The classic form of drydock, properly known as graving dock, is a narrow basin, usually made of earthen berms and concrete, closed by gates or by a caisson, into which a vessel may be floated and the water pumped out, leaving the vessel supported on blocks. The keel blocks as well as the bilge block are placed on the floor of the dock in accordance with the "docking plan" of the ship.
Some fine-tuning of the ship's position is done by scuba divers while there is still some water left to manoeuvre it about. It is extremely important that supporting blocks conform to the structural members so that the ship is not damaged when its weight is supported by the blocks. Some ASW warships have protruding sonar domes, requiring that the hull of the ship be supported several meters from the bottom of the drydock.
Once the remainder of the water is pumped out, the ship can be freely inspected or serviced. When work on the ship is finished, water is allowed to reenter the dry dock and the ship is carefully refloated.
Modern graving docks are box-shaped, to accommodate the newer, boxier ship designs, whereas old drydocks are often shaped like the ships that are planned to be docked there. This shaping was advantageous because such a dock was easier to build, it was easier to side (support) the ships, and less water had to be pumped away.
Drydocks that are used for building Navy vessels may occasionally be built with a roof. This is done to prevent spy satellites from taking pictures of the drydock and any ships or submarines that may be in it. During World War II covered drydocks were frequently used by submarine fleets to protect them from enemy air raids, however their effectiveness in that role diminished after the Second World War. Today covered drydocks are usually used only when servicing or repairing a fleet ballistic missile submarine. Another advantage of covered drydocks is that one can work independent of the weather. This can save considerable time.
A floating drydock is a sort of pontoon for dry docking ships that has floodable buoyancy chambers and a "U" shaped cross-section. The walls are used to give the drydock stability when the floor is below the water level. When valves are opened the chambers are filled with water, the dry dock floats lower in the water, allowing a ship to be moved into position inside. When the water is pumped out of the chambers, the drydock rises and the deck is cleared of water, allowing work to proceed on the ship's hull.
Shipyards operate floating drydocks, among other means of hauling or docking vessels. The advantage of floating drydocks is that they can be moved all over the world and also can be sold second-hand. During World War II, the U.S. Navy used such (floating) drydocks extensively to provide maintenance in remote locations. One of these, the 850-foot AFDB-3, saw action in Guam, was mothballed near Norfolk, Virginia, and was eventually towed to Portland, Maine, to become part of Bath Iron Works' repair facilities.
Other ways of drydocking ships
Apart from graving docks and floating drydocks, ships can also be drydocked and launched by:
Mobile boatlift (also called Travelift, for vessels up to 1000 metric tons)
Slipway, patent slip — For repair of smaller boats and the newbuilding launch of larger vessels
Marine railway — For repair of larger ships up to about 3000 tons ship weight
Shiplift — For repair as well as for newbuilding. From 800 to 25000 ton shipweight
Uses other than for ships
Some drydocks are used during the construction of bridges, dams, and other large objects. For example, the drydock on the artificial island of Neeltje-Jans was used for the construction of the Oosterscheldekering, a large dam in the Netherlands that consists of 62 concrete pillars weighing 18,000 tonnes each. The pillars were constructed in a drydock and towed to their final place on the seabed.
