Ship’s cargo carrying capacity generally describe in ‘tons’, but it does not refer to the weight ton at all. It was originally derived from the word ‘tun’ , referred to the 252-gallon barrel used in the wine trade in olden days. Then the convenient way of estimating a ship’s size was to calculate how many of such barrels she could carry and that figure was used as ship’s “Register Tonnage”
Gross Tonnage & Net Tonnage
From the above mentioned tradition , the Gross Register Tonnage (GRT) and Net Register Tonnage (NRT) evolved. Though GT & NT (new system, came in force from 1994)are called as "Tonnages' actually they are the cubic measurements based on an international formula for comparing the overall size of ships. GT is used by ports as the basis for levying vessel related expenses like berth hire , pilotage , port dues and NT is used by customs as a basis for calculation for Light House dues (ILH = NRT x Rs8).
GT - The internal cubic measurement of all the enclosed spaces within the ships hull and superstructure.
NT - The GT minus the space which is not used for commercial purpose such as crew accomodation area, engine room etc.
Displacement Tonnage – The actual wt. of a ship, the weight of the body is equal to the weight of water it displaces ( Archimedes Law).
Light Displacement – Actual wt. of the empty ship (without fuel , cargo , stores etc)
Deadweight (dwt) – Difference in tons between the light and loaded displacement. Represents the total weight a ship can carry (cargo + fuel + stores + fresh water etc)
Measurements of ship
Below given are the important dimensions of a ship which determines whether ship can safely enter canals,locks , port approach chanels, berths etc.
Length Over All (LOA) - The length of a a ship from the foremost point on the bow to the mostaft point on the stern.
Beam - The maximum breadth of the ship, which is in the amidships( the area between forward and the aft portion) body.
Draft - The distance between the bottom of the ship (the keel) to the level of water on the ships side (water line)
ü The more cargo the ship loads, greater her draft would be. Every state of the ship’s draft there is a corresponding total dead weight
ü The draft of a port refers to the depth of water available at the port channels / terminal basin
Air Draft - The distance from the waterline to the highest point on the ship's super structure ,usually the top of the mast.