Dry Dock

A dry dock is a facility for lifting a ship out of the water so that the parts of the hull normally submerged can be reached. In the common graving-dock form it is a rectangular basin cut into the shore, closed off from the sea by a gate or caisson. The dock is flooded, the ship is floated in and positioned precisely over a prepared line of keel blocks and side supports, and then the water is pumped out. As the dock empties, the ship settles onto the blocks and its entire hull, rudder, propeller and underwater fittings are left high and dry for work to be carried out. Floating dry docks achieve the same result with a submersible pontoon structure that can be sunk to receive a ship and then raised.

Drydocking is crucial because much of a ship's structure and equipment is underwater and cannot be properly examined or maintained afloat. In dock the hull is cleaned of marine fouling, inspected for corrosion and damage, and repainted with anti-corrosive and anti-fouling coatings that protect the steel and reduce drag. The propeller, rudder, stern gear, sea chests and underwater valves are overhauled, the hull plating and welds are gauged and repaired as needed, and the anchors and cable are ranged and inspected. Major steel renewals and modifications are also done at this time.

Drydocking is tied to the ship's survey and certification cycle. Classification societies require the underwater hull to be examined in dry dock at intervals — traditionally twice in a five-year period, though extended intervals with in-water surveys are allowed for some ships, usually coinciding with the class special survey. A docking is a significant, planned event: the ship is taken out of service, the work is scheduled in advance, and the docking plan showing the position of blocks and services is prepared carefully, because getting a large, heavy ship safely onto the blocks and back into the water demands precision.