A semi-submersible or semisubmersible is a watercraft that is designed with a large portion of its volume (bulk) underwater. Unlike a submarine it never is entirely underwater in normal seas.

Types and Applications

Tourist Underwater Viewing

An example of this type is the "Capitan Nemo's Submarine Ride" at the amusement park Anaheim, California Disneyland. This consists of a surface craft resembling a surfaced submarine, in which the pasengers board and descend to seats below the water. Their viewing ports thus present an underwater view, with the intended illusion during the ride to be that of riding in a submerged craft. Similar craft have been used commercially to view natural oceanic reefs.

Deep Sea Ocean Studies

A tilting craft was designed to form a stable platform in rough seas. This was constructed with a conventional bow and stern (forward and after parts) of a ship, but separated and joined by a tubular structure. Unlike a conventional ship, the stern part was built with decks, ladders, and fittings suitable for use if the forward part of the ship was sunk, leaving the entire craft pointing vertically downward toward the ocean bottom, with the stern portion well above the water line. The narrow tubular portion had the effect of transmitting very small forces to the craft as large ocean swells and waves passed by, so the platform was extremely stable in comparison to a conventional ship.

Military Craft

The U.S. Civil War warship USS Monitor is considered to be the earliest semi-submersible ship, having very little freeboard (the distance between deck and the water) and containing all of its propulsion machinery, fuel, and crew acommodations below the waterline. Only a small pilot station, cylindrical two gun turret and a smokestack protruded above its flat deck.

The presentation of a small cross sectional area at the waterline can produce a veri stable watercraft, less subject to the effects of waves and swells. It has also been observed that submarines can be very efficient craft when operating underwater, as they do not create a bow wave. Craft have been proposed and prototypes constructed that consist of two submarine-like structures underwater, with streamlined pylons supporting a superstructure. This offers both high efficiency and stable operation in rough seas (up to a limit determined by the size of the vessel).

Lockheed Corporation designed and built a craft, the Sea Shadow, in many ways similar to the above description, although rather than mounting the superstructure on pylons the superstructure was joined to the hulls with continuous members, with sloped sides, forming a stealthy structure less visible on radar.

Offshore drilling

Semisubmersible rigs are used for offshore drilling.

See also

A semi–submersible salvage ship, used to carry disabled ships clear of the water, is described in the dry dock article. Some forms of dry dock (floating dry docks), used for the repair and maintenance of ships, are also semi–submersible.