Principle of Buoyancy

Steel has a density of 487 pcf, about eight times that of water. Steel boats float, however, because they are hollow and shaped to displace a volume of water that weighs more than the boat's weight. Ships are often rated by their displacement. Displacement is measured in units called tons, which are the weight of the water displaced by the ship. As a ship is loaded with cargo, it settles deeper into the water. This displaces an additional volume of water and produces the greater buoyant force required to support the added load. Plimsoll marks are painted onto the hull of cargo ships to indicate the depth to which the ship could be loaded. The different marks refer to fresh and saltwater and to the various seasons where temperature also effect water density.

Fish can alter their buoyancy by changing the volume of their internal swim bladder. Scuba divers can inflate their external buoyancy compensator vest to change its volume. Both of these changes alter the amount of displaced water and, thus, the buoyant force acting on the body. With this control, divers and fish can ascend or descend at will as they observe each other at play.

The principle of buoyancy applies to all fluids, including gases. A blimp is filled with very light helium gas with a density of 0.01 pcf. As a result, the weight of the blimp is less than the weight of the air that it displaces and the blimp will float in air. By dropping ballast or venting helium, the blimp can control its buoyancy and, thus, its altitude. A hot air balloon gets it buoyancy because hot air is less dense than cold air. The density of air at 200°F (93°C) at sea level is 0.06 pcf and serves the same function as the light helium gas in the blimp. Although highly flammable, hydrogen gas is much less dense than helium and was used for lift in dirigibles up until 1937, when the German airship Hindenberg burned and crashed.