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After two days of luis vuitton racing it's very interesting to see how the foil design moved from fully submerged foils in previous america's cup to "surface piercing" foils. I allways thought they have a drag penalty due to surface wake and ventilation. Does anybody here have a theory about the gains?
Thanks

Are they aerating the foil intentionally? The only thing I can think is the boat is going fast enough to not need the full area of the foil perhaps they aerate the tips to decrease drag...but it seems pretty far fetched lol

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.

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isandoval said..

After two days of luis vuitton racing it's very interesting to see how the foil design moved from fully submerged foils in previous america's cup to "surface piercing" foils. I allways thought they have a drag penalty due to surface wake and ventilation. Does anybody here have a theory about the gains?
Thanks

Maybe we will see moth's main foil with anhedral so they can have one tip out easily. Windfoiling as well. This is so opposite to all previous development...
Will see how it goes...

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.

I believe they are going fast enough for cavitation to be a limiting factor so this might be using it to their advantage like certain high speed torpedoes.

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Grantmac said..
I believe they are going fast enough for cavitation to be a limiting factor so this might be using it to their advantage like certain high speed torpedoes.

Cavitation is more of an issue over 50 knots, and most of the racing seems to be at lower speeds. You can see the foils quite well at times in the videos, and I did not see any sign of cavitation.

airshaper.com/blog/hydrofoil-design

I found this article, explains the advantages and challenges of surface piercing foils.

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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Cool thread. I remember reading something about angle of foil relative to surface has an impact on ventilation and recover rate but can't remember what was better...

Also consider the impact of waves. Big changes in wetted area as you go through swell.