2.1 BALLAST WATER
The term "ballast" usually refers to any heavy material placed in a ship that lowers the centre of gravity. Ballast provides vessel stability and ensures that ships sit deeply enough in the water to enable efficient and effective operation of their propellers1
Ballast is particularly important in ships carrying little or no cargo. When fully laden, stability is primarily conferred by the cargo. Empty or lightly laden, the buoyancy and therefore the instability of a ship increases. By adding weight to the lower part of a ship in the form of ballast, positive stability is achieved by drawing the centre of gravity below the centre of buoyancy.
Originally, chains and other heavy materials such as sand and beach boulders were used as ballast. From the late 1870s they were replaced by water2 The advantages of liquid over solid ballast proved considerable. Sea water provides a good weight to volume ratio; it is readily taken on-board and discharged; and it can be shifted between ballast tanks within a vessel. It is easily obtained at no cost other than that associated with pumping.Ballast water is used to alter the draft, trim, manoeuvrability and stability of a vessel during cargo loading and unloading operations in port and at sea. It is also used to compensate for changes in vessel draft that result from fuel consumption during a voyage.
Although the majority of vessels use salt water ballast, some use fresh water or solid ballast. In Victorian waters, for example, a number of coastal vessels carry solid ballast permanently in one or more of their cargo holds. This is possible because the types and amounts of cargo they carry are consistent and patterns of loading and unloading are predictable3 Other vessels use fresh water ballast which is rarely discharged: the water is shifted from one tank to another by an internal pumping system to control trim and list4
