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How a sail (and wing) works

A wing deflects air down. As the air is pushed down, the wing is pushed up.
The force that pushes the air down is the same force that pushes the wing up.
Compare to: When you are standing on the floor there is a force supporting you, and a force pushing the floor down.
This is the same force.

Now the question remains, why the air is deflected.
At the lower side of the wing (=windward side of a sail) the air is deflected in a very simple way: Going straight is not possible because the wing is blocking this option.
The leeward of the sail (=upper part of a wing) is also changing the direction of the air passing it.
This is due too the Coanda effect: Air tends to follow a curved surface as long as the curvature is not too much.
Something as sketched down here is happening.

A wing is deflecting air with the bottom as well as the top of the wing.

The air being deflected at the lower side of the wing is for most people easy to understand.
The air being also deflected by the upper side of the wing, by the Coanda effect, is harder to understand.
Later I continue about the Coanda effect, first something about what this theory results in.

One could imagine from the theory above that a flat plate works as well as a curved profile.
Unfortunately this is not completely true.
That is because the air going up the wing (leeward of the sail) has to make a very tight turn around the sharp leading edge when under an angle.
If this turn is too sharp the air can not follow the upper surface of the wing (the leeward side of the sail), and will be deflected less.
In aviation flow separation of a wing is called "stall".
Curvature makes this turn less sharp.

Beside the sharpness of the turn also the amount of not moving air at leeward matters.
If the wing (or sail) has a rough surface it tends to slow down the air by friction, resulting in not moving air staying at leeward.
This explains why sail are never rough like a carpet.

Again, a sail works by deflecting the air.
This deflection results in a force -The sailforce- working perpendicular to the sail.

That this force is perpendicular to the sail is important to understand and apply couple and forces

Now in more detail the Coanda effect:

In the right picture one can see the start situation.
From the dotted area the air is being "washed" away by the friction of tf the fastmoving air.
This is viscosity.
De dotted air is disappearing. This would imply that there would be no air at all and that there would be a very low pressure.
The air from the fast-moving air is thus being sucked to this area, and thereby deflects the air.

So why does the flow separate from the curved surface sometimes?
That is due to the friction between air and the surface.
This friction slows down the air, and "refills" the dotted area with not moving air.
This area then sucks less and if the fast-moving air is then deflected too much it can not be sucked completely to the surface.

The air close the surface that has been slowed down is called boundary layer.
This boundary layer grows when moving to the trailing edge.
This implies that there can be more curvature in the front(=luff) of the wing as there can be at the aft part of the wing(=leech).

A flow follows a curve because friction between air (=viscosity) and separates if there is too much friction between the air and the surface.
Some simple experiments to show this can be found here

Using this theory.

When one wants as much force as possible directed forward one has to deflect as much as possible air aft.
If one can achieve something as down here one is doing it right.

Often confused with this too much deflecting to the windward side:

Also one has to watch out for stall.

In above stalling sail the air has to make a tight turn at the leading edge.
This can be too much. If one can give a little bit more curve in the forward part, the air moves around smoother, and will not easy separate.

Off course one also has to take care to sheet in the sail enough, else the forward part will not do anything.

When having less curvature one needs to pull in the sheet less to have the luff doing anything.
A flat sail can be used to improve pointing, but is gives less force.

above: when one would go more to the wind the luff would start flapping. If then one sheets in the sail more it would be deflecting air to the windward side.

Until now we only discussed curvature in length direction, but a sail often also has curvature in height direction: Twist.
The upper part of the sail is more pointing outward then close to the bottom.

A little twist is good, since the higher one gets the harder the wind gets, and thus the apparent wind changes more to abeam.
Een beetje twist is gunstig, aangezien hoe hoger je komt hoe harder de wind, en dus hoe ruimer de wind inkomt.

Interesting is that with some heeling and twist one can make the sail more flat, from the winds point of view.
The blue line represents the path the air is going along the sail.

In the right picture the air is only deflected a little.

without twist the path of air is not much changed:

Because one can sail more upwind with a flat sail one can see that with some twist and heeling one can sail more up wind.
Unfortunately this will reduce the sailforce.

Nice theory, but how can you see how and if the flow is following your sail?
I recommend telltales and gentry tales, more about them in tuning
but before you start there I recommend to first read couples and Forces
The theory of deflecting as much wind as possible aft has an error.
When one is sailing close to the wind and the trailing edge of the sail is directed aft one is doing it right.
However, this does not mean when one is sailing before the wind one should have the sail sheeted in almost completely.
In that case the sail is not working as a wing but as a parachute, slowing as much air as possible.

The air moves faster on the leeward side.

This is only for the real theory cracks, Probably you can better continue with reading couples and forces

As explained above deflecting the wind results in the sailforce.
This force is given to the sail by pressures, A lower pressure at the leeward side and a higher pressure at the windward side.
Air has the tendency to be sucked to the lower pressure area from the higher pressure area.
The air in front of the sail is also sucked to the lower pressure area on the leeward side.
This means that there will be more air flowing on the leeward side as on the windward side.
This results in that deflection on the leeward side is more important as on the windward side, because there is more air to deflect.
The ropes in the sail show that the speed is much higher on the leeward side.

Some people say that the air is speeding up because of the Bernoulli effect. They say the lower pressure is created by the higher speed.
They are right, but it does not help understanding the flow around a sail to my opinion.
Also the Law of Bernoulli is applied wrong because it is not a very simple law.
My advice is not to use Bernoulli law to explain sailing.
For people who want to know more about Bernoullis law please click here.


(Also only for the real theory cracks)

Beside the air "leaking" from the higher windward pressure around the mast to the leeward part as explained above air is also leaking around the lower side of the sail.
This leaking is a deflection to the wrong side and is not helping.

The force that results from this deflection is in the wrong direction. It results in less driving force and more sideway force.
See the picture below (I should have drawn only the red force, or only the orange force, and not all together)

(For airplanes the increase in sideway force is called "induced drag", and the loss in driving force "loss of lift")

To reduce this leaking one needs to have a sail with only a small area where this leaking occurs, so a real high sail with a short length. In general such a sail is called a "high Aspect sail"
This leaking of air can be easily detected by holding a windex just below your sail. It will point to the higher pressure level -the windward side- much more as the windex on top of your mast.
This leaking also explains the needed twist:
If one would hold a very small sail just below the sail it should be sheeted in much less as the mainsail, because it is in the "leaking" air.
same goes for the upper end of your sail, what is actually being much smaller because the sail is tapered.
Sometimes this is called "taper induced twist"
The leaking air in front of the sail explains why the jib is sheeted in less as the main.
The jib is in the leaking air of the mainsail.
Of course the main is also in the pre-deflected air of the jib, making it also necessary to sheet in the main more as the jib.

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