The effect of the wind on a surface is the velocity pressure. The formula is velocity divided by 4005, the quantity squared where velocity is in feet per minute, standard density of air is assumed, and the answer is in inches of water. Twenty miles an hour (1760 fpm) is a velocity pressure of 0.19 inches of water. that results in a pressure per square foot of 1 lb. A 250 sq. ft. sail would then have a pressure of 250 lbs. At 40 miles an hour (3520 fpm) the v.p. is 0.77 " of water. That is 4 lbs, per sq. ft., a pressure of 1000 lbs. on the sail. At 100 mph v.p. is 4.83" of water, 25.1 psf or almost 6300 lbs. on a 250 sq. ft. sail. If the boat is going with the wind you would have to subtract its velocity to get the relative velocity.

This will at least give an order of magnitude to the wind strength discussion. Basically force varies with the square of the velocity.

Howard