Submarines

A submarine is a watercraft that can operate underwater. Military submarines were first widely used in World War I and are used by all major navies today. Civilian submarines and submersibles are used for scientific work at depths too great for human divers.

The word submarine was originally an adjective meaning "under the sea". Some firms who make diving gear but not parts for submarines, called their work "submarine engineering". "Submarine" as a noun meaning a submersible craft originated as short for "submarine boat" and older books such as Twenty Thousand Leagues Under the Sea always use this term. Also, some people simply say 'sub' instead of saying the entire word 'submarine'.

Submarines encompass one of the largest ranges in capabilities of any vessel. They range from small one- or two-man vessels that can examine the sea floor for a few hours to the Russian Typhoon class, which can remain submerged for 6 months and carry nuclear missiles capable of destroying multiple cities. There are also specialized submarines such as rescue submarines (like the DSRV or Priz) and tiny one-person human powered subs intended for competitions between universities. An older device for use in underwater exploration, salvage, construction and rescue is the diving bell. A specialized form of submarine capable of extremely deep dives is the bathyscaphe.

Submarines are typically referred to as "boats" even though most modern submarines should technically be called "ships". The term U-Boat is sometimes used for German submarines in English. This comes from the German word for submarine, `U-Boot`, itself an abbreviation for Unterseeboot ('undersea boat').

The vertical structure, usually located amidships, houses communications and sensing devices as well as periscopes. In modern submarines it is the "sail" in American usage ("fin" in the Royal Navy). A submarine's "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.
The roles of military submarines are almost limitless. From delivering supplies and facilitating troop movements undetected, to launching a surprise nuclear attack from just a few miles off of the coast. Submarines have been used as spy platforms and as underwater research vessels.

Submarines are useful militarily because of their ability to remain concealed: they are difficult to find and destroy when deep below the surface. Because water's dense molecular structure makes it an excellent conductor of sound, a submarine is more likely to be detected from its sound patterns than its visual appearance. To maintain stealth, a great deal of attention in the design of a submarine is devoted to making it travel through the water as silently as possible. Some submarines conceal their sound so well that they actually create a silent area in their environment, which can be detected if it blocks other ambient sounds. If a submarine remains undetected, it is able to strike at close range.

A concealed submarine can force an enemy Navy to waste resources defending large areas of ocean against possible attack, while in reality only threatening a small area. This advantage was vividly demonstrated in the 1982 Falklands War when the British SSN HMS Conqueror sank the Argentine cruiser General Belgrano. After the sinking the Argentine Navy realised that they were vulnerable to submarine attack, and that they had no defense from it. Thus the Argentinian fleet remained in port for the remainder of the war.

Submerged submarine seen from a planeAll surface ships, as well as surfaced submarines, are in a positively buoyant condition, weighing less than the volume of water they displace. To submerge hydrostatically, a ship must gain negative buoyancy, either increasing its own weight or decreasing displacement of the water. To control their weight, submarines are equipped with ballast tanks, which can be filled with either outside water or pressurized air.

For general submersion or surfacing, submarines use the forward and aft tanks, called Main Ballast Tanks or MBTs, which are opened and completely filled with water to submerge, or filled by pressurized air to surface. Under submerged conditions, MBTs generally always stay flooded, which simplifies their design, so on many submarines these tanks are simply a section of interhull space. For more precise and quick control of depth, submarines use smaller Depth Control Tanks or DCTs, also called hard tanks due to their ability to withstand higher pressure. The amount of water in depth control tanks can be controlled either to reflect changes in outside conditions or change submersion depth. Depth control tanks can be located either near the submarine's center of gravity, or separated along the submarine body to prevent affecting trim.

When submerged, the water pressure on submarine's hull can reach 4 MPa for steel submarines and up to 10 MPa for titanium submarines like Komsomolets, while the pressure inside stays the same. This difference results in hull compression, which decreases displacement. Water density also increases, as the salinity and pressure are higher, but this does not compensate for hull compression, so buoyancy falls with depth. A submerged submarine is in an unstable equilibrium, having a tendency to either fall down to the ocean floor or float up to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces.

Submarines in a neutral buoyancy condition are not intrinsically stable in trim. To sustain desired trim, submarines use specialized forward and aft trim tanks. Pumps can move water between these tanks, changing the weight distribution and therefore creating a moment to turn the sub upwards or downwards. A similar system is sometimes used to maintain stability.

Sail of the French nuclear submarine Casabianca; note the diving planes, camouflaged masts, periscope, electronic warfare masts, door and windows.The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several surfaces, which can be turned to create corresponding hydrodynamic forces when a submarine moves at sufficient speed. The stern planes, located near the propeller and normally oriented horizontally, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on many submarines. The fairwater planes on the sail and/or bow planes on the main body, both also horizontal, are located closer to the centre of gravity, and are used to control depth with less effect on the trim.

When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the sub may even partially jump out of the water, but it inflicts serious damage on some submarine systems, primarily pipes.

Modern submarines use an inertial guidance system for navigation while submerged, but drift error unavoidably builds up over time. To counter this, the Global Positioning System will occasionally be used to obtain an accurate position. The periscope - a retractable tube with prisms allowing a view to the surface - is only used occasionally in modern submarines, since the range of visibility is short. The Virginia-class submarines have "photonics masts" rather than hull-penetrating optical tube periscopes. These masts must still be hoisted above the surface, and employ electronic sensors for visible light, infrared, laser range-finding, and electromagnetic surveillance.

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