A submarine is a specialized watercraft that can operate underwater. Submarines, first widely used in World War I, are used by all major navies today, especially the American, Russian and British navies. Civilian submarines and submersibles are used for marine and freshwater science and for work at depths too great for human divers.

Submarines are always referred to as "boats" except in some formal documents. 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 German "Boot" rhymes with the English "boat."

Submarines encompass one of the largest ranges in capabilities of any vessel. They range from small two-man vessels that can examine the sea floor for a few hours to the Russian Typhoon class, which can remain submerged for half a year 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.

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.

All small modern submarines and submersibles, as well as the oldest ones, have a single hull. Large submarines generally have an additional hull or hull sections outside. This external hull, which actually forms the shape of submarine, is called the outer hull or light hull, as it does not have to hold any pressure difference. Inside the outer hull there is a strong hull, or pressure hull, which withstands sea pressure and has normal atmospheric pressure inside.

Starting from the times of World War I it was realized that the optimal shape for withstanding pressure conflicted with optimal shape for seaworthiness and minimized water resistance, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two hulls shapes, internal for holding pressure, and external for optimal shape. Until the end of the World War II most submarines had additional partial cover on top, bow and stern, built of thinner metal, which was flooded when submerged. Germany went further with Type XXI, the general predecessor of modern submarines, fully enclosing the pressure hull inside the light one, however optimised for submerged navigation unlike earlier designs.

After World War II, approaches split. The Soviet Union changed their designs, basing them on the latest German developments. All post-WWII heavy Soviet and Russian submarines are built with a double hull structure. American and most other Western submarines retain a single-hull approach. They still have light hull sections in bow and stern, which house main ballast tanks and provide hydrodynamically optimized shape, but the main cylindrical hull section has only a single plating layer.

Despite being no longer needed for different shapes, the double-hull approach still has a number of advantages. The ring stiffeners and longitudinals are located between hulls, and the light hull also can be used to mount equipment not requiring constant pressure to operate, while attaching it directly to the pressure hull could cause dangerous local stress. These measures save a lot of space inside the pressure hull, which is much heavier and longer to build than the light hull. In case the submarine is damaged, the light hull can take most of the damage, which does not compromise the boat's integrity, as long as the strong hull is intact. Light hull also can be acoustically decoupled from the pressure hull, significantly reducing noise from internal equipment, improving stealth or allowing to use simpler internal layout and equipment mounting.

The major downside of double-hull structure is the significantly greater amount of manual work required to construct it. The Soviet Union had implemented the welding technology earlier and had enough cheap qualified workforce available, but the high cost of manual labor in the United States made less expensive single-hull approach preferable. Another reason for double-hull construction in the Soviet Union was operation under the Arctic Ocean, where submarines had to break thick ice to launch their missiles, which could damage the hull. However, the double-hull approach is today being considered for future submarines in the United States as well, to increase payload capacity, stealth and operational reach.

The first military submarine was ''Turtle'', a hand-powered egg-shaped device designed by the American David Bushnell, to accommodate a single man. It was the first verified submarine capable of independent underwater operation and movement, and the first to use screws for propulsion. During the American Revolutionary War, Turtle (operated by Sgt. Ezra Lee, Continental Army) tried and failed to sink a British warship, HMS ''Eagle'' (flagship of the blockaders) in New York harbor on September 7, 1776.


In 1800, France built a human-powered submarine designed by Robert Fulton, the ''Nautilus''. The French eventually gave up with the experiment in 1804, as did the British when they later considered Fulton's submarine design.

During the War of 1812, in 1814, Silas Halsey lost his life while using a submarine in an unsuccessful attack on a British warship stationed in New London harbor.

In 1851, a Bavarian artillery corporal, Wilhelm Bauer, took a submarine designed by him called the Brandtaucher (incendiary-diver) to sea in Kiel Harbour. This submarine was built by August Howaldt and powered by a treadwheel. It sank but the crew of 3 managed to escape. The submarine was raised in 1887 and is on display in a museum in Dresden.

During the American Civil War, the Union was the first to field a submarine. The French-designed ''Alligator'' was the first U.S. Navy sub and the first to feature compressed air (for air supply) and an air filtration system. She was the first submarine to carry a diver lock which allowed a diver to plant electrically-detonated mines on enemy ships. Initially hand-powered by oars, she was converted after 6 months to a screw propeller powered by a hand crank. With a crew of 20, she was larger than Confederate submarines. Alligator was 47 feet (14.3 meters) long and about 4 feet (1.2 meters) in diameter. She was lost in a storm off Cape Hatteras on April 1, 1863 while uncrewed and under tow to her first combat deployment at Charleston.

