stye arrangement would use up 4 of those, then you do need a rudder or two and a keel.

The control surfaces could be rigidly connected to the foils. I would count that as one appendage. The IMOCA class association may count it differently.

John Perry wrote:As for wether Trifoiler style foils would be better than the current crop of shallow Vee foils I would have thoght that they would be more complicated and more prone to damage.

For Moth-style foils, I don't know why there would be a difference. The lever arm to the farthest point looks pretty similar to the Dali foils in use now. The trifoiler style foils would be able to give a bit by rotating aft, and so may be less vulnerable, unless the sudden pull down is a problem.

John Perry
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Re: Trifoiler-style foils on IMOCAs?

Postby John Perry » Sun Nov 18, 2018 4:38 pm

When you mentioned the Trifoiler I somehow had the idea that you were talking about the Rave Windrider trimaran which I think was the same vintage, not the Hobie product that I think was a production version of the Longshot record holder - sorry about that.

Thinking about this again, I think there is a basic problem in applying Trifoiler type foils, or wand controlled foils, or indeed any surface tracking foils to IMOCA 60s. The problem is that the IMOCA boats are currently sailed at large heel angles, so a Trifoiler type foil mounted from the side of the hull would frequently be forced deep below the surface hence the foil angle would mostly be at maximum incidence, making the incidence control system largely redundant. Only in light airs would the heel angle and rig heeling moment be small enough that the foil might operate in a surface tracking mode rather than being forced deep below the surface. Of course, the above would not apply if the hydrofoil righting moment could be large enough to control the heel of the boat, essentially maintaining a constant heel angle regardless of sailing conditions. In the case of the new AC75 class, the intention is that the hydrofoil system will operate in this way since it will be the primary means of providing righting moment, but I would question whether this could be feasible within the restrictions of the IMOCA rule, particularly the maximum beam restriction.

Robert Biegler
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Re: Trifoiler-style foils on IMOCAs?

Postby Robert Biegler » Tue Dec 04, 2018 6:39 pm

John Perry wrote:Thinking about this again, I think there is a basic problem in applying Trifoiler type foils, or wand controlled foils, or indeed any surface tracking foils to IMOCA 60s. The problem is that the IMOCA boats are currently sailed at large heel angles, so a Trifoiler type foil mounted from the side of the hull would frequently be forced deep below the surface hence the foil angle would mostly be at maximum incidence, making the incidence control system largely redundant.

I have thought about this for a while, and i can't decide. You can adjust the slope of the function relating the angle of attack of the foil to the heel angle through both the length of the lever arm to the forward control surface, and through how far from the centreline that control surface meets the water. So it should be possible to get any desired angle of attack at maximum heel.
The thing I can't work out is whether it's worth doing. I am not convinced that a constant angle of attack, perhaps at optiumum L/D of the foil, is best, because that neglects that at low speed, hulls can carry weight with less drag, and at high speed, a slightly higher angle of attack might still pay off by lifting the hull further out of the water, and by providing more righting moment.

John Perry wrote:Only in light airs would the heel angle and rig heeling moment be small enough that the foil might operate in a surface tracking mode rather than being forced deep below the surface. Of course, the above would not apply if the hydrofoil righting moment could be large enough to control the heel of the boat, essentially maintaining a constant heel angle regardless of sailing conditions.

I suspect the boats already need to limit their angle of heel such that the outrigger that holds the shrouds doesn't hit the water, at least not often. If they can do that with foils that change their lift only be immersion, it should work better if the foil can also change angle of attack.

John Perry wrote:In the case of the new AC75 class, the intention is that the hydrofoil system will operate in this way since it will be the primary means of providing righting moment, but I would question whether this could be feasible within the restrictions of the IMOCA rule, particularly the maximum beam restriction.

The new IMOCA Charal supposedly carries up to 80% of her weight on her foil. (I don't know either the original source or the method by which that estimate was obtained. Therefore I have no idea how reliable it is.) If true, that should give plenty of scope for increasing the lever arm of the boats weight by moving lift to lee by 2 metres or so.

