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Measurement of tonnage can be less than straightforward, not least because it is used to assess Fee s on Commercial Shipping . TONNAGE MEASUREMENTS Gross Register Tonnage ('''GRT''') represents the total internal , 2006 . Net Register Tonnage ('''NRT''') is the volume of cargo the vessel can carry; ie. 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 Passenger s. It was replaced by ''net tonnage'' in 1994, under the Tonnage Measurement convention of 1969. Gross Tonnage ('''GT''') refers to the volume of all ship's enclosed spaces (from Keel to Funnel ) measured to the outside of the hull framing. It is always larger than ''gross register tonnage'', though by how much depends on the vessel design. . It was a measurement of the enclosed spaces within a ship expressed in "tons" – a unit which was actually equivalent to 100 cubic feet. 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 :). GT is consequently a measure of the overall size of the ship. 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. 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 . Retrieved May 10 , 2006 . Thames Measurement Tonnage is another volumetric system, generally used for small vessels such as Yacht s; it uses a formula based on the vessel's length and beam. Many people in many countries, including those professional people working in maritime industries for many years or even in their lifetime, often confuse "Tonnage" and "Ton". Please note that "Tonnage" refers to the unit of a ship's volume in measurement for registration and "Ton" refers to the unit of weight. They are totally different in concept. WEIGHT MEASUREMENTS While not "tonnage" in the proper sense, the following methods of ship measurement are often incorrectly referred to as such: ''' of the hull to get the hull volume in Cubic Feet . Then multiply this figure by 64 (the weight of one cubic foot of seawater) to get the weight of the ship in pounds; or divide by 35 to calculate the weight in Long Ton s. Using the SI or metric system : displacement (in tonnes) is volume (in m3) multiplied by the specific gravity of sea water (1.025 nominally). The word "displacement" arises from the basic physical law, discovered by Archimedes , that the weight of a floating object equates exactly to that of the water which would otherwise occupy the "hole in the water" displaced by the ship. Lightship measures the actual weight of the ship with no Fuel , passengers, Cargo , water, etc. on board. Deadweight (often abbreviated as '''DWT''' for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, , 2006 . ORIGINS Historically, tonnage was the tax on ''tuns'' ( Cask s) of Wine that held approximately 252 gallons of wine and weighed approximately 2,240 Pounds . This suggests that the unit of weight measurement, Long Ton s (also 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 straight-forward and transparent method of assessment. SEE ALSO REFERENCES
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