The Confederate States of America fielded several human-powered submarines including ''H. L. Hunley'' (named for one of her financiers, Horace Lawson Hunley) . The first Confederate submarine was the 30-foot long Pioneer which sank a target schooner using a towed mine during tests on Lake Pontchartrain but she was not used in combat. She was scuttled after New Orleans was captured and in 1868 was sold for scrap.

Hunley was intended for attacking the North's ships, which were blockading the South's seaports. The submarine had a long pole with an explosive charge in the bow, called a spar torpedo. The sub had to approach an enemy vessel, attach the explosive, move away, and then detonate it. It was extremely hazardous to operate, and had no air supply other than what was contained inside the main compartment. On two occasions, the sub sank; on the first occasion half the crew died and on the second, the entire eight-man crew (including Hunley himself) drowned. On February 18, 1864 Hunley sank USS ''Housatonic'' off the Charleston Harbor, the first time a submarine successfully sank another ship, though she sank in the same engagement shortly after signaling her success. Another Confederate submarine was lost on her maiden voyage in Lake Pontchartrain; she was found washed ashore in the 1870s and is now on display at the Louisiana State Museum. Submarines did not have a major impact on the outcome of the war, but did portend their coming importance to naval warfare and increased interest in their use in naval warfare.

The first submarine that did not rely on human power for propulsion was the French Navy submarine Plongeur, launched in 1863, and using compressed air at 180 psi. The 14 meter long craft was designed to carry a crew of two, dive 30 metres (96 feet), and demonstrated dives of two hours. When on the surface it ran on a steam engine, but underwater such an engine would quickly consume the submarine's oxygen. So Monturiol turned to chemistry to invent an engine that ran on a reaction of potassium chlorate, zinc, and manganese peroxide. The beauty of this method was that the reaction which drove the screw released oxygen, which when treated was used in the hull for the crew and also fed an auxiliary steam engine that helped propel the craft under water. In spite of successful demonstrations in the Port of Barcelona, Monturiol was unable to interest the Spanish navy, or the navy of any other country.

In 1870, the French writer, Jules Verne, published the science fiction classic 20,000 Leagues under the Sea, which concerns the adventures of a maverick inventor in Nautilus, a submarine more advanced than any that existed at that time. The story inspired inventors to build more advanced submarines.

In 1879, the Peruvian government, during the War of the Pacific commissioned and built a submarine. That was the fully operational Toro Submarino, which nevertheless never saw military action before being scuttled after the defeat of that country in the war to prevent its capture by the enemy. The first submarine built in series, however, was human-powered. It was the submarine of the Polish inventor Stefan Drzewiecki — 50 units were built in 1881 for Russian government. In 1884 the same inventor built an electric-powered submarine.


Discussions between the English reverend, George Garrett, and the industrially and commercially adept Swede, Thorsten Nordenfelt, led to a series of steam powered submarines. The first was the Nordenfelt I, a 56 tonne, 19.5 metre long vessel similar to Garret's ill-fated Resurgam (1879), with a range of 240 kilometres and armed with a single torpedo in 1885. Like the Resurgam, it operated on the surface by steam, then it shut down its engine to dive. Greece, fearful of the return of the Ottomans, purchased it. Nordenfelt then built the Nordenfelt II (Abdülhamid) in 1886 and Nordenfelt III (Abdülmecid) in 1887, a pair of 30 metre long submarines with twin torpedo tubes, for the Ottoman navy. Abdülhamid became the first submarine in history to fire a torpedo while submerged under water. Nordenfelt's efforts culminated in 1887 with the Nordenfelt IV which had twin motors and twin torpedoes. It was sold to the Russians, but proved unstable, ran aground and was scrapped.



Another fully capable military submarine was the electrically powered vessel built by the Spanish engineer and sailor, Isaac Peral, for the Spanish Navy. It was launched on September 8 1888. It had two torpedoes, new air systems, a hull shape, propeller and cruciform external controls anticipating much later designs. Its underwater speed was ten knots, but it suffered from the short range and difficulty of recharging at sea of battery powered systems. In June 1890 Peral's submarine launched a torpedo while submerged under the sea. The Spanish Navy scrapped the project. Many more submarines were built at this time by various inventors, but they were not to become effective weapons until the 20th century.

The turn of century era marked a pivotal time in the development of submarines, with a number of important technologies making their debut, as well as the widespread adoption and fielding of submarines by a number of nations. Diesel electric propulsion would become the dominant power system and things such as the periscope would become standardized. Large numbers of experiments were done by countries on effective tactics and weapons for submarines, all of which would culminate in them making a large impact on coming World War I.

In 1895, the Irish inventor John Philip Holland designed submarines that, for the first time, made use of internal combustion engine power on the surface and electric battery power for submerged operations. In 1902, Holland received . Some of his vessels were purchased by the United States, the United Kingdom, the Imperial Russian Navy, and Japan, and commissioned into their navies around 1900 (1905 for Japan, too late to serve in the war).


