If you throw a crowbar in the ocean, it sinks. Yet, a ship, which has a weight many times that of a crowbar, manages to float in the water. This interesting phenomenon known as buoyancy is the reason that boats float. In short, boats float because their design exploits the principle of buoyancy. The properties of buoyancy were first described by the Greek mathematician Archimedes, in what we now call Archimedes' Principle. This principle states that any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

What is displacement? Archimedes is said to have made this famous discovery in the bathtub, which makes sense, because a bathtub is a great place to research buoyancy and displacement. He discovered that when you place an object (such as a bathing scientist) into a fluid, the level of the fluid rises, because the object has displaced some of the fluid. Archimedes realized that any object placed in water will displace its own weight or volume in water, which ever comes first. This is called the weight to surface area ratio.

He realized that different materials having different densities but equal volume will have a different buoyancy. A bowling ball will sink while a balloon filled with air will float, even though they occupy the same volume in space. This is due to the greater density of the bowling ball. unlike a balloon, a bowling ball weighs more than the weight of the water it displaces. The bowling ball has a low surface area to weight ratio. A boat is essentially a hollow shell filled with air, meaning that it has a large surface area to weight ratio. Therefore, the boat will still be well above the surface when it has displaced its equivalent weight in water.

Boats float because of their design. When a boat is heavily laden, it will settle lower in the water, because its surface area to weight ratio is different than when the boat is light. A boat will remain floating and stable provided it is not overloaded.

Now that you know how boats float, you can probably imagine how it is that other objects float also. From pieces of wood to ducks, an object's weight to surface area ratio dictates its buoyancy. You can also see why boats sink; if the hull of a boat is breached, it begins to take on water, which makes it denser, therefore causing it to displace more water. If the boat takes on enough water, it will become too heavy to remain buoyant.