Ship Tonnage Explained - Deadweight, Cargo, Gross, Net, Displacement
Everyone who has looked at specifications for steamships is often bewildered by the many different tonnages used for the same vessel. An ocean liner may have different gross tonnage, depending on which country's rules were used in determining the weight. Below is an article from 1932 that provides a good explanation on just what the tonnages for ships really are.
There are five kinds of tonnage in use in the shipping business.
- Net and
- Displacement Tonnages
- Deadweight Tonnage: expresses the number of tons of 2,240 pounds that a vessel can transport of cargo, stores, and bunker fuel. It is the difference between the number of tons of water a vessel displaces "light" and the number of tons it displaces when submerged to the "load line." Deadweight tonnage is used interchangeably with deadweight carrying capacity. A vessel's capacity for weight cargo is less than its total deadweight tonnage.
- Cargo Tonnage: is either "weight" or "measurement." The weight ton in the United States and in British countries is the English long or gross ton of 2,240 pounds. In France and other countries having the metric system a weight ton is 2,204.6 pounds. A "measurement" ton is usually 40 cubic feet, but in some instances a larger number of cubic feet is taken for a ton. Most ocean package freight is taken at weight or measurement (W/M) ship's option.
- Gross Tonnage: applies to vessels, not to cargo. It is determined by dividing by 100 the contents, in cubic feet, of the vessel's closed-in spaces. A vessel ton is 100 cubic feet. The register of a vessel states both gross and net tonnage.
- Net Tonnage: is a vessel's gross tonnage minus deductions of space occupied by accommodations for crew, by machinery, for navigation, by the engine room and fuel. A vessel's net tonnage expresses the space available for the accommodation of passengers and the stowage of cargo. A ton of cargo in most instances occupies less than 100 cubic feet; hence the vessel's cargo tonnage may exceed its net tonnage, and, indeed, the tonnage of cargo carried is usually greater than the gross tonnage.
- Displacement: of a vessel is the weight, in tons of 2,240 pounds, of the vessel and its contents. Displacement "light" is the weight of the vessel without stores, bunker fuel, or cargo. Displacement "loaded" is the weight of the vessel plus cargo, fuel, and stores.
For a modern freight steamer the following relative tonnage figures would ordinarily be approximately correct:
- Net tonnage 4,000
- Gross tonnage 6,000
- Deadweight carrying capacity 10,000
- Displacement, loaded, about 13,350
A vessel's registered tonnage, whether gross or net, is practically the same under the American rules and the British rules. When measured according to the Panama or Suez tonnage rules most vessels have larger gross and net tonnages than when measured by British or American national rules.
Source: Article appearing in the American Export Lines, Passenger List from June 28, 1932.