Gross register tonnage

Gross register tonnage (GRT, grt, g.r.t.) a ship's total internal volume expressed in "register tons", each of which is equal to 100 cubic feet (2.83 m³). Gross register tonnage uses the total permanently enclosed capacity of the vessel as its basis for volume.

Net register tonnage subtracts the volume of non-revenue-earning spaces (i.e. spaces not available for carrying cargo, for example engine rooms, fuel tanks and crew quarters) from gross register tonnage.

Gross register tonnage is not a measure of the ship's weight or displacement and should not be confused with terms such as deadweight tonnage or displacement.

History

Gross register tonnage was defined by the Moorsom Commission in 1854. Gross and net register tonnages were replaced by gross tonnage and net tonnage, respectively, when the International Maritime Organization (IMO) adopted The International Convention on Tonnage Measurement of Ships on 23 June 1969. The new tonnage regulations entered into force for all new ships on 18 July 1982, but existing vessels were given a migration period of 12 years to ensure that ships were given reasonable economic safeguards, since port and other dues are charged according to ship's tonnage.

Tonnage

Tonnage is a measure of the cargo-carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Tonnage should not be confused with displacement, which refers to the actual weight of the vessel.

Measurement of tonnage can be complicated, not least because it is used to assess fees on commercial shipping.

Tonnage measurements

Gross tonnage (GT) is a function of the volume of all of a ship's enclosed spaces (from keel to funnel) measured to the outside of the hull framing. The numerical value for a ship's GT is always smaller than the numerical values for both its gross register tonnage and the GRT value expressed equivalently in cubic meters rather than cubic feet (for example, 0.5919 GT = 1 GRT = 2.8316 m³; 200 GT = 274 GRT = 775,88 m³; 500 GT = 665 GRT = 1,883.07 m³; 3,000 GT = 3,776 GRT = 10,692.44 m³), though by how much depends on the vessel's design (volume). There is a sliding-scale factor. Gross tonnage is therefore a kind of capacity-derived index that is used to rank a ship for purposes of determining manning, safety, and other statutory requirements and is expressed simply as GT, which is a unitless entity, even though its derivation is tied to the cubic meter unit of volumetric capacity.