A submarine is built with ballast tanks (“trim tanks”) to control its depth and its horizontal orientation. The ballast tanks have large openings, called flood-ports, at the bottom and vent valves at the top. When the submarine is on the surface the ballast tanks are partly filled with air to provide positive buoyancy. The submarine is submerged by opening the vent valves, allowing air to escape and water to fill the tanks through the flood-ports and thus provide negative buoyancy.
A submarine is also controlled by rudders and by diving planes that extend from the sides. The diving planes are like small airplane wings that can be tilted up or down to make the submarine go up or down through the water.
Because of the depth at which submarines cruise—usually about 1,000 feet (300 m) below the surface—they require reinforced hulls, called pressure hulls, to withstand the water pressure. Submarines may be built with single hulls or double hulls. The United States, Great Britain, France, and Germany generally build single-hull submarines; Russia prefers double-hull submarines. (In double-hull submarines, the inner hull forms the pressure hull. The space between the inner and outer hulls is divided into compartments for fuel and ballast.)
Most submarines have streamlined hulls for maximum efficiency while submerged. The most common streamlined design is the teardrop shape pioneered by the U.S. Navy for its nuclear-powered submarines and gradually adopted for both nuclear and conventional submarines by all navies. The teardrop shape sacrifices speed on the surface for underwater efficiency. Some navies of minor powers are equipped with old conventional submarines that have prominent bows and other features that make them more efficient on the surface than underwater.
Conventional submarines are about 310 feet (94 m) long with a beam (width) of about 27 feet (8 m). Nuclear submarines range in length from 250 feet (76 m) to about 560 feet (171 m); their hulls have beams ranging from 25 to 40 feet (8 to 12 m).
Old conventional submarines have a surface speed of about 14 to 18 knots and a submerged speed of about 10 to 12 knots. Modern conventional submarines have surface speeds of 10 to 12 knots and submerged speeds of about 16 to 20 knots. Nuclear submarines have surface speeds of about 15 to 20 knots and submerged speeds of about 30 to 45 knots.
A conventional submarine has a diesel-electric propulsion system. In such a system, a diesel engine is connected to an electrical generator that powers an electric motor, which drives the propeller shaft. At the same time the generator charges a bank of storage batteries. Because diesel engines require air, conventional submarines usually operate on the surface or approximately 50 feet (15 m) below the surface using a snorkel, an air tube that extends above the water. The disadvantage of a snorkel is that it creates a telltale wake.
To run submerged at greater depths diesel-electric submarines derive power from storage batteries. On battery power they can stay submerged up to a week while running at low speed; at high speed most can operate for only a few hours.
Power for nuclear submarines comes from a nuclear reactor. In the reactor a controlled chain reaction of uranium heats water under high pressure. This pressurized water is circulated through heat exchangers (boilers) which generate non-radioactive steam at moderate temperature and pressure. The steam drives turbines that, in turn, drive generators for supplying ship's electricity. The ship's propellers are driven by the turbines through a mechanical linkage or by electric motors powered by the turbine's generators. Heavy shielding around the reactor and boiler protects the crew and surrounding environment from radiation.
A single charge of uranium fuel can produce enough power to drive the submarine for many months. The nuclear power plant does not require oxygen and can therefore be used underwater as well as on the surface. Also, the engine is much more powerful than the electric motors of conventional submarines.
When the submarine is near the surface, periscopes are used to see above the surface. Most submarines have two periscopes, a large-aperture one for search and scanning and a small-aperture one for use in attacks. The periscopes may incorporate television cameras and infrared sensors to enhance vision; videotape recorders may be used to provide a record of sightings and attacks.
During long periods underwater the air inside a nuclear submarine is purified by equipment that removes carbon dioxide and replaces oxygen breathed in by the crew members. The oxygen is generated from sea water by an electrical process. (Conventional submarines, which must surface periodically to charge their batteries, do not have this equipment.)
The size of a submarine crew ranges from 18 to 22 for a small coastal submarine, from 25 to 80 for a conventional submarine, and from 90 to 150 for a nuclear submarine. The quarters on a nuclear submarine are roomier and more comfortable than those on a conventional submarine because crew morale becomes a concern when a submarine spends weeks submerged. The U.S. Navy has two crews for each nuclear submarine, one replacing the other at the end of a cruise.
For underwater detection of ships and for some navigational purposes, submarines use two kinds of sonar. Active sonar sends out sound to bounce off a target and then receives its echo. It is used to locate underwater hazards and at times to determine the range and direction of target ships. Because active sonar transmits a revealing signal, however, submarines generally avoid using it to locate enemy vessels. For this purpose they rely mainly on passive sonar, which uses an array of hydrophones (listening devices) embedded in the hull to listen to various noises from the target ship, particularly its propeller.
Inertial guidance (called Ships Inertial Navigation System, or SINS, in the U.S. Navy) is a system of navigation that permits a submarine to fix its position without coming to the surface. Inertial guidance was originally developed for missiles and is explained in the article missiles and rockets, section “Military Missiles,” subtitle Guidance Systems: Missiles Guided in Flight (Preset Guidance).
ELF (extremely low frequency) radio waves are used for long-distance communication with submerged submarines. Unlike the radio waves used for conventional radio communications, ELF waves can penetrate deep into seawater. However, ELF radio waves are inefficient for sending long messages and they require that the submarine use a very long trailing-wire antenna.
Until after World War II, most submarines had deck guns; today only older submarines carry these weapons. All submarines carry torpedoes, which are used against other vessels. On most new submarines they are fired from tubes along the sides near the bow, but on older submarines they are fired from forward tubes (tubes in the bow itself) and aft (rear) tubes.
Submarines may also carry guided, cruise, and ballistic missiles. Guided missiles have a range of some 70 miles (110 km), and cruise missiles have a range of some 1,550 miles (2,500 km). Both are launched underwater, either from torpedo tubes or from vertical tubes in the bow, and then fly through the air toward surface targets.
Ballistic-missile submarines can launch their weapons while submerged and are capable of hitting land targets up to 4,600 miles (7,400 km) away. These missiles usually have multiple warheads, each of which can be directed at a different target. These submarines have few tactical weapons. The U.S. Navy's fleet ballistic-missile submarine, with only 4 torpedo tubes, has as its principal armament 16 intermediate-range Poseidon or Trident missiles. A few are armed with 24 Trident missiles.