My apologies, Tony, for overstating your hypothesis. I initially thought, because we can't point all that high, that we would be dealing with reasonably large angles of sideways force. As you point out, though, we are not concerned here simply with the angle of the boat relative the wind, but the angle of boat relative to its movement in the water, and much of the time that angle, the leeway angle, will be reasonably small, much smaller than the angle to the wind.
Has anyone tried to measure this angle on an Island? It would require good measures of wind direction and boat track. The latter, over a reasonably long tack, could be obtained from GPS tracking, but measuring local wind direction within a few degrees, and having it consistent over the course of the course is beyond my instruments.
However, sailing yesterday, pointing about as I could with good telltale flow, I noticed that there was a fair amount of pressure trying to push one of my peddles backward. Since the drive had both fins, if this was sideslip pushing, I don't know why it would not have pushed more or less equally on both fins, and balanced out. But it made me pretty sure that if I ever conduct the one-fin experiment, I will find that that fin is pushing at almost all angles of sail. But, I don't have any good way to compare that with other forces resisting side slip.
I have sailed without the centerboard down (usually because I forgot to put it down
), and could make upwind progress, so the hull alone offers considerable lateral relative to longitudinal resistance.
While I am thinking about that, do you think the reason a daggerboard (or centerboard) is long and narrow instead of short and wide is only so that it is pulled entirely out of the water when the boat heels, or is there some other consideration? It would seem that maximizing lateral vs longitudinal resistance would call for minimal front surface and maximal side surface, best satisfied by a thin, wide, short board, but this is not what has been developed.