What makes the boat go to windward??

It is easy to see how the wind can push anything across the water. If you throw an empty sardine can overboard it will take off downwind, pronto (till it sinks) . A sailboat is a little more sophisticated than that, because it has some control over its direction of movement. The controls on a sailboat are the sails and the rudder, but there is a big difference between a sailboat and any other watercraft: the underwater profile of the boat. The depth of the boat below the water has an important effect on how the boat will sail. The lowest part of a boat underwater is called the “keel”. This depth of keel provides what is known as “lateral resistance”, i.e., something to keep the boat from moving sideways. The deeper the keel, the more lateral resistance.

When the sails fill with wind one side of the sail has a higher pressure than the other. Notice in the diagram below that due to the angle of the sail in relation to the boat, some of this low pressure is on the forward part of the boat, and some of it is on the side of the boat

click on pic for larger view

What happens is that the keel, or the lateral resistance, cancels out the force pushing on the side of the boat, but not the force pushing on the front, and so the net result is that the boat moves forward. Note that the boat in the diagram below is moving to windward (or “going to weather”).

Without a keel, the boat above would be sliding off sideways. This sliding off sideways is called “leeway” Remember, the deeper the keel, the more lateral resistance, hence a better windward performance. The amount of leeway any given boat will make is important to know, as it has a direct effect on how you steer a course.

Terms:
Keel
Lateral Resistance
Leeway

in a sentence:
Her deep keel gives so much lateral resistance she makes very little leeway