Ballast is a heavy substance employed to give a ship sufficient immersion in the water, to insure her safe sailing with spread canvas, when her cargo and equipment are too light. The amount of ballast required by a ship depends not only on her size and cargo, but also on her build; some forms of construction requiring more ballast than others. It is not merely the quantity of ballast which a skilful shipmaster has to consider; he is required also to take into account its distribution. If a heavy mass of ballast be deposited within a small compass near the keel, it locates the centre of gravity very low down, in which circumstances the ship will sail sluggishly, and heavy and uneasy rolling may result, causing discomfort, and engendering severe strains in the hull and rigging. If, on the other hand, the ballast be massed too high up, the ship becomes unstable or 'crank,' and cannot carry much sail without danger of being upset.
In ballasting a ship, the cargo and ballast are considered together, the quantity and distribution of the latter being made dependent on the former. In a ship of war, the ballast is made subservient to the requirements of the necessary stores and war matériel; in a merchant or passenger vessel, to the efficient storage of the cargo and the comfort of its passengers.
The substances used as ballast are various—chiefly iron, stone, gravel, sand, and water. In ships of any considerable size, or engaged in any service of importance, iron has long superseded stone, gravel, or other variety of dry ballast, but within recent years the adoption of water ballast has become very general in almost all classes of vessels. Its first application, about the beginning of the century, in the ships engaged in the coal-trade between the Tyne and London, consisted in the use of waterproof bags arranged on the floor of the vessel, which were filled or emptied as circumstances demanded, by pump and hose. When steamers began to be employed in the trade, special ballast tanks, forming part of the vessel's structure, were introduced, which served for cargo carrying, when the vessel was loaded and not requiring ballast. This developed into using the whole extent of a vessel's bottom exclusively for water ballast, a second or inner bottom being fitted two or three feet above the bottom proper. The practice of fitting vessels with double bottoms, divided into cellular spaces by continuous longitudinal keelsons and transverse bracket floors—of which the Great Eastern is an early and notable example—had been instituted by Mr Scott Russell between 1850–58. The object then in view, however, was that of enhanced structural strength, but about 1874–78 several vessels were built on the north-east coast of England on the cellular-bottom system, the end in view being chiefly the carriage of water ballast. From that time the system has been continuously and increasingly adopted, its later development having been more closely associated with Clyde-built vessels. Steamers only, for a time, were thus constructed, because of the special pumping facilities they possessed for rapidly emptying the ballast tanks. Within very recent times, however, many sailing-ships have been built with water-ballast bottoms; the presence of steam donkey-boilers, cargo-winches, windlass, &c., in well-equipped modern ships, incidentally furthering the adoption of water ballast. The bottoms of typical modern merchant-steamers are divided into numerous watertight compartments, which can be filled or emptied separately through inlet or suction pipes, by means of pumps worked by the main engines. The system has many advantages which highly commend it. Vessels are ballasted automatically, without any manual intervention further than the opening and closing of the inlet valves. Ballasting may proceed simultaneously with the discharge of cargo, and quite irrespective of that work. It may be discharged also at any time, even while the vessel is under way. The subdivision of the bottom into self-contained compartments admits of ballasting being done either at the fore or the aft end of the vessel, just as may be required for its equal or desired trim. The inner bottom, it may be stated, adds immensely to the safety of the vessel in the event of its striking on sunken rocks.
The customs laws relieve merchant-ships from certain formalities and payments when leaving a port in ballast. To prevent captains from filling up or injuring the entrances to rivers, ports, havens, and roadsteads by the discharge of ballast, regulations have been made at most places for its disposal.
The term ballast is employed by civil engineers to signify the broken stone, cinders, or gravelly material which is laid as a packing between railway sleepers, in order to give them solidity, and at the same time prevent the lodgment of water. No English railway is considered to be complete or safe for transit until it is dressed and finished by ballasting; the bed of ballast being usually about two feet thick. The possibility of procuring ballast at a cheap rate, considerably affects the cost of railway undertakings.