I don't hate to say it but you and PIM are BOTH incorrect.

You both have only a laymans idea of how a sail works.

Fact: Venturi effect is proven and accepted
Fact: Venturi effect causes lower pressure in the area of the venturi.
Fact: Lower pressure is cause by the lower density of the gas molecules
in the area.
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.

THAT is how a sail works. All that other crap is extraneous to the situation
at hand aboard a sailboat. There is no high-horsepower motor available to
make the sail act as a plane like PIM suggests. All the force is generated
by a difference in pressure between the leeward and windward sides of
the sail which force acts against the foil in the water and the planeform
of the hull to produce forward motion.


"jlrogers" wrote in message .com...
I hate to be the one to tell you this, but PIM is correct. Lift is to air as buoyancy is to water.

"Simple Simon" wrote in message ...
I think you should do a little research into Bernouilli and Venturi.


"PIM" wrote in message ...

"Simple Simon" wrote
The convex side of the sail or the leeward side directs the air
over a longer path. This longer path for the air lowers the pressure
because said air must move faster to cover the increased cord.
Faster flow equals lower pressure. This low pressure combined
with the normal or slightly increased pressure on the windward
side is what creates lift.

Dear Simon,
I am not neglecting what happens on the leeward of the sail,
the leeward of the sail is also deflecting air and thereby creating lift.
as I am explaining in www.sailtheory.com/sail.html

According the "longer path theory" you use it would not matter how
much you pull your sail in, since there is always the same difference
in pathlength between windward and leeward.
Please read www.sailtheory.com/wrongtheory.html for more examples
why it is wrong.

The "longer path theory" is not easy to use and is wrong.
It is the reason I started my site.
Clearly I am not clear enough at this point

Many agree with me that the longer path theory is wrong:
jeff raskin:
http://tinylink.com/?RD9ntJyTuz
university frankfurt (under 2.2)
http://www.informatik.uni-frankfurt..../MIS/mis6.html
University of washinton:
http://www.aa.washington.edu/faculty/eberhardt/lift.htm
Bill beaty:
http://www.amasci.com/miscon/miscon4.html#wing

And so do the most aerodynamicists as far as I know.

I tried to adjust above theories from a wing into a sail.

Best Regards

Pim






--
Posted by news://news.nb.nu

You are making a mistake: why the air speeds up at leeward.
It has nothing to do with the length difference.

If you do not want to leave your good old theory,
please consider the one I suggest as an extra.
my theory :
"the sail deflects the air (on the windward side as well as on the leeward
side)"
It is very satisfying, it explains a lot.

Any way, all theories are a simplification of the reality.
more theories to describe the same are possible.
The best theory is the one that is the simplest and explains the most.
It does not mean all other theories are rubbish!

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)


In reply of your previous post:

"Simple Simon" wrote in message
Fact: Venturi effect is proven and accepted
Venturi effect is in a pipe, in a venturi, There it is indeed proven and
accepted.
A sail is no venturi.

Fact: Venturi effect causes lower pressure in the area of the venturi.
yes.

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.

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.

All that other crap is extraneous to the situation
at hand aboard a sailboat.
oh.

There is no high-horsepower motor available to
make the sail act as a plane like PIM suggests.
I do not want to suggest a motor. Sorry if I did.
(with "power" I mean energy/time, or force x speed, so not a motor,
on my site you can read "force" instead of "power" in most places.
It is a small translation error. "power" is in Dutch different from
"motorpower"
thanks for the hint. This helps improving my site )

All the force is generated
by a difference in pressure between the leeward and
windward sides of the sail
yes, and these pressures are created by changing the direction of the wind
locally.

which force acts against the foil in the water and the planeform
of the hull to produce forward motion.
yes.

Best regards,

Pim
www.sailtheory.com






--
Posted by news://news.nb.nu

"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.

Will move in the direction of the lower pressure

Why in the hell couldn't you see that when we were talking about weather
systems?

OT