Windsurf raked sail

Closing the gap cancel the bottom root vortex and increase the aspect ratio, increase the lift and decrease the drag. However the aero drag is much less than hydrodynamic displacement drag, the increased verocity of AC 75 is provided by foiling !

 

Windsurfing technique has been experimentally developed by the athletes. They may not know why it works, but there is no doubt that it does.
One simple observation is that inclining the sail to Windward makes it possible to hold onto the sail. Spectacular crashes happen when the sail gets to vertical and launches the sailor forward and to leeward. Moth class hydrofoils tip ove of they fail to heel to windward by the correct amount. Try holding a sheet of plywood upright in a strong wind. You end up with the top inclined windward.

Sealing the sail to the deck and or water surface reduces tip losses at the bottom of the sail and causes the center of effort to be lower when the top twists. This is established by testing on hydrofoil racing boats.
SHC