The word ‘ton’ in shipping can denote both weight and capacity. Loaded displacement tonnage is the actual weight of the ship and cargo. Light displacement tonnage is the actual weight of the ship. The difference between the loaded displacement and the light displacement is the weight that the ship can actually carry and is known as the Deadweight Tonnage (DWT).
In the shipping business, the word “ton” has many different meanings and ship tonnage can be based on either “weight” or on “volume”. The actual weight of the ship plus the weight of all it is carrying is termed its ‘load-displacement tonnage’ or, simply ‘displacement tonnage’. Displacement tonnage is used to describe the size of certain ship types not built for cargo carrying (e.g. icebreakers or naval ships) but has little practical value in the dry cargo market.
The weight of an empty ship is called ‘Light Displacement Tonnage’ (LDT), is equally of little value to dry cargo chartering personnel, although it is of particular interest to those engaged in the sale and purchase of ships, for the ship demolition prices are based on this tonnage, which is used to establish a ship’s steel weight.
The two tonnage descriptions of particular value to the dry cargo market sector are a ship’s ‘Deadweight Tonnage’ (DWT), which not only happens to be the difference between a ship’s loaded and light displacement tonnages but, more importantly, represents the total weight a ship can carry. This total weight will include, of course, not only cargo but bunkers, fresh water, stores, spare parts, etc. Those engaged in chartering activities sometimes describe this tonnage as ‘Deadweight All Told’ (DWAT), to distinguish it from ‘Deadweight Cargo Capacity’ (DWCC) which is found after deducting the amalgamated weights of bunkers, fresh-water, stores, spare parts, etc. from the ship’s ‘Deadweight All Told’ (DWAT). Therefore, ‘Deadweight Cargo Capacity’ (DWCC) represents the quantity of cargo a ship should be able to load.
Ship Deadweight Tonnage (DWT) and Cargo Size:
Infrequently a shipowner offers to carry an exact cargo size for example ‘50,000 metric tonnes minimum/maximum coal in bulk, stowage factor around 47 cubic feet per tonne’. However, sometimes a margin is negotiated to enable a master to maximize the ship’s lifting which varies depending upon the bunkers on board, for example ‘50,000 metric tonnes coal in bulk, 5% more or less in shipowner’s option’. It may be that this margin is at ‘charterers’ option” although such an arrangement precludes the certainty that the ship’s master can maximize cargo lifting, and means that the shipowner must estimate the minimum cargo quantity when calculating the viability of such a prospective future. Where a shipowner contracts to load or charterer to provide about a certain quantity for example ‘about 100,000 metric tonnes bagged fishmeal’ – the word ‘about’ is construed to mean within, say, a reasonable margin of 5%; in other words, between 95,000 and 105,000 metric tonnes. However where the word ‘about’ is replaced by ‘without guarantee’ it means just that. There is ‘no guarantee’ and the cargo can legally be of any size. Infrequently a stated margin is agreed for example ’50/52,000 metric tonnes’. Here it is understood that the cargo to be loaded and/or supplied will be between 50,000 and 52,000 tonnes of co make matters absolutely clear, the words ‘minimum/maximum’ or similar might be added for example ‘within 50/52,000 tonnes min/max’ whilst the additional phrase ‘in owners/shippers’ option’ defines whose right it is to decide upon the exact cargo quantity within the agreed limitations. While a shipowner will usually be looking to maximize a ship’s intake there are other elements to consider. On no account must the ship’s maximum DWT (Deadweight Tonnage) be exceeded, nor must the load-line be submerged at any point during the voyage. Even if a ship starts its voyage in a zone where cargo intake can be maximized, the ship may have to pass through another zone during the voyage. Sometimes a ship may have to sail with empty holds for safety reasons because to do otherwise would mean part cargoes in one or more holds that might be liable to shift.