Sizemore points out the key: foil-borne efficiency. I sailed the hydrofoil Rave trimaran extensively, and once the hulls come out of the water, it takes off like a catapult, or a 2-stroke racing motorcycle getting "on the pipe", or a turbo or supercharger kicking in with a vengeance. Likely full wattage is required from the 3.7KW Torqeedo to get foil-borne with a full load, but once "flying" at a "cruising" speed of perhaps 15-25 knots, it should last 100km. That is still slow enough through the water to minimize parasitic drag from the underwater portion of the foils, but fast enough to also minimize the dragging portion of the foils. They are slightly "progessive" or "piercing" foils: the faster it goes, the higher it rides.
Forget 100 knot speeds though. Hydrofoils usually "high speed stall" around half that speed (depending on the foil profile and water depth (hence pressure)). The water has too much mass to quickly close in behind the too-fast foil, so the extreme low pressure vaporizes the sluggish water, the foil cavitates, and lift is suddenly and dramatically lost. Same thing with jetliners up at altitude: they have to be very careful to not fly too fast or too slow, since both extremes result in stalls.
The Rave trimaran (http://www.windrider.com/windrider_rave.aspx) "flew" between 10mph and 47mph; anything else and the hulls hit the water. I only got it up to 38mph mid San Francisco Bay with pretty big waves (no motor, 195 sq ft of main and jib only).
Mark Stafford
--- In electricboats@yahoogroups.com, James Sizemore <james@...> wrote:
>
>
> If you can hydrofoil with a small sail then a it obviously does not much power to do it :
>
> http://en.wikipedia.org/wiki/Moth_(dinghy)
>
No comments:
Post a Comment