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> Wind velocity increases logarythmically above surfaces, including
> the waters surface and any obstructions (i.e. cabin tops and
> decks).
True, but from a couple texts I've read, the large reduction of induced drag
at the foot of the mainsail (from spillage between the foot and cabintop)
and reduced heeling moment of a lower main supposedly more than makes up for
the slight change of windspeed in a higher sail. This effect is supposedly a
factor in the great light-air performance of deck-sweeping genoas, and is
similar in theory to using winged keels.
> Placing the sail as high as possible avoids
> surface-induced drag and eddies. I can furnish references for
> equations but they can be found in any transport phenomena text.
I'll try to find my references for the low-sail theory. My limited
experience/gut-feeling/seat-of-the-pants-observation is that lower works
better, but I haven't exactly run it through a wind tunnel.
Someone mentioned high-aspect-ratio sails - these do not, as I recall, gain
their power because so much because they reach into higher wind, but because
they have a relatively shorter foot and therefore lots less induced drag.
Tom Whidden (president of North Sails) goes on and on and on about induced
drag and sail shape in "The Art and Science of Sails" and since North is
arguably the most technically advanced loft with extensive computer analysis
and wind tunnel testing, I'm inclined to believe him.
I sure wish sail/trim testing was easier, but until wind tunnels get cheap
or we find a place with absolutely consistent wind and seas, a lot of it is
trial and error. I hope if we get the NW potters out on a group sail this
year we can run some tests, with one boat holding steady as others tweak and
trim to see what works and what doesn't on potters.
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