Tonnage - Weight Measurements

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 (mostly without pay load). 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 (i.e. the volume of water it is displacing) by the specific gravity of the water. (Note that the specific gravity 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 or Lightweight measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.

Deadweight tonnage (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.

Metric Tonnes per Centimetre Immersion (usually abbreviated to TPC or TPCMI ) is the number of Metric Tonnes (1,000KG) that need to be loaded on the ship for the salt water draft to increase by one centimetre.

Imperial Tons per Inch immersion (usually abbreviated to TPI) is the number of Imperial Long Tons (2,240 lbs) that need to be loaded on a vessel for the draft to increase by one inch. Old imperial TPI measurements are still occasionally used within the USA and the Panama canal. As no ship has been measured by a Classification Society since the 1950s using Imperial measures, modern TPI figures are therefore a conversion from the original Metric measurements and should not be relied upon to be accurate.

The TPCMI figure is used to calculate the draft of the vessel with a given DWT of cargo loaded. On a typical Panamax Bulk carrier with a TPCMI of 80, it would mean that the ship will sink (Draft increase) by one centimetre for every 80 tonnes of cargo loaded.

Draft The distance, usually measured in meters, between the lowest point of the keel and the waterline which varies dependant on the load (DWT) the vessel has on board