"I wonder if a sail shouldn't be consider "mechanical" propulsion though.
=|:-)
The mast and rigging are could be consider parts of a "machine" as well as
the
sail. A vessel under sail is certainly not human powered. Not sure how you'd
rate the horsepower!!!!! "

I believe that the hp is rated at the square of the wind speed with some
formula for the size of the sail, angle of attack, and some constant that
allows the engineers to fudge everything into working.

OK, so I didn't go to the reference books like I clearly should have, I am
not in school today. ;-)

Brian