Dry dock is a dock in which a vessel can lie out of the water while repairs are being made below its water line (see Dock ). The two chief kinds of dry docks are graving docks and floating docks.
Graving docks
are used chiefly to repair large ships in shipyards. Graving was a term used in the days of wooden ships to mean cleaning a vessel’s bottom and coating it with tar. A graving dock looks like a huge, concrete bathtub sunk into the ground. One end of the dock opens onto a harbor, river, or other variety of waterway.
When a ship enters a graving dock, shipyard workers place a huge floating or sliding caisson, or gate, against the open end. Pumps suck the water out and the vessel slowly sinks. Its keel, or bottom, comes to rest on wooden blocks placed on the floor of the dock. Spars, or long pieces of wood wedged between the ship and the sides of the dock, also help support the vessel. When repairs are completed, workers flood the dock until the water reaches the same level as the water outside the gate. It is opened and the ship leaves.
Floating docks
can be self-propelled or towed from place to place. They are important in war to repair ships in forward battle areas. A floating dock looks like a shoebox with the top and ends removed. Some types are built in U-shaped sections that can be assembled to make one large dock.
The hull, or bottom, and wingwalls, or sides, of a floating dock contain compartments. Water enters these compartments, making the dock sink low enough to allow a ship to enter. Pumps then suck the water out and the dock rises, lifting the ship out of the water. Wooden blocks and spars help support the vessel. When repairs are completed, the compartments are flooded again until the dock sinks enough to allow the ship to float.
