Dock (structure)

Dock (structure), a type of harbour structure used for loading, unloading, or repairing ships. In accurate usage the term applies either to the water channel in which a ship is berthed beside a pier, or to a dry dock, in which a ship is placed for repairs. Commonly, dock refers to a pier or quay. A pier is a structure that extends out into the water, usually perpendicular to the shoreline; a quay is constructed parallel to the shoreline.

In harbours that have a large tidal range, ships are usually berthed in wet docks. These docks are actually basins that can be shut off from the rest of the harbour by movable gates that hold the water in the dock at the high-tide level. Wet docks are a necessity in most ports of the British Isles and in other localities where the height of the tide is more than about 3 m (10 ft). The London wet-dock system is the largest in the world; the combined length of the quays and piers in London amounts to about 72 km (45 mi).

In most United States seaports the rise and fall of the tide is small enough to permit the use of tidal docks. Used almost exclusively in the United States, these consist of a series of rectangular water spaces between projecting piers. Because changes in water level are relatively small, there is no need to isolate the dock basins from the rest of the harbour.

Today, enormous container ships are primarily responsible for transporting cargo overseas (see Shipping Industry); as a result, major shipping ports of the world have container ship docks. Container ships transport the trailer components of tractor-trailer trucks, or large standard-sized rectangular boxes. The cargo is already packed in these large boxes, so at the docks these containers need only be placed on or removed from the ships. Container ship docks are often fitted with large cranes that can load and unload the containers. The docks must also be wide enough to accommodate vehicles that deliver and pick up the containers.

Dry docks are generally used to make repairs beneath a ship's waterline. The ship is manoeuvred into a dry dock, then the structure is closed and pumped free of water, leaving the entire ship exposed for repairs. Dry docks are of two types: graving docks, fixed basins lined with concrete; and floating dry docks, usually made of steel. Floating dry docks have certain advantages over graving docks in that they can be built more quickly and economically than fixed docks, and can also be towed into place to meet a ship unable to make it to a port.

See also Breakwater.