what ship

A ship is a large watercraft that travels the worlds oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished from boats based on size, shape, load capacity, and purpose. They have supported exploration, trade, warfare, migration, colonization, and science. Ship transport is responsible for the largest portion of world commerce. The term "ship" can apply generally to a seagoing vessel or particularly to a full-rigged sailing ship with three or more masts, all square-rigged. Other rigs on seagoing vessels included brig, barque, and barquentine. Some large vessels are traditionally called boats, notably submarines. Ships can be designed to carry a specified weight of cargo, plus such necessary supplies as fuel, lubricating oil, crew, and the crew’s life support. These combine to form a total known as deadweight. To deadweight must be added the weight of the ship’s structure, propulsion machinery, hull engineering (nonpropulsive machinery), and outfit (fixed items having to do with crew life support) . These categories of weight are known collectively as lightship weight. The sum of deadweight and lightship weight is displacement—that is, the weight that must be equaled by the weight of displaced water if the ship is to float.