How a sail produces a force
Probably the single most important phenomenon to understand about sailing is how a sail produces a force
For a minute we'll switch to airplanes, as things are easier to figure out this way
As shown on the below figure, the air partly flows above and partly below a wing
The distance the air covers above the profile is slightly longer than below, just like the distance outside a road curve is longer than inside the curve
Because the distance is longer above the wing, the air flows faster in order to reach the end at the same time as the air flowing below
This faster air flow causes a depression on the upper part...
... that "sucks" the profile upward
This force ( which is perpendicular to the plane of the wing ) is what allows an aircraft to fly and a sail to produce a force
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