"PIM" wrote in message ...
By the way, how do you explain that a flatter sail sails
better "pointing" close hauled as a deep(=curved) sail?
The more curve the more force according your theory, so what is the use of
flattening the sail?
(my answer: www.sailtheory.com/sail.html#ch)

You forget an important concept that of drag. A foil
is a compromise between lift and drag. NACA did
extensive studies on differently shaped foils and it
was all about lift vs.drag. One can make a foil with
maximum lift but if it also has maximum drag it will
not function very well. If a foil is to full in higher
winds it creates more drag. a flatter foil creates
less lift but proportionately less drag so overall
it is more efficient.

A sail is no venturi.

Yes it is. Take a venturi. Cut it and unroll it so it is spread out.
It is still a venturi but just not a cylindrical venturi. Air molecules
don't know if they are passing over a curve in a closed cylinder
or a curve in a spread out cylinder. View the sail as a spread
out cylinder.

Fact: Lower pressure is cause by the lower density of the gas molecules
in the area.
density difference is not of importance. A keel works just as a sail
prducing sideways lift, but
density of water stays the same.

Density is all important because things happen on a molecular
level. It is a fact of physics that lower density equals lower
pressure. The measure of pressure of a gas tells you its
density.

Fact: An object with low pressure on one side and higher pressure on the
other side if free to move will move in the direction of the low pressure.
Yes.


THAT is how a sail works.
There is no venturi. There is a sail. That is the error.

Incorrect as explained above with respect to the opened
and spread out cylinder and how molecules of air see no
difference.