Tonnage Definitions: Deadweight, Gross, Net, Displacement, and Cargo.
There are five different kinds of tonnage. Ordinarily, three tonnage terms are commonly used in discussing shipping: deadweight tonnage, gross tonnage, and net tonnage. The other two kinds are displacement tonnage and cargo tonnage.
Deadweight 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. It is expressed in terms of the long ton of 2,240 pounds or the metric ton of 2,204.6 pounds.
Deadweight tonnage forms customary 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
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.
This book's chapter on Measurement and Tonnage Laws has itemized nine different spaces exempt under United States rules from gross tonnage measurement.
Net Tonnage
Net tonnage construes the net ton as equaling 100 cubic feet of carrying capacity, excluding deductions for space occupied by cabins, machinery, fuel, etc. 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 Customs House statistics of ship entrances and clearances and those of many other governments are reported regarding 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, the deadweight tonnage from gross tonnage is multiplied 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 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, 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 the actual earning capacity of a vessel instead of net tonnage. The proposed regulations will likely cover deck cargoes as well as other cargoes.
There is no uniformity in tonnage among the world's different nations reporting on their merchant marine. Some countries give statistics of gross tons. Others report in net tons. This diversity needs to be clarified for comparative statistics. Shipping men believe an international standard is advisable to ensure better understanding and accuracy in the world's tonnage returns.
Displacement Tonnage
Displacement tonnage signifies the weight of a vessel, in tons of 2,240 pounds, and is equal to the weight of water the ship displaces. A merchant vessel's displacement "light" usually means its weight with its crew and supplies 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 draft. The term displacement tonnage is familiar because of its typical application to war vessels.
Cargo Tonnage
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.
