DRY DOCK

type of dock (q.v.) consisting of a rectangular basin dug into the shore of a body of water and provided with a removable enclosure wall or gate on the side toward the water, used for major repairs and overhaul of vessels. When a ship is to be docked, the dry dock is flooded, and the gate removed. After the vessel is brought in, and properly positioned and guyed, the watertight gate is placed in its seat and the dock is pumped dry, bringing the craft gradually to rest on supporting blocks anchored to the floor. In older installations, in which the basins were relatively small, the dock structure was built mainly of massive stonework, or in a few instances, heavy timber framing. Later, these materials were supplanted by concrete, first in the ordinary mass form and later reinforced with steel. Modern dry docks are considerably larger in size and correspondingly more complex than their prototypes. A dry dock gate, with its removable watertight barrier, has many forms and arrangements. In some, two leaves form a mitre gate hinged to the side walls of the dock. In others, the leaves roll on a track into recesses in the dock walls. In still others, a one-piece gate is hinged at the bottom sill so it may be lowered to allow a ship to enter. The type most commonly used, however, is the floating gate, which is held in its seat by its weight when the dock is empty and can be removed simply by floating it out of the way when the dock is filled with water. While most ship repair work is carried out in stationary dry docks, there are some services that can be performed by mobile or floating structures. The principal such facility, the floating dry dock, is a trough-shaped cellular structure, used to lift ships out of the water for inspection and repairs. The ship is brought into the channel of the partly submerged dock, which is then floated by removing ballast from its hollow floor and walls and draining the dock so that it supports the craft on blocks attached to the dock floor. A typical floating dry dock is built of steel, with a framing system similar to that of a ship, although both timber and reinforced concrete have been used. Floating dry docks ordinarily are operated in sheltered harbours where wave action presents no problem.