Dry dock
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The dry dock is a place where ships come to be serviced or repainted. Many ships are built in dry docks, allowing them to be launched in a more gradual fashion than the gravity-assisted launches of yore.
Dry docks can be freefloating structures or can be carved into the shoreline (generally called "graving docks").
The process of dry docking a ship begins with the construction of supports, called "blocking", that will hold the ship upright when the water is drained from the dry dock. These concrete, steel, or wooden posts must conform to the shape of the hull. After they are arranged on the dry dock floor, the dock is flooded.
The ship enters the dry dock, usually with the help of tugboats. Large hydraulic gates close, separating the dry dock, and the ship, from the sea. Powerful pumps start to remove the water, sending it back into the harbour. This can take a long time.
Some fine-tuning of the ship's position is done by scuba divers while there is still some water left to manoeuvre it about. Each ship will have a diagram of the shape of its bottom. It is extremely important that supporting blocks conform to this shape 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, people can walk around in the dry dock, and 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.
Dry docks are usually excavated from dry land on the shore. The largest dry dock in the world is in Belfast, Northern Ireland. The oldest dry dock is in Portsmouth Naval Base (in England) and dates back to 1495, though the Chinese had invented and used dry docks by at least 1070.
Floating drydock
A floating drydock is a specialised ship used in salvage which is constructed with floodable buoyancy chambers in a "U" shaped cross section that can support the hull of a ship needing repair. Valves can be opened to allow the chambers to fill with water. When this happens the dry dock floats lower in the water. A ship can then be moved into position inside the "U" of the dry dock. The water is pumped out of the chambers and the dry dock rises in the water. It can then be towed, carrying the ship, to a repair facility.
Floating drydocks were used extensively during the Second World War to establish maintenance facilities in remote locations. Several of these docks could be joined to accommodate a long ship.
See also
de:Dock (Schifffahrt) he:מבדוק יבש nl:Droogdok nds:Dock pl:Suchy dok sv:Torrdocka