Floating depends on two things: displacement and density. Archimedes' principle, which explains the concept of buoyancy, states that in order for an object to float, it must displace an amount of water equal to its weight. As a sailboat's weight pushes downward and displaces water beneath it, an upward force equal to that weight holds the boat up.
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Here's where density comes into play. To displace enough water to remain afloat without becoming submerged, a boat must have an average density less than water. For that reason, the hull of the boat is hollow. Whether the boat is made of concrete or fiberglass, its average density is less than water. Think about it: If you put a basketball and a bowling ball in a swimming pool, the air-filled basketball has an average density much less than that of water, so it will float. The solid bowling ball, however, will sink immediately. This is how anything from a small sailboat to an aircraft carrier can manage to stay on top of the water.
Surface area also helps to keep the boat afloat. More surface area gives an object a better chance to displace enough water to offset its own weight. For instance, a small ball of clay likely will sink before it can displace the amount of water equal to its weight. But if you flatten the ball into a thin pancake, there is more surface area to distribute the weight across and displace the water, so it will float. For more information on precisely how a steel ship can float, read how boats made of steel float on water when a bar of steel sinks.
Now that we know how sailboats can float, we can learn how they zip through the water.