TANKER

ship designed to carry liquid cargo in bulk. Its cargo is usually a petroleum product, either crude oil being carried from oil fields to refineries or gasoline being carried from refineries to distribution centres. The liquid is piped into the cargo space of the ship and transported without the use of barrels or other containers. Special tankers carry other liquids such as molasses, asphalt, wine, or liquefied natural gas. Tankers differ from general cargo ships in that they normally carry a full load in one direction and return without cargo. Tankers vary in size from small coastal vessels about 60 m (200 feet) long, carrying from 1,500 to 2,000 tons deadweight, up to huge vessels that are more than 500,000 tons deadweight, reach lengths of more than 400 m (1,300 feet), and are the largest ships afloat. Experience with these supertankers has shown that the direct cost of transporting oil goes down as the size of the tanker increases, apparently without limit; an obstacle to building larger tankers is the lack of suitable shore facilities for them. Objection to these huge vessels has been raised by conservationists; when tankers suffer damage at sea, large oil spillages may result that can cause great damage to fish and other wildlife and to nearby coasts. The propelling machinery of a tanker is almost invariably located in the stern. The cargo-oil space occupies about 60 percent of the ship's length; usually there is a general-cargo hold in the bow, along with ballast or fuel tanks. The cargo space is isolated from the forward and after portions by cofferdams, watertight compartments that are kept empty. There is a forecastle (additional topside deck at the bow) and a house amidships, containing the pilothouse, other navigational spaces, and usually quarters for some of the crew. A poop, an additional topside deck at the stern, extends over the machinery space and, together with higher levels of deckhouses, provides quarters for the rest of the crew.