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Started this because I couldnt find the right one, let me know if there is already one.
Disclaimer, I am certainly not educated in this field hence the question to the aero experts
In the below photo of an AC40 there is a vertical twist in the trailing edge of the sail.
This appears to spill the air towards the top.
Does this produce a downward force that would be countering the lift from the hydroplanes?
From many years back when I raced sailing boats, you tend to reduce the angle of attack at the top of the sail so as to reduce the lateral force and hence tipping force the sail produces. You make more power from further down the sail and there's usually plenty to use - any more and you often have trouble keeping the boat from tipping over.
It's a shame this topic never got the attention it deserved from users.
Good idea overall
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Twist in sails is there to serve various purposes.
One is to take in account vertical wind gradient.
Wind hitting the sails has two velocity component. One due to the "real" wind, the other due to the forward speed of the boat.These two components have different direction, and the "real" one has also an intensity gradient from zero at sea level to the full wind speed higher. Combining them results in a different angle hitting the sail, with the flow more aligned to the boat motion the lower you go.
This gradient is related to the boundary layer effect of the wind blowing over the sea, air molecules touching the water cannot have relative velocity to the sea surface, so you start with zero and you have a vertical gradient up to full speed, usually in some couple of meters.
Another one is to keep the boat upright. Usually sailboats become overpowered with increasing wind speed, meaning that you can't fully use the power of all the sail area without running out of righting moment (from crew weight shift, keel ballast, foils...) and tipping over. When approaching this condition, you start depowering the sail from the top (the area with more moment arm for tipping the boat), adding twist and spilling excess pressure.
There is also a purely mechanical factor: sails are usually unsupported in the top end, so wind pressure will naturally twist the tip. You can exert some control adding tension to the sail, but there is nothing directly keeping the trailing edge of the tip from opening downwind.
