From submeged foils to surface piercing as an advantage?

Well, keeping in the realm of "theory", mine is that surface drag is much less a penalty than submerged drag. That's where the gain might be. They've probably got it figured already, but I would think ventilation is probably the more pertinent worry.

they've got a much better management system in place so far as foil control goes, so I can't see this being something that will benefit us windfoilers.

Interesting. Seems they are using larger foils than last time but are getting a bit higher speeds, so surface area reduction may be required. Seems like up to about 1/4th of the foil was out of the water in one race I watched.

I wonder to what extend tip effects play a role. They seem to be keeping the foil very close to the surface, probably to reduce drag from vertical components (the "mast" equivalent). In general, turbulence at the wing tips is a major source of drag (one reason, if not the main reason, why high aspect foils have better lift ratios). It seems plausible that the tip drag would be even worse with tips very close to the surface. By this logic, having the tips come out of the water would be to minimize tip-induced drag.

The races seem to contradict some papers that stated that foils have a reduced lift-to-drag ration near the surface, mostly due to induced drag. One paper had drag go up about 3-fold when going from deep submersion (h > 2 c) to near surface (h = 1/2 c), and lift going below 0 (hal.science/hal-03963200/document). Makes me wonder of the calculations were off a lot, or if angling the foil makes a big difference. But that was a 2D simulation with a symmetrical foil. I bet that the ACA75 foils are rather asymmetrical, and run at an angle of attack close to 0, which should make a big difference for surface distortions that cause drag.

thanks , very interesting, pasting the relevant bit below :

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Surface-piercing foils are the concept we see in AC75, the America's Cup craft: with these foils, when you are going at high speeds, lesser wing area is required to produce the same lift, i.e., the weight of the boat, compared to at lower speeds. Thus, there is a possibility to reduce the wing surface in water and gaining more efficiency by piercing. And as the wing pierces more out of the water, it results in valuable drag savings. The lift requirement stays the same as weight stays the same; as speed increases, lift increases by the second power of velocity, so you need less wing surface to generate lift, which is automatically done as it lifts out of the water. As you slow down, you sink again, having more wetted foil area, so it is self-balancing as well. And when looking for performance advantage, this is what can make a difference.However, one of the disadvantages of this configuration is ventilation, and it is extremely important to take it into account during a design phase. Ventilation is the phenomenon occurring at the air-water interface of the hydrofoil piercing the water when air gets sucked down the lifting surface of the foils and therefore lift is compromised, and the hull falls back on the water. At the moment we are relying on some empirical methods and real-world testing to predict and better understand the ventilation behavior. CFD is a work in progress, as it is quite difficult and computationally expensive to simulate this effect of air and water together. It has been done but it is not as simple as simulating pure water or air.
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