A technique used to remove a ship from the water so that the underwater portion may be inspected, repaired, maintained, or altered. Occasionally underwater repairs may be undertaken while a ship is afloat; however, at regular intervals, or as dictated by emergency, it may be necessary to expose all of the underwater portion, regardless of whether the ship is a small harbor tug or a large transoceanic liner.
The four types of dry docks are known as marine railways, floating dry docks, graving docks, and mechanical lift docks. The size of the ship usually determines which type is used.
The marine railway consists of a cradle of wood or steel with rollers on which the ship may be hauled out of the water along a fixed inclined track leading up the bank of a waterway. The advantages of a marine railway lie in the economy of the original construction and the relative low cost of maintenance. A marine railway is ideal for ships up to 5000 tons.
The floating dry dock may be constructed of wood, steel, or concrete. The dock is submerged, to provide the required depth of water over the keel blocks, by partially filling its tanks with water. The ship to be drydocked is then positioned within, the tanks of the dock are rapidly pumped out by powerful pumps located within the dock walls, and the ship is lifted out of the water.
The graving dock consists of an excavation in the ground with a thick concrete base supported, if necessary, by piling and surrounded on three sides by earth held back by timbers, stone, cement, or steel supports, or a combination of these materials. The entrance, or seaward end of the dock, is usually closed by a caisson of the pontoon type which, when flooded, is trimmed down into position. The dock is flooded, the caisson is floated, and the ship enters the dry dock and is positioned over the keel blocks. The caisson is then replaced and submerged, the dock is pumped out, and the ship settles on the keel blocks. This process is reversed when the ship is ready to leave the dock.
The mechanical lift dock is somewhat similar in action to the floating dry dock. The vessel, after taking up on the keel and bilge blocks in the dock, is bodily lifted clear of the water. The mechanical platform dock has much more flexibility than other types and has increased greatly in size and use.
