Tonnage is a measure of the capacity of a

ship A ship is a large watercraft, vessel that travels the world's oceans and other Waterway, navigable waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally disti ...

, and is commonly used to assess

fee A fee is the price one pays as remuneration for rights or services. Fees usually allow for overhead, wages, costs, and markup. Traditionally, professionals in the United Kingdom (and previously the Republic of Ireland) receive a fee in contrad ...

s on

commercial Commercial may refer to: * (adjective for) commerce, a system of voluntary exchange of products and services ** (adjective for) trade, the trading of something of economic value such as goods, services, information or money * a dose of advertising ...

shipping Freight transport, also referred to as freight forwarding, is the physical process of transporting commodities and merchandise goods and cargo. The term shipping originally referred to transport by sea but in American English, it has been ...

. The term derives from the taxation paid on '' tuns'' or casks of wine. In modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Although tonnage (volume) should not be confused with

displacement Displacement may refer to: Physical sciences Mathematics and physics *Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...

(the actual mass of the vessel), the

long ton The long ton, also known as the imperial ton, displacement ton,Dictionary.com - ''"a unit for measuring the displacement of a vessel, equal to a long ton of 2240 pounds (about 1016 kg) or 35 cu. ft. (1 cu. m) of seawater."'' or British ton, is a ...

(or imperial ton) of 2,240 lb is derived from the fact that a " tun" of wine typically weighed that much.

Current maritime units

Tonnage measurements are governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which initially applied to all ships built after July 1982, and to older ships from July 1994.''International Convention on Tonnage Measurement of Ships, 1969''
, International Maritime Organisation. Retrieved May 10, 2006. A commonly defined measurement system is important, since a ship's registration fee, harbour dues, safety and manning rules, and the like may be based on its gross tonnage (GT) or net tonnage (NT).

Gross tonnage

Gross tonnage (GT) is a function of the volume of all of a ship's enclosed spaces (from

keel The keel is the bottom-most longitudinal structural element of a watercraft, important for stability. On some sailboats, it may have a fluid dynamics, hydrodynamic and counterbalancing purpose as well. The keel laying, laying of the keel is often ...

funnel A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening. Funnels are usually made of stainless steel, aluminium, glass, or plastic. The material used in its constructi ...

) measured to the outside of the hull framing. The numerical value for a ship's GT is always smaller than the numerical values of '' gross register tonnage (GRT).'' Gross tonnage is therefore 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 it derives from the volumetric capacity in cubic metres.

Net tonnage

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.

Panama Canal/UMS

The Panama Canal/Universal Measurement System (PC/UMS) is based on net tonnage modified for

Panama Canal The Panama Canal () is an artificial waterway in Panama that connects the Caribbean Sea with the Pacific Ocean. It cuts across the narrowest point of the Isthmus of Panama, and is a Channel (geography), conduit for maritime trade between th ...

purposes. PC/UMS is based on a mathematical formula to calculate a vessel's total volume; one PC/UMS net ton is equivalent to of capacity.

Suez Canal Net Tonnage

The Suez Canal Net Tonnage (SCNT) is derived with a number of modifications from the former net register tonnage of the

Moorsom System The Moorsom System is a method created in the United Kingdom of calculating the tonnage Tonnage is a measure of the capacity of a ship, and is commonly used to assess fees on commercial shipping. The term derives from the taxation paid on '' ...

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.

Thames measurement tonnage

Thames measurement tonnage (TM) is another volumetric system, generally used for small vessels such as

yacht A yacht () is a sail- or marine propulsion, motor-propelled watercraft made for pleasure, cruising, or racing. There is no standard definition, though the term generally applies to vessels with a cabin intended for overnight use. To be termed a ...

s; it uses a formula based on the vessel's length and beam.

Historical maritime units

Traditional casks

Historically in

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

, tunnage was the medieval import duty on tuns of wine. A tun was a large size of

cask A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids ...

s used for

wine Wine is an alcoholic drink made from Fermentation in winemaking, fermented fruit. Yeast in winemaking, Yeast consumes the sugar in the fruit and converts it to ethanol and carbon dioxide, releasing heat in the process. Wine is most often made f ...

), used in the wine trade. The number of tuns that a ship could carry was used as a measure of the size of the ship. The wine trade to England originated in France, which is where the tuns were made. A French standard tun cask size was established about 1450. The 15th century Bordeaux wine tun was between . When measuring the tonnage of a ship, the approximately cylindrical cask would have air space around it when stowed in a ship. Therefore the volume of hold space required for several tuns was greater than the total of the capacity of those tuns. 252 imperial gallons is just over . British practice by the 19th century was for a ton of cargo volume to be . This is derived from an estimate of the hull space needed to fit the roughly cylindrical tun and is broadly consistent with the much earlier French definition given below. The Bordeaux tun was used as a measurement elsewhere in Europe. By the 16th century, multi-decked ships which were loaded through hatchways (as opposed to earlier undecked or single decked ships) found it more convenient to use a smaller size of standard barrel. This was the , measuring a quarter of the size of the tun (in English, this barrel was termed a hogshead). For instance,

