Admeasurement and Tonnage
The tonnageof ships and boats is determined in several different ways; it may be a measurement of either (1) weight, or (2) size without regard to weight (volume). Persons unfamiliar with the term as it is used in connection with vessels are likely, in error, to think only of the common ton which is a measure of weight - either 2000 pounds in a short ton or 2240 pounds in a long ton. This is a part, but only a part, of the situation when ships are considered.
The determination of the "tonnage" of a vessel is called measurement or admeasurement, the latter term being found mainly in legal documents.
Gross and Net Tonnage
The documentation law repeatedly refers to vessels of a certain tonnage. Net tons are derived from gross tonnage which is a measurement of volume rather than weight.
Gross tonnage is the total enclosed space or internal capacity of a vessel, calculated in terms of "tons" of 100 cubic feet each. This was agreed upon many years ago as the average space or volume required by a ton (by weight) of general merchandise. Goss tonnage includes all spaces below the upper deck as well as permanently closed-in spaces on the deck.
Net (or registered) tonnage is a measurement of the earning power of a vessel when carrying cargo. Thus to arrive at a net tonnage figure it is necessary to deduct from the gross tonnage the volume of such spaces as would have no earning capacity or room for cargo. For example, on a ship there would be deducted the volume of the fuel compartments, engine room, crew's quarter, bridge, etc. Many charges against vessels, such as canal tolls, harbor dues, etc., are based on net tonnage.
Displacement and Deadweight Tonnage
Displacement tonnage is the actual weight, in tons of 2240 pounds, which a vessel displaces when floating at any given draft, such as "light" or "loaded." The displacement is calculated by figuring the volume of the vessel under the water in cubic feet and dividing by 35, as 35 cubic feet of sea water weighs one long ton.
Deadweight tonnage is the carrying capacity of a vessel figured by weight in terms of tons of 2240 pounds. If her displacement were calculated when the vessel was "light" (with fuel wnd supplies but no cargo) and again when she was "loaded" (with the same fuel and supplies aboard), the difference would express the deadweight tonnage.