[I]ITC has nothing to do with displacement. It is a measure of volume calculate to a given set of regs.
From Wikipeda, since someone else took the time to write it out:[/I]
[B]Gross tonnage (GT)[/B] is a function of the volume of all shipâs enclosed spaces (from keel to funnel) measured to the outside of the hull framing. The numerical value for a shipâs GT is always smaller than the numerical values for both her gross register tonnage and the GRT value expressed equivalently in cubic meters rather than cubic feet, for example: 0.5919 GT = 1 GRT = 2.8316 m3; 200 GT = 274 GRT = 775,88 m3; 500 GT = 665 GRT = 1,883.07 m3; 3,000 GT = 3,776 GRT = 10,692.44 m3), though by how much depends on the vessel design (volume). There is a sliding scale factor. So GT is 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 its derivation is tied to the cubic meter unit of volumetric capacity.
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.
[B]Net tonnage (NT)[/B] is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vesselâs 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 shipâs registration fee, harbour dues, safety and manning rules etc., are based on its gross tonnage, GT, or net tonnage, NT.
[B]Gross register tonnage (GRT)[/B] represents the total internal volume of a vessel, where a register ton is equal to a volume of 100 cubic feet (2.83168 m3), which volume, if filled with fresh water, would weigh around 2,800 kg or 2.8 tonnes. The definition (and calculation) of the internal volume is complex; a shipâs hold can, for instance, be assessed for bulk grain (accounting for all the air space in the hold) or for bales (omitting the spaces into which bulk, but not baled cargo would spill). If V stands for the total internal volume in m3, then the GRT equals V / 2.83168, so for a ship of 10,000 m3 total internal volume, the gross register tonnage is 10,000 / 2.83168 = 3531.47 GRT. Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, and is no longer a widely used term in the industry.[1][2]
[B]Net register tonnage (NRT)[/B] is the volume of cargo the vessel can carry; i.e., 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.
[B]Lightship or Lightweight [/B]measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.
[B]Deadweight tonnage[/B] (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 is often expressed in long tons or in metric tons.
Historically, tonnage was the tax on tuns (casks) of wine that held 954 litres (252 gallons) of wine and weighed 1016 kilograms (2,240 pounds). This suggests that the unit of weight measurement, long tons (also 1016 kilograms or 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 shipâs deadweight rather than counting the tuns of wine. In 1720 the Builderâs 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.
âHarbour Duesâ 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 straightforward and transparent method of assessment.