Ship measurements

Measuring the size and other aspects of ships is a confusing field, full of non-intuitive terms that can be very confusing to non-specialists. This article provides definitions for many of the terms used in measuring ships.

Definitions

Beam - The width of the ship

Complement - The full number of people required to operate a ship. Includes officers and crewmembers; does not include passengers. For warships, the number of people assigned to a ship in peacetime may be considerably less than her full complement.

Cube - The cargo carrying capacity of a ship, measured in cubic feet. There are two common types:

Bale Cube - The space available for cargo measured in cubic feet to the inside of the cargo battens, on the frames, and to the underside of the beams. It is a measurement of capacity for cargo in bales, on pallets, etc., where the cargo does not conform to the shape of the ship.

Grain Cube - The maximum space available for cargo measured in cubic feet, the measurement being taken to the inside of the shell plating of the ship or to the outside of the frames and to the top of the beam or underside of the deck plating. It is a measurement of capacity for cargo like grain, where the cargo flows to conform to the shape of the ship.

Displacement - A measurement of the weight of the vessel, usually used for warships. (Merchant ships are usually measured based on the volume of cargo space; see tonnage). Displacement is expressed either in long tons of 2,240 pounds or metric tonnes of 1,000 kg. Since the two units are very close in size (2,240 pounds = 1,016 kg and 1,000 kg = 2,205 pounds), it is common not to distinguish between them. To preserve secrecy, nations sometimes misstate a warship's displacement.

Displacement, Light - The weight of the ship excluding cargo, fuel, ballast, stores, passengers, and crew, but with water in the boilers to steaming level.

Displacement, Loaded - The weight of the ship including cargo, passengers, fuel, water, stores, dunnage and such other items necessary for use on a voyage, which brings the vessel down to her load draft.

Deadweight Tons (DWT) - The difference between displacement, light and displacement, loaded. A measure of the ship's total carrying capacity.

Cargo Deadweight Tons - The weight remaining after deducting fuel, water, stores, dunnage and such other items necessary for use on a voyage from the deadweight of the vessel.

Draft, Loaded - The depth of water necessary to float a vessel fully loaded.

Length - The distance between the forwardmost and aftermost parts of the ship.

Length Overall (L.O.A.) - The maximum length of the ship

Length at Waterline (L.W.L.) - The ship's length measured at the waterline

Shaft Horsepower (SHP) - The amount of mechanical power delivered by the engine to a propeller shaft. One horsepower is equivalent to 746 watts in the SI system of units.

Ton - The unit of measure often used in specifying the size of a ship. There are three completely unrelated definitions for the word. One of them refers to weight, while the others refer to volume.

Measurement Ton or Ship Ton Calculated as 40 cubic feet of cargo space. (Abbreviated M/T). See Bale Cubic - example, a vessel having capacity of 10,000 M/T has a bale cubic of 400,000 cubic ft.

Register Ton - A measurement of cargo carrying capacity in cubic feet. One register ton is equivalent to 100 cubic feet of cargo space.

Weight Ton - Calculated as a long ton (2,240 pounds) (abbreviated W/T)

Tonnage - A measurement of the cargo-carrying capacity of merchant vessels. It depends on not on weight, but on the volume available for carrying cargo. The basic units of measure are the Register Ton, equivalent to 100 cubic feet, and the Measurement Ton, equivalent to 40 cubic feet. The calculation of tonnage is complicated by many technical factors.

Gross Tons - The entire internal cubic capacity of the ship expressed in tons of 100 cubic feet to the ton, except certain spaces with are exempted such as: peak and other tanks for water ballast, open forecastle bridge and poop, access of hatchways, certain light and air spaces, domes of skylights, condenser, anchor gear, steering gear, wheel house, galley and cabin for passengers.

Net Tons - Obtained from the gross tonnage by deducting crew and navigating spaces and allowances for propulsion machinery.

References

• The Oxford Companion To Ships & The Sea, by I. C. B. Dear and Peter Kemp. Oxford University Press, 1979. ISBN 0-19-860616-8
• Ton types, by Gregory M. Walsh, from Ocean Navigator Online.
• U.S. Military Sealift Command Glossary of ship terms
• How do you "weigh" a ship? from navweaps.com