On Sun, 7 Nov 2004 at 20:11:39 in message
, John Bell
wrote:
In addition to the issue of boats traveling through water with a current,
there is actually the issue of sailboats traveling through the water at a
different angle from which they are pointed due to the push on the sail.
This slip angle creates an angle of attack on the centerboard, daggerboard,
or keel to provide an opposing force.

An additional way of looking at this is to visualise that a boat sails
because it has one lifting part (the hull, keel and rudder) in the water
and another lifting system (the sails) in the air. The boat can move
because there is a relative velocity between the two fluids. It is
obvious you can sail if there is no wind at all provided you are in a
tidal flow.

Every one knows a sailing boat can tack up wind but can it tack down
wind? The answer is not much, mainly I believe because of the much
higher drag of the water. Sand yachts and ice yachts can, and they can
sail on a reach, downwind faster than the wind is blowing.
--
David CL Francis