mbounds,

You are correct about acting perpendicular to a flat plan (or the projected flat area of curved surface) but a boat in 100 mph wind is not going to be pointing up or on a reach.

The foil, like an airplane wing, causes another force by converting static pressure to velocity pressure on the convex side which causes a difference in the static pressure on either side of the sail. That is an additional force on the boat to make it go forward but not applicable on a run.


My experience in wind forces is in HVAC, not in sail dynamics, but the idea is to give an order of magnitude to this discussion.

Howard