I am not persuaded that heel control is worse with the self-adjusting than the fixed foil. Whether self-adjusting foils would be faster is a different question. One practical problem could be that foils shallow enough to lift out the weather foil are not efficient, but if the foils are deeper, then the weather foil stays in the water. With only one degree of freedom, it could be lifted out of the water only when the boat is nearly stopped.

John Perry
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Re: Trifoiler-style foils on IMOCAs?

Postby John Perry » Sat Dec 15, 2018 1:00 pm

I am not convinced that a constant angle of attack, perhaps at optiumum L/D of the foil, is best, because that neglects that at low speed, hulls can carry weight with less drag, and at high speed, a slightly higher angle of attack might still pay off by lifting the hull further out of the water, and by providing more righting moment.


Robert - the way I would look at is as follows:
Start by assuming that lift from the foil contributes to righting moment only as much as does lift from the heeled displacement hull. (I realise that this is not the case for the IMOCA60) On that basis it becomes simply a matter of how best to support the weight of the boat, you can use solely the hull or you can have some lift from the hull and some from the foil, but you can't have 100% lift from the foil unless you can think of some way to stabilise the boat when the hull is clear of the water. At low boat speed, lift from the foil incurs a bigger drag penalty than lift from displacement, i.e. the lift to drag ratio of the hull is higher than that of the foil. (at really low speeds it is ever so much higher!) So, at those low speeds you simply want to get the foil out of the water if it is retractable (as IMOCA foils are required to be under the class rules) or if it is not retractable you want to adjust the foil to the angle of incidence that gives lowest drag regardless of lift. This would be zero degrees for a symetrical foil section, but not exactly zero degrees for asymetric foil sections. Then as boat speed increases, the drag of the hull rises so the lift to drag ratio of the hull reduces and there comes a speed at which the best possible lift to drag ratio of the foil becomes equal to the lift to drag ratio of the hull. That is the speed at which the foil should be deployed, i.e. lowered into the water, and at that speed its angle of incidence should be adjusted for best lift to drag ratio. Taking the H105 foil section as an example, this being a section that Tom Speer has suggested as suitable for sailing hydrofoils, I think the maximum lift to drag ratio occurs at about 4 degrees incidence. Then, as boat speed continues to increase and hence the lift to drag ratio of the hull continues to reduce, the foil angle of incidence should be increased beyond the angle that gives the best lift to drag ratio provided that (a) this further increase in incidence does not make the lift to drag ratio of the foil less than that of the hull and (b) that this further increase in incidence does actually reduce the foil lift coefficient - there comes a point when the foil is stalled (or maybe it ventilates or cavitates) and increasing angle of incidence actually reduces the lift coefficient. So, to put it another way, once it becomes worthwhile to use the foil at all, the foil angle of incidence should be continuously adjusted to that the lift to drag ratio of the foil tracks that of the hull up to the point at which further increase in foil incidence would reduce rather than increase foil lift coefficient, at speeds beyond that the foil incidence should remain constant.

All the above is on the basis that the foil lift is no more effective in providing righting moment than is the bouyancy force of the heeled hull. For an IMOCA 60 with the foil extended sideways from the hull, foil lift is clearly going to provide more righting moment than the same amount of hull lift and since under most sailing conditions you want all the righting moment you can get, that will result in some increase in the optimum foil angle of incidence at any given boat speed.

However, it's probably not going to be practical to continuously make tiny adjustments to foil incidence to always keep the foil at the perfect angle of incidence for maximum boat speed. Also, looking again at Tom Speer's lift to drag plots, for the H105 and similar sections, there is not all that much additional lift to be gained by going to foil incidence angles above the angle that gives best lift to drag ratio, i.e. about 4 degrees. So, for a boat with retractable foils operating in 'foil assist mode' (i.e. with both hull bouyancy and foil forces contributing to lift and righting moment - hull off the water is a different matter) you might not give away much performance by using just one foil angle of incidence, this being the angle that gives the best lift to drag ratio. If your foil(s) is not retractable then you need a second incidence setting that gives the option to go to minimum drag at low speed.


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