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 with wings than with sails
As shown on the figure below, the air partly flows above and partly below a wing
As soon as the profile receives the airflow according to some angle, a relative void is created near the end of the wing on its upper side
This relative vacuum calls for more air to pass above the wing than below, so as to compensate it, and ultimately makes the air flow faster on the upper side of the wing
This faster airflow 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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