Ship measurements

Measuring the size of ships is a confusing field, filled with non-intuitive terms that are confusing to non-specialists. This article provides definitions for many of the terms used in measuring ships.

Definitions

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

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 unit of measure is the Measurement Ton, equivalent to 100 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.

Register Tons - Register tonnage is applicable to both gross and net, in other words it can be expressed as gross register tonnage or net register tonnage.

Displacement - A measurement of the weight of the vessel, in long tons of 2,240 pounds. The laws of physics say that a floating object such as a ship displaces a weight of water equal to its own weight.

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.

Deadweights Tons - The Difference between displacement, light and displacement loaded, in tons of 2,240 pounds

Cargo Deadweight Tons - The number of tons (2,240 pounds per ton) which remain after deducting fuel, water, stores, dunnage and such other items necessary for use on a voyage from the deadweight of the vessel.

Cubic - Cargo space measured in cubic feet.

Grain Cubic - The maximum space avaiable for cargo measured in cubic feet, the measurements 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 beams or under side of deck plating. In other words, if a bulk cargo were loaded such as grain, it would flow in between the frames and beams and occupy the maximum space avaiable.

Bale Cubic - The space avaiable for cargo measured in cubic feet to the inside of the cargo battens, on the frames, and to the underside of the beams. In a general cargo of mixed commodities the bale cubic applies. The stowage of the mixed cargo comes in contact with the cargo battens and as a general rule does not extend to the skin of the ship.

Ton - The unit of measure often used in specifying the size of a ship. There are two completely unrelated definitions for the word.

Measurement Ton or Ship Ton - Calculated as 40 cubic feet, volumetric or space meassurement. (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.

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

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.