Submarine is a ship that can travel underwater. Most submarines are designed for use in war—to attack enemy ships or to fire missiles at enemy countries. These submarines range in length from about 200 feet (61 meters) to more than 550 feet (168 meters). Their rounded hulls are about 30 feet (9 meters) in diameter. More than 150 crew members can live and work in a large submarine.
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Some submarines are used for scientific research. These underwater craft explore the ocean depths and gather scientific information. They are smaller than military submarines and carry only a few crew members. See Ocean (Exploring the ocean) .
In war, a submarine usually attacks from beneath the surface of the water. A submarine must remain underwater to be effective. Early submarines could not stay submerged for long periods. They had to surface every few hours for air for their engines and crews. Enemy planes and ships could then attack them. Today, nuclear submarines can stay underwater for months at a time. Nuclear engines do not need oxygen to operate, and modern submarines can produce air.
A submarine’s long, cigar-shaped body enables it to move swiftly underwater. Its working and living spaces, weapons, and machinery are encased in a pressure hull made of high-strength steel or titanium (a strong, lightweight metal). At operating depths, a pressure hull cannot be crushed by the pressure of the water around it. Built into the bow and stern of the pressure hull are tanks that, when filled with water, give the submarine ballast (weight) for diving. Submarines that are not built in the United States usually have a second, outer hull. The space between the hulls is used to store ballast tanks and equipment that does not need protection from water pressure.
A tall, thin structure called the sail rises from the middle of a submarine’s deck. The sail stands about 20 feet (6.1 meters) high. It holds the periscopes and the radar and radio antennas. The top of the sail also serves as the bridge, from which the captain directs the craft when on the surface. Steel fins called diving planes stick out from both sides of the sail or bow and from the stern. They guide the ship to different depths. One or two propellers in the stern drive the submarine. Rudders mounted above and below the propellers steer the craft.
Kinds of submarines
There are two main kinds of submarines, attack submarines and ballistic missile submarines.
Attack submarines
are designed to search out and destroy enemy submarines and surface ships. They also are used to attack targets on land and to gather information about enemy vessels. 
Most attack submarines in the United States Navy range in length from about 290 to 360 feet (88 to 110 meters). They have about 130 crew members. Most of these submarines have nuclear-powered engines and carry torpedoes and guided missiles. Attack submarines track and locate their underwater targets with sonar (_s_ound _n_avigation _a_nd _r_anging) equipment, which detects sounds underwater. They use periscopes and radar equipment to identify enemy ships on the surface.
Modern U.S. attack submarines fire their torpedoes from four tubes located along the sides of the hull. Torpedoes have homing devices that follow the target and guide the torpedo to it (see Torpedo ). Older submarines had their torpedo tubes in the bow. But in modern submarines, the sonar is located in the bow—far away from the noise of the ship’s propeller.
Some submarines can also fire antisubmarine missiles from the torpedo tubes. These short-range weapons have a torpedo or a nuclear warhead that can destroy submerged submarines from as far away as 30 nautical miles (57 kilometers). Other submarines can attack surface ships and onshore targets with cruise missiles, which have short wings that open after launching. Cruise missiles can be directed to avoid enemy defenses.
Ballistic missile submarines
are designed to attack enemy cities and military bases ashore. They carry long-range missiles that can strike targets from about 1,500 to 4,000 nautical miles (2,780 to 7,410 kilometers) away. 
Ballistic missile submarines are larger than attack submarines, measuring from about 380 to 560 feet (115 to 170 meters) long. Their crews number about 150 members. Missiles are fired from silos (launching tubes) in the submarine’s hull. The missiles can carry multiple warheads (explosive sections). Each warhead can be aimed at a separate target. Ballistic missile submarines also carry torpedoes for defense.
Special equipment aboard the submarine plots the craft’s exact location and determines the path of the ballistic missile to its destination. This equipment, called the inertial navigation system, consists of accurate measuring devices linked to computers. The system helps navigate the submarine by recording its starting position on a voyage and its movement in all directions. This information is fed into a guidance system in the missile to provide the precise distance and direction to the target. After launching, the missile’s own inertial navigation system guides the weapon. See Inertial guidance .
The power plant
The engine of a nuclear submarine consists of a nuclear reactor and a steam generator. The reactor uses uranium for fuel and splits uranium atoms in a controlled process called fission. This process produces intense heat. See Nuclear energy .
