What is Deadweight?
The ship is an inanimate object hence the term deadweight does not relate to the presence of absence of life but to something else. Well, jokes apart, the deadweight has something to do with the carrying capacity of the ship. We know from our previous articles that commercial ships of various types are mainly used for transport of either passengers or cargo of various types from one port to another. Hence they have a certain carrying capacity which is measured in terms of tonnes which is equivalent to a thousand kilograms.
Basically the deadweight of the ship is the totally load it can carry safely. Now this total load not only includes the load of the cargo which it intends or is designed to carry but also the other non-cargo loads and these include bunkers, stores, fresh water, lub oil and even crew members. Now as you must have noticed that the term “carry safely" is a very vague term not capable of exact definition, hence the deadweight is usually measured in relation to some particular marking of the plimsoll line and the most commonly used marking is that of the summer line.
In actual practice the deadweight is measured as the difference between the lightweight and the weight when it is fully loaded, as this gives the maximum carrying capacity of the ship.
Let us take an imaginary example in which the ship has a lightweight of say 5000 tonnes and when fully loaded upto the summer line, say its weight is around 8000 tonnes. This means to say that the deadweight of the ship is 8000 – 5000 = 3000 tonnes. Now these 3000 tonnes include the weight of the crew members, fresh water and so forth. Just assume that the weight of these non-cargo items is around 300 tonnes, so the actual cargo carrying capacity of the ship will be around 2700 tonnes which is roughly 10% less than the actual deadweight.
Hence the deadweight gives an approximate measurement of the amount of cargo it can carry since in most cases, the percentage of the weight of non-cargo items is very less as compared the weight of the cargo that it is designed to carry. Even in real ships this percentage approximately lies between say around 3 to 10 %. This should not be surprising since the main aim of the ship is to transport maximum possible cargo safely, while the other paraphernalia exist for the sake of making that transport safe and secure.
Although each ship has its own deadweight depending on its size and design, but just to give you a rough idea that the deadweight of a huge ship such as an ULCC or Ultra Large Crude Carrier can be as massive as of the order of 500, 000 tons. The deadweight of dry cargo vessels is usually lesser than tanker ships. Comparatively the deadweight of a Handymax bulk carrier is of the order of ten times smaller in the region of less than 50,000 tonnes. The deadweight of an MBC or a mini bulk carrier can be as less as a couple of thousand tonnes.