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On 05 May 2004 12:33:50 GMT, ospam (Stephen Baker)
wrote:

Parallax asks:

Why not underwater sails?

Because the size of the "keel" would be about the size of a normal sail, and
would have much the same effect. You already have one, in other words... ;-)

Steve

More on this theme:

where a sail boat reacts the force from an airfoil against a waterfoil
for traction,
to react the force from a waterfoil against an airfoil would call for
a rather huge sail, because the speed potential from most water
currents is less than 5 knots and to provide considerable resistance
by an airfoil to a 5 knot movement calls for area - lotsa area.

And if an adverse wind springs up, you still need to strike sail.

Brian W