Gross Tonnage

Although these two measurements are called ‘tonnages’ they are actually cubic measurements based on an international formula (International Tonnage Convention) for comparing the overall size of ships.  These tonnages are frequently used by ports and statutory bodies as the basis of levying dues or taxes on vessels. Gross Tonnage – The internal cubic measurement of all the enclosed spaces within the ships hull and superstructure.  Calculated to a formula one GT is a little less than 3M3. It is a convenient way of measuring the total bulk of the vessel and therefore is often used to describe passenger ships and ferries. Net Tonnage – The Gross tonnage less those spaces such as engine room and crew accommodation that cannot be used for commercial, that is revenue earning, purposes. These are the important dimensions of the vessel which must be known to determine whether the vessel can safely enter locks, canals, docks etc. Length over all (LOA) – The length of the ship from the foremost point on the bow to the most aft point on the stern. Beam – The overall width of the ship. Moulded depth – The height of the ship from the bottom of the keel to the top of the main deck. Draft (sometimes also spelt ‘draught’) – The distance from the waterline to the bottom of the keel. This determines the minimum depth of water the ship needs to be afloat.  Air draft – The distance from the water line to the highest point on the ship’s superstructure, usually the top of the mast.  This is important when making passage under bridges or cross river power cables etc.  Grain Cubic Capacity – The measurement in cubic metres or cubic feet of the internal capacity of all cargo spaces when they are filled with a free flowing material such as bulk grain. This measurement includes such spaces as the recess between the frames of the ship’s hull. Bale Cubic Capacity – The measurement in cubic metres or cubic feet of the internal capacity of all accessible cargo spaces if the holds are full of baled cargo. The smaller spaces and recesses are excluded.