The dry dock was invented in Egypt by a Phoenician, some years after the death of Ptolemy IV Philopator, who reigned from 221 to 204 B.C.E. His method of launching a ship consisted of digging a trench under it close to the harbor, then making a channel from the sea to fill the excavated space with water.
Dry docks continued to be used throughout antiquity. In Europe the first dry dock was commissioned in 1495 by King Henry VIII at Portsmouth, England. Dry docks are mainly used for the maintenance and repair of ships, and more rarely for their construction because the time required to build a ship is so long. While early dry docks were often used for launching ships, slipways are more frequently used in modern times.
There are two types of dry docks: graving docks, where "graving" is the term for scouring aship's bottom, and floating dry docks. The graving dry dock consists of a water-filled narrow basin, usually made of concrete, with gates that can be opened and closed, into which a vessel may be floated. The water is then pumped out, leaving the vessel supported on blocks, so that the ship can be serviced. When the work is finished, water is let back into the dock and the ship refloated. Earlier dry docks were built in the same shape as the ships that were to be docked there, but more recently, graving docks have been built in a box- shape, to conform to boxier ship designs.
A floating dry dock is usually built of hollow steel. The dock is first submerged, the ship is brought into its channel, and the dock is then floated by removing ballast from the hollow floor and walls. The fully drained dock supports the craft on blocks attached to the dock floor. Floating dry docks are usually operated in sheltered harbors to avoid wave damage.
