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cammd said..
The only time you would ever sail directly downwind in a race is in non planing conditions, regardless of the board. This would also apply to many dinghy classes such as skiffs and cats. If you go direct downwind when its planing you will most times overtake the wind and lose power in the sail, then as board speed drops the sail will load up again and want to pull you over the front. Everyone always goes down on angles in planning conditions, its faster.
It is not so obvious. Suppose that we have wind 25 knots.
Average board is planing possibly at 15 knots already.
That give you 10 knots apparent wind still pushing into your sail.
Completely another aspect is that in such instance your sail works like bag of potato not as airfoil.
Sail is constructed in such way for stream of air flowing laminar on both surfaces - like a wing.
Position 90 degree to the wind works like obstacle creating nothing but turbulence behind - an effect will be very unpleasant then.
To rectify the problem you could create hole in the middle of the sail and then such improved sail should be quite comfortable down the wind.

( similarly to the vent in parachute)