Cavitation as we know is due to extreme low pressure that allows the spontaneous formation of a gas bubble starting at the leading edge and progressing to the trailing edge on the low pressure side of a fin. The fin loses about two thirds of its lift and kablooey!

I was thinking about twinnies. The pressure bulb from the upwind fin must impinge on the downwind fin increasing the pressure on the low pressure side of the fin. Would it be possible to put this effect to good use in speed sailing to enable higher speeds before cavitation?

I imagine a standard speed fin plus a much smaller fin to the windward side and a bit forward of it. The smaller fin has a tiny bit of toe in so that it doesn't cavitate but just elevates the pressure on low pressure side of the other fin a bit to delay cavitation.

It would be more draggy and less efficient for sure but maybe not so much that it matters.

Our fins are so good now that cavitation is really only a problem when we go faster than 45-46knots or in choppy water. Raising the cavitation threshold whilst reducing drag is the key to developing good fins, I understand. Slowie and Mal Wright would be able to answer your question more ably than I could.

There have been several discussions on this topic in Seabreeze and GPSSS.

DL said...


If you put the canard on the low pressure side, wouldn't that just eliminate your lift in the main fin?

Not eliminate but interfere with for sure. It would appear to be a method of trading additional drag for higher fin speed. Not something for your everyday sailor, but something to enable Finian or Antoine with masses of weight in a hurricane to beat the kites.

DL said...

decrepit said...


I built a rotating canard many years ago, trouble was I incorporated it in an experimental balsa-glass fin.

sweet! so the leading foil could rotate depending on what tack you were on?

Doesn't a slotted fin do the same sort of thing as a canard?
There is some argument that the slot effect for sails is imaginary but I thought that too was a canard of sorts.

In any case I would expect a canard to lower the pressure on the low pressure side even more and lower the speed of the inception of cavitation - not so good for very high speeds.