A term of maritime law and taxation law.
Tonnage was first used as a feature of tax law: a custom or tax paid by an importer to the government of the receiving country according to a specified rate per ton of merchandise.
Now mostly used in the context of maritime law, the term has different meanings as was pointed out by Edgar Gold in his book Maritime Law:
"In general, tonnage can refer to any of three shipping aspects: the size of the vessel, the amount of cargo carried by a ship, and the ship's spatial (volumetric) capacity."
In most instances of maritime law, tonnage is not an actual weight measurement, but a cubic measurement of a ship's capacity.
Tonnage is measured very carefully and formal certificates are issued in regards thereto.
The International Convention on Tonnage Measurement of Ships, 1969 includes a complex formula (for measuring tonnage and for which an International Tonnage Certificate is issued, and to which many nations now subscribe.
Other legal usages of tonnage:
- Cargo tonnage: the cargo capacity of a vessel, expressed in weight or volume.
- Deadweight tonnage: the actual cargo carrying capacity of the ship, when she is fully loaded with fuel, passengers, workers and cargo; the maximum weight of cargo the ship can carry.
- Displacement tonnage: the weight of water displaced by the ship when she is fully loaded with all her crew, bunkers, stores and equipment. Although often used to describe a warship, displacement tonnage is rarely used in regards to cargo ships.
- Gross tonnage, also referred to as Gross register tonnage (GRT): the cubic capacity of a ship as a function of the ship's length, width, height and under and over-deck dimensions.
- Limitation tonnage:the vessel's registered tonnage, together with engine room space, but excluding space occupied by seamen.
- Net tonnage, also referred to as net registered tonnage: space on a ship available for cargo and passengers. The space on a ship that can be used to earn revenue.
- Tonnage tax: taxes paid by a shipping company based on the total tonnage of its ships. Tonnage tax is also used to tax liquids such as, for example assessing import taxes on wine or to determine mining taxes.
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