There are five different kinds of tonnage. Ordinarily, in discussing shipping, three tonnage terms are commonly used: deadweight tonnage, gross tonnage, and net 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 particular application to cargo vessels carrying coal, iron, or other bulky commodities, enabling the ship's operator to know the maximum cargo weight that the ship can hold and thus determine the extent of loading.
Gross tonnage is commonly used with 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 gross tonnage.
In certain countries, gross tonnage is used as the basis for fixing ship subsidy payments. The term gross tonnage is held in official reports to express in units of 100 cubic feet the total cubical capacity of the vessel, including the spaces occupied by cabins, engines, boilers, and coal bunkers.
In this book's chapter on Measurement and Tonnage Laws, 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 the actual carrying capacity of cargo and passengers.
Net-register tonnage has been the basis for tonnage taxes and other tonnage dues worldwide. 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.
Estimating 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 calculate gross tonnage from deadweight tonnage, divide the deadweight tonnage by 1.6. One can give no precise rule for passenger and cargo-carrying ships in general.
Rules and usages vary in the computation of net tonnage from gross tonnage. Under the Suez Canal rules, the average deduction made from gross tonnage in determining net tonnage has been about 28 or 29 percent.
Under the national measurement rules of Great Britain, which have differed from the Suez, the deduction has been about 39 percent and the same percentage under those of Germany. Norway has allowed 37 percent, Denmark 41 percent, and France 42 percent, deduction. For the first year or so of the Panama Canal operation, the deduction was 30 percent.
In contrast, in the United States itself, where another official rule prevailed, the deduction was about 34 percent. 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 regulations evidently will cover deck cargoes as well as other cargoes.
There is no uniformity in terms of tonnage among the different nations of the world reporting on their merchant marine. Some countries give statistics of gross tons. Others report in net tons. This diversity tends to confuse comparative statistics. Shipping men believe that an international standard is advisable to ensure better understanding and accuracy in the world's returns of tonnage.
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. A merchant vessel's displacement "light" usually means its weight with its crew and supplies, but before any fuel, stores, cargo, or passengers have been taken aboard.
Displacement "loaded" is the vessel's weight, 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 typical 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. In the United States and British countries, a weight ton is the English long or gross ton of 2,240 pounds.
Countries that utilize 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 used in American overseas trade where goods are shipped as weight cargo.
However, a large amount of ocean freight 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.