Displacement and Tonnage
There is a natural confusion between displacement and tonnage when used as measures of vessel size. In part, this arises from the unique use of the word “tonnage” in nautical settings—a ton is not necessarily a measure of weight!
Displacement, or displacement tonnage, is a measure of the weight of a vessel; more accurately, it is the weight of the fresh water displaced by the vessel (fresh water weighs less than salt water), expressed in long tons (one long ton = 2240 pounds). Tonnage is a measure of the cargo-carrying capacity of a vessel--one ton equals 100 cubic feet of capacity. This use of the word comes from the old English word tun, meaning a cask or barrel. Vessels entering English ports paid port entry taxes based on “tunnage,” defined as the number of standard-size (252 gallon) barrels that could be carried.
Measurement of tonnage is a precise calculation following strict international rules. The Gross Registered Tonnage of a vessel is the number of hundreds of cubic feet of space in enclosed places (hull, deckhouse, pilothouse). The Net Registered Tonnage is the gross tonnage less space used for propulsion machinery (engines, shafts, etc), piloting, living space and other exempted areas.
Myeerah has a displacement of 204,000 pounds, or about 91 long tons. Her registered tonnage is 157 gross tons and 47 net tons.
For additional information, select the "Tonnage" section at www.uscg.mil/hq/msc.