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.
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.
Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the convention, the correct term is now GT, which is a function of the moulded volume of all enclosed spaces of the ship.
GT is calculated by using the formula : , where V = total volume in m3 and K = a figure from 0.22 to 0.32, depending on the ship's size (calculated by : ). For a ship with a total volume of 10,000 m3, the gross tonnage would be 0.28 × 10,000 = 2,800 GT. GT is consequently a measure of the overall size of the ship. For a ship with a total volume of 80,000 m3, the gross tonnage would be 0.2980617 × 80,000 = 23,844.94 GT.
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