"Simple Simon" wrote in message
...


You're as bad as PIM in forgetting what angle of attach
does to the lengths of the sides of single sided foil. Think
of the distance the air molecules must move on the leeward
side as opposed to the windward side.

The nice curves indicating the movement of individual air
molecules demonstrate this nicely. Measure them and
see for yourself. Print it out and use a divider. You will
be surprised.

Nonsense!

There is an absolutely negligible difference in length between the windward
and leeward paths.

The wind pushes the windward side of the sail. The keel stops the boat from
moving sidways, and the pressure is relieved by the forward movement of the
boat.



Try pushing a knife through butter to see what I mean.


If your theory were correct, then a boat would go *faster* upwind.



Regards


Donal
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"Donal" wrote in message
...

"Simple Simon" wrote in message
...
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. 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. The air above that air on the surface of the
foil acts like a venturi also to a small degree. The net effect is
the air moves faster, the molecules become spaced farther apart
and the pressure is decreased. This decreased pressure is a greater
factor causing lift than the air pushing up on the windward side of
the sail.

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.


If I didn't already know the answer, I'd be tempted to measure both
sides
of my mainsail - to see if you were right.

Fortunately, I am not stupid enough to think that there is a significant
difference between the lengths of the windward and leeward surfaces.
Where
do you get this rubbish from? Are you reading a "definitive" book on
the
subject?



Regards


Donal
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