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.