15 August 2010
Some of you have been a great help in alerting me to speed-sailing topics of interest. I want to thank Paul Flados for this one. Longshot (Trifoiler) is evaluating the feasibility of another run at the sailing speed record; this time with wings in place of sails. Take a look at this address:
As many of you know, early on in this blog, I posted my views on Longshot's chances of breaking 50 knot's. I still hold the view that the horizontal tips of the hydrofoils are going to be the limiting factor, but that an increase in efficiency will result in an increase in speed. The only point in question is how much increase in speed can be obtained? The limit will be when the lee foil cavitates due to pressure and velocity overload. Note that those foils do not drive the craft but lift the hull out of the water and provide for roll and pitch stability. The vertical portions of the foils are what drive the craft (think center-boards, lee-boards, etc).
Apparently, the crew of Longshot will be evaluating their chances of breaking the record, before making any decision to attempt it. I hope we can find out more about this if they decide not to go after the record; if they do attempt it, I'm sure we'll know.
So what does efficiency have to do with this? After stability (which I've discussed at some length earlier) efficiency is the key to going really fast. Note that of the top speed contenders, Longshot is the only craft that has mastered stability and its limit has been efficiency. The other craft have all crashed due to stability factors. The efficiency is described by the Lift/Drag ratio (L/D). For sailors this means the power (lift) from the airfoil (sail, wing) and from the hydrofoil (keel, centerboard, etc). This power being the result of the "squeeze" I described earlier. The drag is the total drag on the craft (part being aero-dynamic drag and part being hydro-dynamic drag). Once stability is conquered, L/D is the key to speed.
One note about the stability of Trifoiler type craft; if the foils are compromised (snag debris, get forced out of the water by a wave, etc) stability of the craft is instantly gone. I personally observed this while Greg Ketterman was testing a Trifoiler prototype. At the address given above, scroll to the bottom, click on "more here", scroll down to post #9 (07-27-2010 07:00 pm) and watch the "video" (a series of stills). At 1:01 into the video the windward foil is in the air and roll stability is gone (this because of the craft getting pitched up by crossing the camera boat wake earlier. The craft then "crashes", but recovers. We are missing a lot of information because of the missing frames. I suspect that the foil got a lot more "air" than shown and we see it starting up and after it comes down. Ketterman's launch was quite similar and more spectacular (I saw the whole thing).