High-Speed Sailing

Mac Stevens posted a good comment about my Trifoiler discussion. He is correct, Trifoiler's hydrofoil cavitated which is a bit different than stalling. The result was essentially the same -- reduced lift and increased drag. Cavitation is a situation where the pressure in the water gets so low that bubbles of vapor form. This is what the video of the water tunnel test showed on Trifoiler's hydrofoil.

Mac's post was timely because the Aussies are at it again, with WotRocket! Ya gotta love those Aussies, with a name like that I want to join them!

I've been looking for info on WotRocket but haven't found much yet. They have designed super-cavitating foils in which some part of the foil travels in the cavitation vapor bubble, reducing hydrodynamic drag. Great idea if they can get it to work. I have not been able to find any photos or drawings that show the overall set-up, but it sounds like they will be using a fairly conventional approach to stability - by using weight for balance.

Personally I think WotRocket is solving the wrong problem at this point, however if they get the foils to work as planned, we will all benefit by learning a way to reduce foil drag.
I haven't heard that anyone has encountered a cavitation problem from the forces that drive a sailboat yet. "Whoa dude, you just said Trifoiler did!" Nope, the foil that cavitated had nothing to do with driving (propelling) Trifoiler. The two horizontal foils are only used to balance the craft and raise the hull out of the water. The vertical part of the foil is what drives the boat (by working against the wind's force - per my wet soap analogy). That vertical part of the foil had more speed "in" it. Note that Hydroptere has already sailed faster than Trifoiler and I don't think they are having trouble with cavitation. (I could be wrong on this; it's just that I haven't heard of cavitation being the problem.)

As I've said before, using weight to balance the craft became obsolete with Bernard Smith's book "40 Knot Sailboat". Ketterman was correct, but the top speed was limited. Sailrocket and those experimenting with aligning the airfoil's force with the hydrofoil's force (including myself) are developing the future design model for stability and high-speed sailing.

Bob