Tonnage Totally Explained

Tonnage is a measure of the size or cargo capacity of a ship. The term derives from the taxation paid on tuns of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. The term is still sometimes incorrectly used to refer to the weight of a loaded or empty vessel. Measurement of tonnage can be less than straightforward, not least because it's used to assess fees on commercial shipping.

Tonnage measurements

Gross Register Tonnage (GRT) 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, but is still a widely used term in the industry. Net Register Tonnage (NRT) is the volume of cargo the vessel can carry; ie. the Gross Register Tonnage less the volume of spaces that won't hold cargo (for example 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 (GT) 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. It was a measurement of the enclosed spaces within a ship expressed in "tons" – a unit which was actually equivalent to 100 cubic feet.
Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.
It is calculated by using the formula :

GT = K cdot V

, where V = total volume in m³ and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by :

K = 0.2 + 0.02 cdotlog_V

). GT is consequently a measure of the overall size of the ship. 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.
   A commonly defined measurement system is important; since a ship’s registration fee, harbour dues, safety and manning rules etc, are based on its gross tonnage, GT, or net tonnage, NT.
   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. Thames measurement tonnage is another volumetric system, generally used for small vessels such as yachts; it uses a formula based on the vessel's length and beam.
   Many people in many countries, including those professional people working in maritime industries for many years or even in their lifetime, often confuse "Tonnage" and "Ton". Please note that "Tonnage" refers to the unit of a ship's volume in measurement for registration and "Ton" refers to the unit of weight. They are totally different in concept.

Weight measurements

While not "tonnage" in the proper sense, the following methods of ship measurement are often incorrectly referred to as such:
Displacement is the actual total weight of the vessel. 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's 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, first determine the volume of the submerged portion of the hull as follows: Multiply its length by its breadth and the draft, all in feet. Then multiply the product thereby obtained by the block coefficient of the hull to get the hull volume in cubic feet. Then multiply this figure 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. Using the SI or metric system : displacement (in tonnes) is volume (in m³) multiplied by the specific gravity of sea water (1.025 nominally).
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. Lightship measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board. Deadweight (often abbreviated as DWT for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like Displacement, it's often expressed in long tons or in metric tons.

Origins

Historically, tonnage was the tax on tuns (casks) of wine that held approximately 252 gallons of wine and weighed approximately 2,240 pounds. This suggests that the unit of weight measurement, long tons (also 2,240 lb) and tonnage both share the same etymology. The confusion between weight based terms (deadweight and displacement) stems from this common source and the eventual decision to assess dues based on a ship's deadweight rather than counting the tuns of wine. In 1720 the Builder's Old Measurement Rule was adopted to estimate deadweight from the length of keel and maximum breadth or beam of a ship. This overly simplistic system was replaced by the Moorsom System in 1854 and calculated internal volume, not weight. This system evolved into the current set of internationally accepted rules and regulations.
When steamships came into being, they could carry less cargo, size for size, than sailing ships. As well as spaces taken up by boilers and steam engines, steamships carried extra fresh water for the boilers as well as coal for the engines. Thus, to move the same volume of cargo as a sailing ship, a steamship would be considerably larger than a sailing ship.
"Harbour Dues" are based on tonnage. In order to prevent steamships operating at a disadvantage, various tonnage calculations were established to minimise the disadvantage that the extra space requirements of steamships presented. Rather than charging by length or displacement etc, charges were calculated on the viable cargo space. As commercial cargo sailing ships are now largely extinct, Gross Tonnage is becoming the universal method of calculating ships dues, and is also a more straight-forward and transparent method of assessment.

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