If the wind is the only thing that drives a sailboat, how can this boat navigate against it? There is a very surprising fact: the most important force that drives a sailboat against the wind is suction.
The sail of a sailboat is like the wing of an airplane on its side. From the curved side outwards or leeward side, the wind has to flow around the sail, creating a powerful suction that pulls it. The same principle applies to the aeroplane, which obtains its ascensional force from the suction that is produced on the upper face of its wings.
The suction effect is produced according to the laws of aerodynamics. The air that diverges around the sail is compressed so that it can pass tightly to the other side. When a current of air is compressed its speed increases: this is why the wind under the door can be surprisingly strong. And when the wind speed increases there is a loss of pressure.
The low pressure zone on the leeward side sucks the sail into itself with twice the force that the wind thrust exerts on the windward side.
Thus, the wind forces the sailboat to tilt. However, the keel or central board of the boat resists the side tumble. The force of the wind then becomes part of the forward movement of the sailboat and part of the leeward inclination that the yachtsman has to counteract by carrying his weight on the other side.
No sailboat can sail directly upwind, but a 12 m yacht can do so at only 12 to 15 degrees out of that direction. To go windward, where the wind comes from, the yacht has to zigzag or turn. The further upwind you sail, the slower you go. The pilot can go faster by zigzagging wider, at a greater angle to the wind, but then he has to travel further.
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