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Net tonnage, also known as NT or N.T., is a method of calculation for how much cargo space a ship has. It is not a measure of weight or mass, or the displacement weight of a ship, but instead a volume measurement. Each ton in a tonnage figure is equivalent to 100 cubic feet (2.83 cubic meters) of space. Gross tonnage is the complete physical volume of space a cargo ship's hold has, but net tonnage is the practical or useful region of this space that can actually store cargo.
The rules for net tonnage were put in place to create uniform rules for such considerations as import and export taxes at ports and serviceability standards for shipyards, but the rules weren't actually put into full force by the International Maritime Organization until 18 July 1982. All signatory nations to the Convention must apply by its rules with the exception of any ships flagged by these nations that are categorized as ships of war and ships less than 79 feet (24 meters) in length. Cargo ships that move within certain territorial waters are also exempt, such as in the Great Lakes in North America, the Caspian Sea, and the rivers of Uruguay and Argentina. Ships in foreign ports that are flagged under provision of the Convention agree to inspections by port authorities for confirmation of compliance with correct net tonnage statements and certification as long as there is no delay in the departure of such ships.
As of 1998, at least 86 nations had either ratified or acceded to the Convention. When it was initially put into force in 1982, 48 nations that comprised over 80% of world shipping began complying with its rules. The United States did not fully implement such guidelines, however, until 1986, when the US Congress passed legislation to comply with its provisions for international shipping. The requirements of the Convention generally exclude all domestic shipping regardless of the net tonnage rating of the ships involved.
Calculating the net tonnage for the capacity of a ship's cargo is a rather complicated matter. It involves allowances for gross tonnage, for the number of passengers and passenger berths allowed for the type of ship, and the molded shape of the ship itself. Generally, it is based on 96% of the total length of a ship at its waterline and 85% of its molded depth measured from the top of the keel to the axis of the rudder stock on the waterline.
Ship construction can also be a variable in calculating net tonnage, as certain classes of ships such as oil tankers have segregated ballast tanks that comprise a significant volume of the ship which cannot be used for cargo. All cargo compartments must be labeled with the term “CC” and ship owners are required to report any changes in cargo volume due to ship construction or passenger allotment changes. Allowable draught is also an important factor in determining net tonnage cargo capacity, as it affects the practical maneuverability of a vessel. It is the level to which a loaded ship sinks in water as a measurement from the waterline to the lowest point of the submerged hull.