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Tonnage is a measure of the size or 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 loaded or empty weight of the vessel itself.
Gross tonnage (GT) is a function of the volume of all 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 her 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 design (volume). There is a sliding scale factor. So GT is 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 to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.
It is calculated by using the formula : , where V = total volume in m3 and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by : ), so that, for a ship of 10,000 m3 total volume, the gross tonnage would be 0.28 × 10,000 = 2,800. GT is consequently a measure of the overall size of the ship. For a ship of 80,000 m3 total volume the gross tonnage would be 0.2980617 × 80,000 = 23,844.94 GT.
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 etc., are based on its gross tonnage, GT, or net tonnage, NT.
Gross register tonnage (GRT) represents the total internal volume of a vessel, where a register ton is equal to a volume of 100 cubic feet (2.83168 m3), which volume, 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; a ship's hold can, for instance, 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). If V stands for the total internal volume in m3, then the GRT equals V / 2.83168, so for a ship of 10,000 m3 total internal volume, the gross register tonnage is 10,000 / 2.83168 = 3531.47 GRT. Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, and is no longer a widely used term in the industry.
Net register tonnage (NRT) is the volume of cargo the vessel can carry; i.e., the gross register tonnage less the volume of spaces that will not hold cargo (e.g., engine compartment, helm station, crew spaces, etc., again with differences depending on which port or country is doing 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; a PC/UMS net ton is equivalent to 100 cubic feet of capacity.
The 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 (1,000kg) that need to be loaded on the ship for the salt water draft to increase by one centimetre.
Imperial Tons per Inch immersion (usually abbreviated to TPI) is the number of Imperial Long Tons (2,240 lbs) 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 USA 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.
The TPCMI figure is used to calculate the draft of the vessel with a given DWT of cargo loaded. On a typical Panamax Bulk carrier with a TPCMI of 80, it would mean that the ship will sink (Draft increase) by one centimetre for every 80 tonnes of cargo loaded.
Draft The distance, usually measured in meters, between the lowest point of the keel and the waterline which varies dependant on the load (DWT) the vessel has on board.
Tonnage can be 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 lbs), metric tons or tonnes (1,000 kg), or in long tons (2,240 lbs). 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, long tons (also 1016 kilograms or 2,240 lb) and tonnage both 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 sailing ships. As well as spaces taken up by boilers and steam engines, steamships carried extra fresh water for the boilers as well as 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 minimise the disadvantage that the extra space requirements of steamships presented. Rather than charging by length or displacement etc., charges were calculated 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.