Parallax wrote:
Almost mounting my mast so it would be underwater got me thinking (as
it should). Why not underwater sails? Force on a sail is
proportional to the density of the medium and that makes a 5 knot
water current give about 50X the force of a 5 kt air current on the
same size sail. This means you could sail across the Gulf Stream with
an underwater foil.

Except for one small detail... such a boat could only sail in one
direction, with the current. The ability to sail "across" the Gulf
Stream or any other current would require some counter force, such as
the sails in the wind are countered by the centerboard.

... However, I am not sure where your rudder and
centerboard would be, in the air? Could you make a planing boat with
an underwater foil taking advantage of the water current?

And when it planed, using force from the current instead of the air, it
would do what... submerge?

... That way
you would not be limited to the 1.2Xsqrt(waterline length) in speed
anf might get up to absurd speeds.
Then again, maybe this idea is the result of breathing epoxy fumes.

I think that Steve B. hit the nail on the head... you don't need an
underwater sail, you've already got one.

Fresh Breezes... or Currents, whichever- Doug King