"Simple Simon" wrote
It does depend on the length difference as stated in the principles
of the venturi effect. The speed of the air is increased when it
is forced through a smaller area.
I agree.
The sail acts like one side of
a venturi in that the air has a smaller area to go through because
of the extra length.
No, there is no real smaller area.
The air above that air on the surface of the
foil acts like a venturi also to a small degree.

!!The distance of the leeward and windward part of the sail is the same!!
only when you have some distance from the sail you have some
length difference because of taking the "inner turn" !
This means the further away from the sail the bigger the lenght difference
so the more the speed difference?
I do not think so.

The net effect is
the air moves faster, the molecules become spaced farther apart
and the pressure is decreased.
the molecules being spaced further apart are no issue.
A wing, sail, or keel can also produce lift under water, and water can not
be compressed.

This decreased pressure is a greater
factor causing lift than the air pushing up on the windward side of
the sail.
Why?

You might know a lot but you need to learn more when it comes
to the practical application of single-surface foils used on sailboats.
Practical application of sails: I am always learning to become better in
sailing.
Theoretical: I rule :-)

Best regards,
Pim


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