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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. 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. Tonnage is commonly used to assess fees on commercial shipping.
Tonnage measurements are governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982.
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 of gross register tonnage (GRT). 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 it derives from the cubic feet of volumetric capacity.
Net tonnage (NT) is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vessel's earning space and is a function of the moulded volume of all cargo spaces of the ship.
A commonly defined measurement system is important, since a ship's registration fee, harbour dues, safety and manning rules, and the like may be based on its gross tonnage (GT) or net tonnage (NT).
Gross register tonnage (GRT) represents the total internal volume of a vessel, where one register ton is equal to a volume of 100 cubic feet (2.83168 m3); a volume that, if filled with fresh water, would weigh around 2,800 kg or 2.8 tonnes. The definition (and calculation) of the internal volume is complex; for instance, a ship's hold may be assessed for bulk grain (accounting for all the air space in the hold) or for bales (omitting the spaces into which bulk, but not baled cargo, would spill). Gross register tonnage was replaced by gross tonnage in 1982 under the Tonnage Measurement convention of 1969, with all ships measured in GRT either scrapped or re-measured in GT by 1994.
Net register tonnage (NRT) is the volume of cargo the vessel can carry—that is, the gross register tonnage less the volume of spaces that do not hold cargo (e.g., engine compartment, helm station, and crew spaces, again with differences depending on which port or country does the calculations). It represents the volume of the ship available for transporting freight or passengers. It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.
The Panama Canal/Universal Measurement System (PC/UMS) is based on net tonnage, modified for Panama Canal purposes. PC/UMS is based on a mathematical formula to calculate a vessel's total volume; one PC/UMS net ton is equivalent to 100 cubic feet of capacity.
Suez Canal Net Tonnage (SCNT) is derived with a number of modifications from the former net register tonnage of the Moorsom System and was established by the International Commission of Constantinople in its Protocol of 18 December 1873. It is still in use, as amended by the Rules of Navigation of the Suez Canal Authority, and is registered in the Suez Canal Tonnage Certificate.
While not tonnage in the proper sense, the following methods of ship measurement are often incorrectly referred to as such:
Deadweight tonnage (often abbreviated as DWT, for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like displacement, it is often expressed in long tons or in metric tons.
Metric tonnes per centimetre immersion (usually abbreviated to TPC or TPCMI) is the number of metric tonnes (1,000 kg) that need to be loaded on the ship for the salt water draft (draught) to increase by one centimetre. The TPCMI is used to calculate the draft of the vessel with a given deadweight tonnage of cargo loaded. For a typical Panamax bulk carrier with a TPCMI of 80, the ship will sink (i.e., its draft will increase) by one centimetre for every 80 tonnes of cargo loaded.
Imperial tons per inch immersion (usually abbreviated to TPI) is the number of imperial long tons (2,240 lb) that need to be loaded on a vessel for the draft to increase by one inch. Old imperial TPI measurements are still occasionally used within the United States and the Panama Canal. As no ship has been measured by a classification society since the 1950s using imperial measures, modern TPI figures are therefore a conversion from the original metric measurements and should not be relied upon to be accurate.
Tonnage can refer to the quantity of a mineral or the mineral ore extracted from a mine. It may refer to the production of any commodity that is normally expressed in tons or tonnes. The term can also apply to the total weight drawn by a railway locomotive, or the total weight of freight passing over a railway line or road.
The tonnage may be expressed in short tons (2,000 lb), metric tons or tonnes (1,000 kg), or in long tons (2,240 lb). Often this distinction is not of any importance, however sometimes it is critical to define the exact units in which the tonnage is expressed.
Historically, tonnage was the tax on tuns (casks) of wine that held 954 litres (252 gallons) of wine and weighed 1016 kilograms (2,240 pounds). This suggests that the unit of weight measurement, the long ton (1,016 kg or 2,240 lb), and tonnage share the same etymology. The confusion between weight-based terms (deadweight and displacement) stems from this common source and the eventual decision to assess dues based on a ship's deadweight rather than counting the tuns of wine. In 1720 the Builder's Old Measurement Rule was adopted to estimate deadweight from the length of keel and maximum breadth or beam of a ship. This overly simplistic system was replaced by the Moorsom System in 1854 and calculated internal volume, not weight. This system evolved into the current set of internationally accepted rules and regulations.
When steamships came into being, they could carry less cargo, size for size, than could sailing ships. In addition to spaces taken up by boilers and steam engines, steamships carried extra fresh water for the boilers and coal for the engines. Thus, to move the same volume of cargo as a sailing ship, a steamship would be considerably larger than a sailing ship.
Harbour dues are based on tonnage. In order to prevent steamships operating at a disadvantage, various tonnage calculations were established to minimize the disadvantage presented by the extra space requirements of steamships. Rather than charging by length, displacement, or the like, charges were calculated based on the viable cargo space. As commercial cargo sailing ships are now largely extinct, gross tonnage is becoming the universal method of calculating ships' dues, and is also a more straightforward and transparent method of assessment.