It' easy to see how a boat sails downwind. The wind pushes on a sail that's kept almost flat and the boat zips along. But how does a boat sail into the wind?
The secret is air pressure. When a sail is angled into the wind it curves. Some of the air in the wind streams along the inside of the sail (the windward side). And some air streams along the curved outside of the sail (the leeward side). The air traveling along the leeward side has a longer way to go than the air on the windward side, so it has to go faster and is thinned out. That means the air pressure is higher on the leeward side. The result is a force that pushes the boat a little bit forward but mostly sideways.
If that were the whole story, a sailboat would keep slipping sideways and never get anywhere. But boats have rudders and either a keel or centerboard, which resist the slipping. (Hold your hand edge downward in the bathtub and try moving it through the water.) The result is that the boat is given a thrust that is more forward than sideways.
Typical sailboats can sail into the wind at an angle of 45 degrees or so.
Ozzie says . . .
You can figure out how a sail works by putting your hand outside the window of a moving car. When you hold your palm open and flat to the wind, your hand will be forced back. (Thats how a sailboat moves forward when the wind is directly behind the boat and the sail is fully let out.)
Now turn the side of your hand toward the front of the car and cup your
fingers. As you try out different angles of approach to the wind, you
will feel more or less pressure on the palm and the back of your hand.
That's how changes in the fullness of a sail can change the degree of
forward motion created.
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