The dimensions of a ship can be expressed by using termsm which descrbe the charateristics of the ship. Each terme has a specific abbreviation. The type of ship determines the term to be used. For instance, the size of a container vessel is expressed in the number of containers it can transport; a roll-on roll-off carrier's size is given by the total deck-area in squre metres and a passenger ship in the number of people it can carry. At the IMO-conference in 1969 the new units "Gross Tonnage" and "Nett Tonnage" were introduced, to establish a world-wide standard in calculating the size of a ship. In many countries the Gross Tonnage is used to determine port dues and pilotage, or to determine the number of people in the crew.

**Register ton**

To determine the volume of a space the register ton is used. One register ton equals 100 cft. or 2.83 m3.

**Gross Tonnage**

The gross tonnage is calculated using a formula that takes into account the ship's volume in cubic metre below the main deck and the enclosed spaces above the main deck.

This volume is then multiplied by a constant, which results in a dimensionless number (this means no values of T or m3 should be placed after the number). All distances used in the calculation are moulded dimensions.

In order to minimier the daily expenses of a ship, the ship owner will keep the GT as low as possible.

**Nett Tonnage**

The Nett Tonnage is also a dimensionless number that describes the volume of the cargo space. The NT ca be calculated from the GT by subtraction the volume of space occupied by:

- crew

- navigation equipment

- propulsion equipment

- workshops

The NT may not be less than 30% of the GT.

**Displacement (in m3)**

The displacement equals the volume of the part of the ship below the water line including the shell plating, propeller and rudder.

**Displacement Δ (in ton)**

The displacement is the weight of the volume of water displaced of the ship. One could also say: the displacement equals the total mass of the ship.

**Displacement Δ (t) = waterdisplacment (m3) * density of water (t/m3)**

Density of fresh water is 1 t/m3;

Density of salt water is 1.025 t/m3.

Biblio: Ship knowldge - A modern encyclopedia