A well deck is a deck at the rear of a ship designed to be periodically filled with water to allow for the launch and recovery of vehicles. This design feature is often seen on vessels designed for amphibious warfare, as well as in some scientific and industrial ships that need to be able to launch vehicles and equipment. This design adds complexity to the overall engineering of the ship, as it must be designed to take on water to lower the well deck and flood it for launch.
When not in use, the well deck is kept closed and dry. Personnel may use it for activities like calisthenics, machining, and so forth during dry periods, as it is usually a large, level space. Equipment intended for launch is also stored in this area, allowing personnel to perform maintenance and other activities as necessary. This can include repairs while a ship is underway, to allow for relaunch of damaged equipment.
For launches, the ship takes on water and opens the gates to the well deck, exposing it to the sea. Boats can float off the surface and launch, while personnel can drive cars and trucks off for activities like beach landings. A ramp may be put in place to facilitate offloading of wheeled equipment. Helicopters, weather balloons, and similar equipment can also be launched from a well deck.
This deck is akin to a hangar, and can vary in size and shape. Large ships may have a very substantial amphibious transport hanger along with accommodations for troops and supporting equipment. The ship can move the troops and equipment to wherever they are needed, open the well deck, offload, and wait for them to return. Once everything is loaded back up, the ballast tanks can be emptied to allow it to rise, taking the deck back above water level.
Ship design of this nature needs to accommodate ballast tanks, the well deck, and any accessory equipment like controls for the doors. Engineers also need to consider the potential laden weight of the ship when the vessel is fully loaded, and what might happen during flooding of the tanks. It is critical to have enough freeboard to keep the ship afloat and prevent swamping in heavy seas. If too much of the hull sinks below the water, the ship can become unstable, and high winds or large waves could pose a threat.