Submarine (or sub) is a watercraft capable of independent operation underwater. Submarine differs from a submersible, which has more limited underwater capability. Submarine is also sometimes used historically or colloquially to refer to remotely operated vehicles and robots, as well as medium-sized or smaller vessels, such as the midget submarine and the wet sub.
Submarines are referred to as “boats” rather than “ships” irrespective of their size.
Modern deep-diving submarines derive from the bathyscaphe, which in turn evolved from the diving bell.
Although experimental submarines had been built before, submarine design took off during the 19th century, and they were adopted by several navies. Submarines were first widely used during World War I (1914–1918), and are now used in many navies large and small.
Military uses include attacking enemy surface ships (merchant and military) or other submarines, aircraft carrier protection, blockade running, nuclear deterrence, reconnaissance, conventional land attack (for example, using a cruise missile), and covert insertion of special forces.
Civilian uses for submarines include marine science, salvage, exploration, and facility inspection and maintenance.
Submarines can also be modified to perform more specialized functions such as search-and-rescue missions or undersea cable repair.
Submarines are also used in tourism and undersea archaeology.
Most large submarines consist of a cylindrical body with hemispherical (or conical) ends and a vertical structure, usually located amidships, which houses communications and sensing devices as well as periscopes. In modern submarines, this structure is the “sail” in American usage and “fin” in European usage. A “conning tower” was a feature of earlier designs: a separate pressure hull above the main body of the boat that allowed the use of shorter periscopes. There is a propeller (or pump jet) at the rear, and various hydrodynamic control fins. Smaller, deep-diving, and specialty submarines may deviate significantly from this traditional layout.
Submarines use diving planes and also change the amount of water and air in ballast tanks to change buoyancy for submerging and surfacing.
Submarines have one of the widest ranges of types and capabilities of any vessel. They range from small autonomous examples and one- or two-person subs that operate for a few hours to vessels that can remain submerged for six months—such as the Russian Typhoon class, the biggest submarines ever built.
Submarines can work at greater depths than are survivable or practical for human divers.
The first military submersible was Turtle (1775), a hand-powered acorn-shaped device designed by the American David Bushnell to accommodate a single person. It was the first verified submarine capable of independent underwater operation and movement, and the first to use screws for propulsion.
Modern military asset submarines rely almost entirely on a suite of passive and active sonars to locate targets. Active sonar relies on an audible “ping” to generate echoes to reveal objects around the submarine. Active systems are rarely used, as doing so reveals the sub’s presence. Passive sonar is a set of sensitive hydrophones set into the hull or trailed in a towed array, normally trailing several hundred feet behind the sub.
Submarines also carry radar equipment to detect surface ships and aircraft. Submarine captains are more likely to use radar detection gear than active radar to detect targets, as radar can be detected far beyond its own return range, revealing the submarine. Periscopes are rarely used, except for position fixes and to verify a contact’s identity.
In an emergency, submarines can transmit a signal to other ships. The crew can use Submarine Escape Immersion Equipment to abandon the submarine. The crew can avoid lung injury from over-expansion of air in the lungs due to the pressure change known as pulmonary barotrauma by exhaling during the ascent.Following escape from a pressurized submarine,[the crew is at risk of developing decompression sickness. An alternative escape means is via a deep-submergence rescue vehicle that can dock onto the disabled submarine.