Basque Basque may refer to: * Basques, an ethnic group of Spain and France * Basque language, their language Places * Basque Country (greater region), the homeland of the Basque people with parts in both Spain and France * Basque Country (autonomous co ...

ships engaged in 16th century whaling in

Labrador Labrador () is a geographic and cultural region within the Canadian province of Newfoundland and Labrador. It is the primarily continental portion of the province and constitutes 71% of the province's area but is home to only 6% of its populatio ...

used this size of barrel (with the name ). The French was legally defined, in 1681, as the cubic space into which four fitted, with the casks aligned two directly above the two below (so not optimising the layout). This redefined ton worked out as 42 cubic (. The difference between this measure and, for instance, the Spanish was calculated by recognised adjustment factors.

Purpose of measuring tonnage

Tonnage measurement was important for an increasing number of reasons through history. In England in the

Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of global history. It began with the fall of the Western Roman Empire and ...

, ships were often impressed by the crown for military use. To do this in an efficient and speedy manner, a measurement of size was needed. The payment to the owner of the requisitioned ship was based on the tonnage. Port dues and various licences were based on tonnage, and it was a useful measure for a ship builder who needed to build a vessel that met the new owner's requirements. In the

Tudor period In England and Wales, the Tudor period occurred between 1485 and 1603, including the Elizabethan era during the reign of Elizabeth I (1558–1603). The Tudor period coincides with the dynasty of the House of Tudor in England, which began with ...

a bounty per ton was paid for the construction of larger ships, so the threshold for that payment had to be determined, as well as the amount paid for those qualifying.

Method of measuring

In the middle ages, the normal way of discovering the tonnage of a ship was to load her with wine and see how many tuns could be fitted in. There is an instance of the owner of a new ship, in 1459, being challenged that his safe-conduct was for a 400 ton vessel, whilst he had already loaded more than 600 tons. The excuse was accepted that he had no idea of her tonnage until she was loaded. In another case, in 1456, a dispute over the actual tonnage of a ship had to be resolved by having coopers part load her with (presumably empty) barrels to estimate what she could carry. The Tudor bounty paid for the construction of larger ship was apparently paid without any measurement system to confirm the actual size. The presumption is that it was taken from the amount of cargo unloaded after the first voyage (which would be recorded in the customs records of the relevant port).

Gross register tonnage

Gross register tonnage (GRT) represents the total internal

volume Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...

of a vessel, where one register ton is equal to a volume of ; a volume that, if filled with fresh water, would weigh around 2.83

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1,000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...

s. The definition and calculation of the internal volume is complex; for instance, a ship's hold may be assessed for bulk grain (accounting for all the air space in the hold) or for

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s (omitting the spaces into which bulk, but not baled cargo, would spill). Gross register tonnage was replaced by ''gross tonnage'' in 1982 under the Tonnage Measurement convention of 1969, with all ships measured in GRT either scrapped or re-measured in GT by 1994.''CWP Handbook of Fishery Statistical Standards''
Retrieved May 10, 2006.

Net register tonnage

Net register tonnage (NRT) is the volume of cargo the vessel can carry—that is, the gross register tonnage less the volume of spaces that do not hold

cargo In transportation, cargo refers to goods transported by land, water or air, while freight refers to its conveyance. In economics, freight refers to goods transported at a freight rate for commercial gain. The term cargo is also used in cas ...

(e.g.,

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...

compartment, helm station, and

crew A crew is a body or a group of people who work at a common activity, generally in a structured or hierarchy, hierarchical organization. A location in which a crew works is called a crewyard or a workyard. The word has nautical resonances: the ta ...

spaces, again with differences depending on which

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Hamburg, Manch ...

country A country is a distinct part of the world, such as a state, nation, or other political entity. When referring to a specific polity, the term "country" may refer to a sovereign state, state with limited recognition, constituent country, ...

does the calculations). It represents the volume of the ship available for transporting

freight In transportation, cargo refers to goods transported by land, water or air, while freight refers to its conveyance. In economics, freight refers to goods transported at a freight rate for commercial gain. The term cargo is also used in ...

passenger A passenger is a person who travels in a vehicle, but does not bear any responsibility for the tasks required for that vehicle to arrive at its destination or otherwise operate the vehicle, and is not a steward. The vehicles may be bicycles, ...

s. It was replaced by ''net tonnage'' in 1994, under the Tonnage Measurement convention of 1969.

Notes

References

Bibliography

* ''The Oxford Companion To Ships & The Sea'', by I. C. B. Dear and Peter Kemp. Oxford University Press, 1979. * ''Ship Design and Construction, Volume II''; Thomas Lamb, Editor. Society of Naval Architects and Marine Engineers, 2004. * {{Authority control Volume Mass Nautical terminology Ship measurements