Warship displacement is generally estimated by totaling individual weights of construction materials, weapons, ammunition, propulsion and auxiliary machinery, and the crew. Where more than one displacement figure is provided, the larger figure also includes fuel, boiler feed water, food, and consumable stores. Different references may have different definitions for how much fuel, water, etc.
Actual weight is computed from knowledge of hull geometry by determining the mass of the volume of seawater displaced by the submerged hull. When interior equipment and hull geometry are kept secret, publications have used the displacement announced by the nation owning the ship and some of these figures were intentionally mis-stated.
Cargo ship tonnage is sometimes computed differently on the basis of volume of cargo holds.
Here is an overview of the tonnages used for cargo ships. In my profession (I work in a maritime company) we use several tonnages:
- Deadweight All Tonnage (DWAT)
This is the total weight of fuel, cargo, equipment, etc., that the vessel can carry when fully loaded.
- Dead weight cargo capacity (DWCC)
This is most important for the Charterers (Whose cargo has to be shipped), because it gives the weight of the cargo that the vessel can carry. The more fuel the vessel needs the lower the DWCC is. This also depends on the water temperature and the water (Salt, brackish, etc.). In the summer, vessels have a higher DWCC.
- Gross Registered Tonnage (GRT)
Measurement of a ship calculated by taking the total enclosed volume of her hull below the upper deck as well as enclosed spaces above it (with certain exceptions, namely; double bottoms used for ballast and accommodation) in cubic feet and dividing by 100. It is measured in accordance with the IMO International Convention on Tonnage Measurement of Ships (1969).
- Net Registered Tonnage (NRT)
Equals gross tonnage minus deductions for space occupied by crew accommodations, machinery, navigation equipment and bunkers. It represents space available for cargo. It is generally used for tax purposes and for harbor/canal dues. For the latter, you have the Suez and Panama Canal tonnages, which are generally higher (in order to collect more in tax payments).
In shipping, the DWCC is the most important figure, although ship owners also look at the NRT, because they have to pay the taxes. Car carriers have a very large GRT, but their cargo capacity is limited. For most modern cargo ships the relation of GRT to cargo capacity is about 1 : 1.4, but for large ore carriers (ore has a high density) this can be as high as 1 : 2.
- 11 April 1999
- Added "Warship Displacement."
- 19 May 2003
- Added "Tonnage Measurements used for Cargo Ships."