I've never had cotton sails, but from what I've heard, weight and the need for closer attention to drying properly are two concerns. Also that sythetic sails hold their shape better longer.
The simplistic (and doubtless somewhat erroneous) explanation is that in a cotton sail the fibers are free to move, and are more prone to inelastic stretching. In a dacron sail, the fibers are less stretchy, more elastic, stronger, and often the sail is impregnated with a binder.
And I'll wager there is a world of difference between a painter's tarp and cotton sailcloth.
[ 12-06-2002, 01:39 AM: Message edited by: ishmael ]
So many questions, so little time.