A graving dock is a facility where people can float ships in and pump water away to expose the entire body of the ship for maintenance and repair. Also known as dry docks, graving docks are found in shipyards and harbors all over the world. The technology for creating them was developed independently in several regions of the world including Ancient Greece and China. While there have been some modern improvements, a graving dock today would be recognizable to sailors who lived and worked centuries ago.
The graving dock is built like a long box with a gate. Ships and boats are floated in through the gate from a harbor or another waterway. The gate is closed, blocks are positioned under the watercraft for support, and the water is pumped away. The positioning of the blocks is critical, as ships are designed to float, not to sit on blocks, and if the blocks are the wrong shape or size or placed in the wrong spots, they can damage the ship. If the ship is not being supported enough, the framing can weaken, or the blocks may punch through the sides of the ship as it settles. Ships are also commonly supported from the side of the graving dock.
Once exposed, a ship can be inspected. If repairs are needed, they are much easier to perform in a graving dock than in the water. The ship can also be scrubbed down to remove algae, barnacles, and other organisms that tend to accumulate, and it can be repainted to protect the hull. Special paints are used on the parts of ships that are routinely underwater to help resist rot, rust, and other problems.
The size and shape of a graving dock varies. Many early facilities were built very narrow to minimize the amount of water that needed to be pumped out. Modern graving docks tend to be larger to accommodate a range of ship designs. The facilities also can be covered if there is a need for confidentiality, as with repairs to military ships. Militaries might not want to alert people to the fact that they are conducting repairs or may want to keep the features of their ships concealed for security reasons.
In addition to being used for maintenance and repair, graving docks are also used during the shipbuilding process. The ship is built inside a graving dock and once the hull is complete, it can be floated and fitted in addition to being taken for sea trials to confirm that it is seaworthy and to determine the performance specifications of the ship.