Ordinarily, in discussing shipping, three different tonnage terms are commonly used: deadweight tonnage, gross tonnage, and net tonnage. There are five different kinds of tonnage. The other two kinds are displacement tonnage and cargo tonnage.
Deadweight tonnage signifies the maximum weight of cargo, bunkers, consumable stores and all weight including passengers and crew that a vessel carries when loaded to its deep-load line. Deadweight tonnage is a term used interchangeably with deadweight carrying capacity, and is expressed in terms of either the long ton of 2,240 pounds or the metric ton of 2,204.6 pounds. Deadweight tonnage forms the customary basis for charter rates on ocean going vessels engaged on time charters. It is of special application to cargo vessels carrying coal, iron or other bulky commodities, enabling the ship’s operator to know the maximum weight of cargo that the vessel can carry and thus determine the extent of loading.
Gross tonnage is the term commonly used in relation to merchant vessels, and applies to vessels, not cargo. The official merchant marine statistics of the United States, Great Britain and some other countries are published in terms of gross tonnage, and in certain countries gross tonnage is used as the basis for the fixing of ship subsidy payments. The term gross tonnage is held in official reports to express in units of 100 cubic feet the entire cubical capacity of the vessel, including the spaces occupied by cabins, engines, boilers and coal bunkers. In the chapter on Measurement and Tonnage Laws in this book there are itemized nine different spaces exempt under United States rules from gross tonnage measurement.
Net tonnage construes the net ton as equaling 100 cubic feet of carrying capacity, exclusive of deductions for space occupied by cabins, machinery, fuel, etc. It is therefore a vessel’s gross tonnage minus certain deductions permitted by law and taken broadly signifies the amount of space available for actual carrying capacity of cargo and passengers. Throughout the world net-register tonnage has been the basis for tonnage taxes and other tonnage dues. Towage, dockage, and wharfage charges are often based on net register-tonnage. The United States Custom House statistics of ship entrances and clearances and those of many other governments are reported in terms of net-register tonnage. Canal tolls are widely based in part or wholly on net tonnage.
The estimating of these different kinds of tonnages varies according to the type of ship. There is no arbitrary standard of measurement. To compute, for a one-type freighter, deadweight tonnage from gross tonnage multiply gross tonnage by 1.6 which gives the deadweight tonnage. To compute gross tonnage from deadweight tonnage divide the deadweight tonnage by 1.6. No precise rule can be given for passenger and cargo carrying ships in general.
In the computation of net tonnage from gross tonnage, rules and usages vary. Under the Suez Canal rules the average deduction made from gross tonnage in determining net tonnage has been about 28 or 29 per cent. Under the national measurement rules of Great Britain, which have differed from the Suez, the deduction has been about 39 per cent, and the same percentage under those of Germany. Norway has allowed 37 per cent., Denmark 41 per cent., and France 42 per cent, deduction. For the first year or so of Panama Canal operation the deduction was 30 per cent., while in the United States itself, where another official rule prevailed, the deduction was about 34 per cent. On October 1, 1919, the House of Representatives passed a bill directing that in Panama Canal tolls rules of measurement based on actual earning capacity of a vessel govern instead of net tonnage. The proposed rules evidently will cover deck cargoes as well as other cargoes.
Among the different nations of the world reporting on their merchant marine there is no uniformity in terms of tonnage. Some nations give statistics of gross tons, others report in net tons. This diversity tends to confuse comparative statistics. Shipping men believe that to insure better understanding and accuracy in the world’s returns of tonnage, an international standard is advisable.
Displacement tonnage signifies the weight of a vessel, in tons of 2,240 pounds, and is equal to the weight of water displaced by the ship. The displacement “light” of a merchant vessel usually means its weight with that of its crew and supplies but before any fuel, stores, cargo, or passengers have been taken aboard. Displacement “loaded” is the weight of the vessel including cargo, fuel, and stores, and when fully loaded to its maximum deep-load line. “Actual” displacement is the vessel’s weight when loaded to any given draft. The term displacement tonnage is familiar because of its customary application to war vessels.
Cargo tonnage means the various forms of cargo tons and tonnage expressing the quantity of cargo and cargo capacity on an ocean-going vessel. Cargo tonnage may be recorded either in weight or measurement tons. A weight ton in the United States and in British countries is the English long or gross ton of 2,240 pounds. Countries having the metric system adhere to a weight ton of 2,204.6 pounds. Although the short ton of 2,000 tons is commonly used on railroad freight shipments in the United States, the long ton is usually the kind used in American overseas trade where goods are shipped as weight cargo. A large amount of ocean freight, however, is shipped not by weight but in units of measurement tons. A measurement ton is usually 40 cubic feet. “Measurement cargo” means light package freight the quantity of which is computed in measurement tons.Bankers Trust Company, “Chapter XXIX: Tonnage Definitions,” in America’s Merchant Marine: A Presentation of Its History and Development to Date with Chapters on Related Subjects, New York: Bankers Trust Company, 1920, pp. 236-239.