Net tonnage is a measure of a ship’s cargo space, not weight, and is calculated based on volume. The International Maritime Organization established rules for net tonnage in 1982, which have been ratified by at least 86 nations. Calculating net tonnage is complex and takes into account factors such as the ship’s length, depth, and construction.
Net tonnage, also known as NT or NT, is a method of calculating how much cargo space a ship has. It is not a measure of weight or mass, or the displacement weight of a ship, but a measure of volume. Each ton in a tonnage figure is equivalent to 100 cubic feet (2.83 cubic meters) of space. Gross tonnage is the entire physical volume of a cargo ship’s hold space, but net tonnage is the practical or usable region of this space that can actually store cargo.
The rules for net tonnage were established to create uniform rules for such considerations as import and export taxes at ports and service standards for shipyards, but the International Maritime Organization did not put the rules into effect until December 18. July 1982. All signatory nations to the Convention must apply its rules with the exception of any ships marked by these nations that are classified as warships and ships less than 79 feet (24 meters) in length. Cargo ships moving within certain territorial waters are also exempt, such as in the North American Great Lakes, the Caspian Sea, and the rivers of Uruguay and Argentina. Ships in foreign ports that are marked under the provision of the Convention agree to inspections by port authorities to confirm compliance with correct net tonnage declarations and certification, as long as there are no delays in the departure of such ships.
By 1998, at least 86 nations had ratified or acceded to the Convention. When it initially came into force in 1982, 48 nations comprising more than 80% of world shipping began to comply with its rules. However, the United States did not fully implement these guidelines until 1986, when the United States Congress passed legislation to comply with its provisions for international shipping. The Convention’s requirements generally exclude all domestic shipments, regardless of the net tonnage classification of the vessels involved.
Calculating the net tonnage for a ship’s carrying capacity is quite a 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. In general, it is based on 96% of a ship’s total length at its waterline and 85% of its molded depth measured from the top of the keel to the rudder stock axis at the waterline.
Ship construction can also be a variable in the net tonnage calculation, as certain classes of ships, such as oil tankers, have segregated ballast tanks that comprise a significant volume of the ship that cannot be used for cargo. All cargo compartments must be labeled with the term “CC” and ship owners must report any changes in cargo volume due to ship construction or changes in passenger allocation. Allowable draft is also an important factor in determining net tonnage carrying capacity, as it affects the practical maneuverability of a ship. It is the level to which a loaded ship sinks in the water as a measure from the waterline to the lowest point of the submerged hull.
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