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 water-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 the net tonnage, and, indeed, the tonnage of cargo carried is usually greater than the gross tonnage.
Displacement - 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.