It is used to determine the weight (mass) of a ship. There are no scales big enough to weigh a ship so what mariners do is calculate the volume of the underwater section of the hull of the ship (length X breadth X height) approximately. The (average) density of sea-water is known (1025 kg/m3). We know from the Law of Flotation (which is derived from Archimedes Principle) that in order for a vessel to float it must displace a volume of water equal to its own weight. Therefore if we can calculate the Mass of the displaced sea-water

__displaced__by the hull we will know the weight of the ship because they are equal. This is the formula: Density = Mass/ Volume Therefore: Mass = Density X Volume en.wikipedia.org