As you know, a sail boat has a long projection on the bottom of the boat called a keel or a centerboard. As the wind blows against the sails, it tries to push the boat in front of it. However, because of the resistance of the keel against the water, and the greater air pressure on the upwind side of the sail, this lateral movement is translated into forward movement. The boat does make some lateral movement called leeway, but most of the wind’s energy is translated into forward movement because there is less resistance to pushing the boat forward than sideways.

Because of the airfoil shape of the sail, a sailboat can move, or point, roughly 90 degrees into the wind. Trying to move more than 90 degrees upwind results in a lowering of the air pressure differential between the two sides of the sail and the boat losing its forward motion. Much of the skill in steering a sailboat involves the ability to sail as “close to the wind” meaning the tightest possible angle upwind without losing any forward motion and speed. For a boat to move into the wind – upwind – it has to zigzag back and forth at 90 degrees to the direction of the wind gaining some distance on each zigzag, or tack.

When a boat moves downwind – sails with the wind behind it- then the sails do act more as large surfaces for the wind to blow against than as airfoils. It is on this downwind tack that you often see boats put up the large colorful balloon like sail called a spinnaker. This sail really is a huge bag designed to simply capture as much wind as possible. However, because it lacks any airfoil like properties, it is useless going upwind. Small sailboats often have a centerboard that can be raised and lowered. When going downwind, there is no need for a centerboard to be used as the boat is going directly before the wind and there is no lateral motion to be translated into forward motion. In this case the centerboard can be raised which will reduce drag and increase speed.

In sailing a boat there are three angles that the person steering the boat must constantly be evaluating. The direction of the wind, the set, or angle, of the sails, and the direction you want the boat to go. These angles are not static but constantly change as the wind shifts. Failure to pay close attention to these angles results in slower speed. Sailing is very much like playing pool. It is all a game of angles and like a pool player, the sailor who can best judge those angles will do the best.