Tonnage

From Free net encyclopedia

Tonnage is a measure of the size or cargo capacity of a ship.

Tonnage varies in meaning depending on the vessel. It can refer to the weight of a loaded or empty vessel, or to its volume or its cargo volume. Measurement of tonnage can be less than straightforward, not least because it is used to assess fees on commercial shipping. Port authorities, naval architects, and owners may have different approaches to calculating tonnage.

1 Systems of tonnage
- 1.1 Volume-based systems
- 1.2 Weight-based systems
2 Origins
3 See also
4 References

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Systems of tonnage

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Volume-based systems

Gross Register Tonnage represents the total internal volume of a vessel, with some exemptions for non-productive spaces such as crew quarters; 1 gross register ton is equal to a volume of 100 cubic feet (2.83 m³). This calculation is complex; a hold can, for instance, be assessed for grain (accounting for all the air space in the hold) or for bales (exempting the spaces between structural frames). Gross register tonnage was replaced by gross tonnage in 1994, under the Tonnage Measurement convention of 1969.Template:Ref Template:Ref

Net Register Tonnage 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 passengers. It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.

Gross Tonnage 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.

Net tonnage is based on a calculation of the volume of all cargo spaces of the ship.

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 cubic feet of capacity.Template:Ref

Thames measurement tonnage was used for smaller, generally unregistered, vessels, used for calculating port dues. It was another volumetric system, based on a formula created by the Thames Yacht Club: (length - beam) x beam x ½beam / 94. The formula was also used in some early handicapping systems for yacht racing.Template:Ref

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Weight-based systems

Displacement Tonnage is the actual weight of the vessel and its contents. It is often used to rate naval vessels, since their weight is fairly constant and they are not subject to the kinds of port fees that are calculated on Register Tonnage. It is often expressed in long tons or in metric tons, and is calculated simply by multiplying the volume of the hull below the waterline (ie. the volume of water it is displacing) by the density of the water. (Note that the density will depend on whether the vessel is in fresh or salt water, or is in the tropics, where water is warmer and hence less dense.) For example, in sea water, multiply the volume in cubic feet 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 tons.

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.

Light ship tonnage measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.

Deadweight Tonnage is the maximum weight that a ship can safely carry when fully loaded. It includes the crew, passengers, cargo, fuel, water, and stores. Like Displacement Tonnage, it is often expressed in long tons or in metric tons.Template:Ref

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Origins

Historically, Tonnage and Poundage was the tax on tuns (casks) of wine shipped to England, mostly from Spain and Portugal, under a subsidy granted to the English crown by Parliament starting in the 14th century.