Why do boats float?
Boats float because they push aside, or displace, a weight of water equal to their own weight. The water pushes back with an upward force called buoyancy.
This idea is known as Archimedes' principle, after the ancient Greek scientist who described it more than 2,200 years ago. Any object placed in water feels an upward force equal to the weight of the water it pushes out of the way. If that force is as large as the object's weight before it is fully submerged, the object floats.
A solid lump of steel sinks because it is much denser than water. But a steel ship is shaped like a hollow bowl filled mostly with air. Its overall density, including the air inside, is less than water. As it sits lower, it displaces more water until the buoyant force balances its weight.
Why boats can sink
If water gets into the hull, it replaces the air and increases the boat's overall density. Once the boat cannot displace enough water to balance its weight, it sinks. That is why large ships have watertight compartments to limit flooding if the hull is damaged.
Did You Know?
It is easier to float in salt water than fresh water because salt water is denser. In the Dead Sea, people float without any effort.