Commissioned in June 1900, the French steam and electric submarine Narval introduced the classic double-hull design, with a pressure hull inside the outer light hull. These 200-ton ships had a range of over 100 miles on the surface, and over 10 miles underwater. The French submarine Aigrette in 1904 further improved the concept by using a diesel rather than a gasoline engine for surface power. Large numbers of these submarines were built, with seventy-six completed before 1914.

The first time military submarines had significant impact on a war was in World War I. Forces such as the U-boats of Germany saw action in the First Battle of the Atlantic, and were responsible for the sinking of the RMS Lusitania, which receives much of the credit for prompting the United States to enter the war.

The U-boats' ability to function as practical war machines relied on new tactics, their numbers, and submarine technologies such as combination diesel/electric power system that had been developed in the preceding years. More like submersible ships than the submarines of today, U-boats operated primarily on the surface using regular engines, submerging occasionally to attack under battery power. They were roughly triangular in cross-section, with a distinct keel, to control rolling while surfaced, and a distinct bow. In 1916, Serbian pilot Konjovic entered the history books as the first pilot who destroyed a submarine from the air, a French submarine Foucault in the Adriatic. When he saw that there were survivors after he dropped the bombs, he came down in his Lohner hydro-plane and saved them. For this heroic act, the French Government awarded him on 14 February 1968 a special recognition for heroism, humanity and compassion in sea battles. Even today at the official site of the French Navy there is a picture depicting Konjovic saving the sailors.

Germany had the largest submarine fleet during World War II. Due to the Treaty of Versailles limiting the surface navy, the rebuilding of the German surface forces had only begun in earnest a year before the outbreak of World War II. Having no hope of defeating the vastly superior Royal Navy decisively in a surface battle, the German High Command immediately stopped all construction on capital surface ships save the nearly completed Bismarck class battleships and two cruisers and switched the resources to submarines, which could be built more quickly. Though it took most of 1940 to expand the production facilities and get the mass production started, more than a thousand submarines were built by the end of the war.


Germany put submarines to devastating effect in the Second Battle of the Atlantic in World War II, attempting but ultimately failing to cut off Britain's supply routes by sinking more ships than Britain could replace. The supply lines were vital to Britain for food and industry, as well as armaments from the USA. Although the U-boats had been updated in the intervening years, the major innovation was improved communications, encrypted using the famous Enigma cipher machine. This allowed for mass-attack tactics or "wolf packs", (Rudel), but was also ultimately the U-boats' downfall.

After putting to sea, the U-boats operated mostly on their own trying to find convoys in areas assigned to them by the High Command. If a convoy was found, the submarine did not attack immediately, but shadowed the convoy to allow other submarines in the area to find the convoy. These were then grouped into a larger striking force and attacked the convoy simultaneously, preferably at night while surfaced.

From September 1939 to the beginning of 1943 ,the Uboatwaffe scored unprecedented success with these tactics, but were too few to have any decisive success. By the spring of 1943, German U-boat construction was at full capacity, but this was more than nullified by equally increased numbers of convoy escorts, aircraft, and technical advances like radar and sonar. Huff-Duff and Ultra allowed the Allies to route convoys around wolf packs when they detected them from their radio transmissions. The result of Allied efforts were devastating: from March to July of that year, over 130 U-boats were lost, one-third of which lost in May alone (41 boats). Allied losses dropped dramatically in the same period, from 750,000 tons in March, to only 188,000 in July. Although the Second battle of the Atlantic would continue to the last day of the war, the U-boat Arm was unable to stem the tide of men and material, paving the way for Operation Torch , Operation Husky, and ultimately, D-Day.


Winston Churchill wrote that the U-boat "peril" was the only thing that ever gave him cause to doubt the Allies' eventual victory.

Main article: Imperial Japanese Navy submarines

Japan had the most varied fleet of submarines of World War II, including manned torpedoes (Kaiten manned torpedo), midget submarines (Ko-hyoteki, Kairyu), medium-range submarines, purpose-built supply submarines (many for use by the Army), long-range fleet submarines (many of which carried an aircraft), submarines with the highest submerged speeds of the conflict (Sen taka ''I-200'' class submarine), and submarines that could carry multiple bombers (WWII's largest submarine, the Sen toku ''I-400'' class submarine). These submarines were also equipped with the most advanced torpedo of the conflict, the oxygen-propelled Type 95.

Overall, despite their technical prowess, Japanese submarines were relatively unsuccessful. They were often used in offensive roles against warships, which were fast, maneuverable and well-defended compared to merchant ships. In 1942, Japanese submarines sank two aircraft carriers among other warships, but were not able to sustain these results afterwards. By the end of the war, submarines were instead often used to transport supplies to island garrisons.