Pipes carry water from the steam generator to the reactor, where the water is heated to about 600 °F (315 °C). This water is kept under pressure so it will not boil. Instead, it returns to the steam generator and boils a supply of unpressurized water that turns to steam. The steam spins large turbines (wheels), producing power to rotate the propeller shaft and run the ship.
Nuclear engines operate without air and consume much less fuel than do other engines. About 4 pounds (1.8 kilograms) of uranium fuel produce more energy than 10 million gallons (38 million liters) of fuel oil.
Some submarines run on diesel engines. However, for combat the U.S. Navy has not built a diesel-powered submarine since 1959. Diesel engines burn fuel oil and need air for combustion. A submarine can use these engines only when on or near the surface. Electric batteries supply power underwater. Late in World War II (1939-1945), the German Navy equipped submarines with an air tube called a snorkel. The snorkel drew air into the submarine when the craft was near the surface. This air replaced oxygen used up by the diesel engines and crew. But the snorkel left a trail through the water and could reveal the submarine’s location.
Submarines did not become true underwater ships until the development of nuclear-powered engines. In the early 1990’s, however, some countries began building submarines with air-independent systems, which allow diesel engines to operate underwater for several weeks at low speeds without taking in air.
How a submarine operates
Surface operation.
On the surface of the water, a submarine performs much like any other ship. A submarine can cruise at about 20 knots (nautical miles per hour) on the surface. However, modern submarines spend little time on the surface.
Diving.
A submarine dives by flooding its ballast tanks with water. The added weight causes the ship to lose its positive buoyancy (ability to stay afloat), and it becomes neutrally buoyant. Then the submarine’s diving planes are tilted down and the craft glides smoothly down into the water.
A submarine can dive to a depth of over 100 feet (30 meters) in less than a minute. Most modern U.S. combat submarines operate at a depth of about 1,300 feet (400 meters) or less. If they were to dive significantly deeper, they would be destroyed by water pressure.
Underwater operation.
A submarine travels underwater somewhat as an airplane moves through the air. The diving planes angle up and down to raise or lower the ship. Two crew members sit at aircraftlike controls. They push the control wheel forward to make the submarine descend or pull the wheel toward them to make the craft rise. Turning the wheel to the right or left moves the rudder and steers the ship.
A nuclear submarine can travel faster than 30 knots underwater. Its sonar warns of any obstacles in its path, and the inertial guidance system keeps a constant check on the position of the submarine. However, submarines normally travel at about 10 knots or less to avoid detection by sonar.
Resurfacing.
A submarine is brought to the surface in one of two ways. Water is blown out of the ballast tanks by compressed air, or the diving planes are tilted so the submarine angles up.
Life aboard a submarine
Attack submarines of the United States Navy go on patrol for two or three months. They frequently stop in ports during the voyage. Ballistic missile submarines stay on patrol for about 60 days and spend almost the entire period underwater.
The sailors on attack submarines and ballistic missile submarines have many comforts during their cruise. For example, large air-conditioning units keep the temperature and humidity at comfortable levels. Libraries, motion pictures, and game rooms help ease the monotony of life beneath the sea.
On most submarines, every member of the crew works a daily four-hour shift called a watch. At the end of their watch, the crew members are relieved and go off duty for eight hours. They may have to do some maintenance work on the ship but are mostly free to relax or study until they return to their stations. The work assignments vary so that all the crew members have days off.
Nuclear submarines produce their own air and drinking water. A process called electrolysis extracts oxygen from seawater and provides all the air needed for the crew. Chemical filters remove any harmful elements from the air in a submarine. Thick lead plates around the nuclear reactor shield the crew from radiation. Machines distill ocean water into pure drinking water.
A submarine returns to port at the end of its cruise. It receives any needed repairs and takes on additional supplies. A ballistic missile submarine also changes crews. Each ballistic missile submarine in the U.S. Navy has two crews, the blue crew and the gold crew. After one crew completes a patrol, it is replaced by the other. The oncoming crew takes the submarine on another mission. The men returning to shore go on leave and then receive additional training. After the crew of an attack submarine completes a patrol, it spends six months in port and in local operations.
History
Early submarines.
The first workable submarine was a wooden rowboat covered with waterproof hides. The builder, a Dutch scientist named Cornelius van Drebbel, demonstrated his invention in England about 1620. Designers constructed many undersea craft during the next century. But little use was made of such ships until the Revolutionary War in America (1775-1783). During that war, David Bushnell, a student at Yale College, designed the Turtle, a one-man submarine powered by a hand-cranked propeller. In 1776, the Turtle failed in an attempt to sink a British warship in New York Harbor. This mission was the first known attack by a submarine.
