It is fairly simple to explain how a sailboat sails when going downwind. (When the wind is behind you.) You simply let the sails out as far as they will go and the wind pushes on them and the boats hull follows along. However, when the wind is coming from the side there are some special physics at work.

    The force that the wind transfers to the sails actually makes a boat move forward for much the same reason a plane flies. If you were to look down on a sailboat from a helicopter you would see what looks like an airplane's wing, except standing on end.

    The air moving across the sails, like air moving across an airplane wing, creates a force called lift. A small amount of this force aims aft and actually pushes the boat forward and some of the force is lost due to friction as it moves over the sail. However, most of the force is sideways which tends to make the boat move sideways away from the wind.

    The keel or centerboard keeps the boat from being pushed sideways by the wind. The resistance from the hull and the keel translate the lifting force to forward motion.

    If you did not have a keel or centerboard the boat would simply move sideways away from the wind. However, with a well-shaped keel the boat will sail mainly forward while sliding slightly to leeward (away from the direction the wind is blowing). The force works on the keel like a thumb on an orange seed. If you put an orange seed on a table and place your thumb on it and push down, it will squirt forward. The keel, which should be shaped similar to the orange seed, does the same thing when a sideways force is placed on it.