The first step in planning a new ship is careful consideration of what it is going to do. The answers to such questions determine a ship's size, its internal layout, and its engines and machinery. Expressing the Size of Ships Oceangoing ships vary greatly in size. Ship size, however, is commonly expressed in tonnage. There are several systems to measure tonnage.
Displacement tonnages express the weight of the ship.
Light displacement is the weight of the ship and its permanent equipment. Load displacement is the weight of the ship when it is filled with fuel and cargo to its designed capacity that is, when it is immersed to its load line. Gross tonnage is the capacity of the spaces in the ship's hull and of the enclosed spaces above the deck available for cargo, stores, fuel, passengers, and crew. Net tonnage is the gross tonnage less the spaces used for the accommodation of the ship's Master, Officers, Crew , and the navigation and propulsion machinery.