In 1800, the American inventor Robert Fulton built the Nautilus, a copper-covered submarine 21 feet (6.4 meters) long. Fulton tried to sell the Nautilus to France and the United Kingdom. But neither nation showed much interest in it, even though it sank several ships in demonstrations.
During the American Civil War (1861-1865), the Confederate submarine Hunley became the first underwater vessel to sink a ship in wartime. The Hunley carried an explosive attached to a long pole on its bow. In 1864, it rammed the Union ship Housatonic in Charleston Harbor off the coast of South Carolina. The explosion sank the Housatonic, but the Hunley went to the bottom with its victim. In 1995, a U.S. search team found the wreckage of the Hunley. The vessel was raised in 2000 and was brought to a nearby laboratory to be restored. After the restoration, which will take several years, the Hunley will be displayed in its own museum.
In 1898, the American inventor John P. Holland launched a 53-foot (16-meter) submarine powered by a gasoline engine and electric batteries. It could reach a speed of 6 knots submerged. The U.S. Navy bought this ship—its first submarine—in 1900 and named it the U.S.S. Holland.
Simon Lake, another American, invented the submarine periscope in 1902. His periscope used magnifying lenses that enabled a submerged submarine to sight distant targets. Lake also built submarines with wheels so they could roll along the bottom of the sea. In 1908, the United Kingdom launched the first diesel-powered submarine. Its engines were more powerful, cost less to operate, and produced fewer dangerous fumes than did gasoline engines. Most submarines used diesel engines until the development of nuclear power in the 1950’s.
World Wars I and II.
During World War I (1914-1918), Germany proved the submarine’s effectiveness as a deadly warship. In 1914, the German submarine U-9 sank three British cruisers within an hour. German submarines, called Unterseeboote or U-boats, blockaded the United Kingdom and took a heavy toll of merchant and passenger ships. U-boats became the terror of the seas by waging unrestricted war on Allied ships.
In May 1915, a German submarine torpedoed the British liner Lusitania. The attack killed 1,201 passengers, including 128 Americans. Public anger increased in the United States as U-boats sank one American merchant ship after another during the next year. These submarine attacks helped lead to the entry of the United States into the war in April 1917.
During World War II (1939-1945), U-boats sank thousands of merchant ships. The U-boats hunted in groups of up to 40. These groups were called wolf packs.
The Allies fought to protect their ships from the German submarines. Merchant ships formed large convoys (fleets) that were protected by destroyers and other ships. The development of radar and sonar helped locate the U-boats and reduced the danger of attack.
The U.S. submarines operated chiefly in the Pacific Ocean. They sank over half of Japan’s merchant ships and many of its warships. The Navy’s submarines also carried troops to raid enemy islands, laid mines in enemy harbors, and performed rescue missions.
Nuclear submarines.
In 1954, the U.S. Navy commissioned the first nuclear-powered submarine, the Nautilus. On its first voyage, the Nautilus broke all previous submarine records for underwater speed and endurance. In 1958, the Nautilus became the first submarine to sail under the ice at the North Pole. In 1960, the Triton traveled underwater around the world. That same year, the Seadragon navigated the Northwest Passage, the northern route from the Atlantic to the Pacific Ocean. 
In the late 1950’s, the U.S. Navy developed the first modern ballistic missile submarines. Each of these submarines had 16 missiles in the hull behind the sail. The early missiles carried nuclear weapons and could strike targets up to 1,200 nautical miles (2,220 kilometers) away.
In 1981, the United States commissioned the first Ohio- class submarine. The last ship of this class was commissioned in 1997. Ohio-class ships are the largest and most powerful U.S. submarines ever built. They are sometimes known as Trident submarines. Ohio- class submarines measure 560 feet (171 meters) long and carry 24 Trident missiles. Each missile has a range of about 4,600 nautical miles (8,520 kilometers) and can hold several individual warheads. Each warhead can be aimed at a separate target. The only longer submarines to ever be built were those of the Soviet Typhoon class of the 1980’s. In 1982, during the conflict between the United Kingdom and Argentina in the Falkland Islands, the British Royal Navy became the first navy to use nuclear-powered submarines in combat.
At the time of its breakup in 1991, the Soviet Union had the world’s largest submarine fleet—about 310 ships. Today’s submarine fleets are much smaller.
