How submarines work: A simple introduction from Explain that Stuff!

What is a submarine?

If you've ever gone snorkelling or scuba diving, you'll know that life underwater is very different from life on the surface. It's dark and difficult to see, there's no air to breathe, and intense water pressure makes everything feel uncomfortable and claustrophobic. Submarines are ingenious bits of engineering designed to carry people safely through this very harsh environment. Although they were originally invented as miltary machines, and most large subs are still built for the world's navies, a few smaller subs do work as scientific research vessels. Most of these are submersibles (generally small, unpowered, one- or two-person submarines tethered to scientific research ships as they operate).

Parts of a submarine

Photo: Despite many technological advances, the basic concept of the submarine has changed little in over a century, since John Holland designed the USS Holland, the US Navy's first submarine. Photo by courtesy of Naval Historical Center.

These are some of the key parts of a typical submarine.

Pressure hull

The pressure of water pushing inwards is the biggest problem for anyone who wants to go deep beneath the ocean surface. Even with scuba tanks, we can dive only so far because the immense pressure soon makes it impossible to breath. At a depth of 600 m (2000ft), the maximum depth subs ever dive to, the water pressure is over 600 times greater than it is at the surface!

How do subs survive where people can't? The hull of a standard ship is the metal outside that keeps the water out. Most submarines have two hulls, one inside the other, to help them survive. The outer hull is waterproof, while the inner one (called the pressure hull) is much stronger and resistant to immense water pressure. The strongest submarines have hulls made from tough steel or titanium.

Photo: An Australian Collins-class submarine, HMAS Rankin (SSK 78), cruising just below the surface with its tower breaking the waves. Photo by David A. Levy courtesy of US Navy.

Ballast tanks

There are spaces in between the two hulls that can be filled with either air or water. These are called the ballast tanks. When they are filled with air, the submarine rises to the surface; with water inside the tanks, the sub sinks towards the seabed. By changing the amount of water or air in the tanks, the submarine alters its buoyancy (ability to float) so it can move close to the surface or deeper down. The tanks at the front (known as the front trim tanks) are usually filled with water or air first, so the submarine's front (bow) falls or rises before its rear (stern).

Engine

Gasoline and diesel engines used by cars and trucks, and jet engines used by planes, need a supply of oxygen from the air to make them work. Things are different for submarines, which operate underwater where there is no air. Most submarines except nuclear ones have diesel-electric engines. The diesel engine operates normally when the sub is near the surface but it doesn't drive the sub's propellers directly. Instead, it powers an electricity generator that charges up huge batteries. These drive an electric motor that, in turn, powers the propellers. Once the diesel engine has fully charged the batteries, the sub can switch off its engine and go underwater, where it relies entirely on battery power.

Photo: Sailors stand on the deck of the fast-attack submarine USS Philadelphia (SSN 690) as it heads out to sea. Photo by Octavio N. Ortiz, courtesy of US Navy.

Early military submarines used breathing tubes called snorkels to feed air to their engines from the air above the sea, but that meant they had to operate very near the surface where they were vulnerable to attack from airplanes. Most large military submarines are now nuclear-powered. They have small nuclear reactors onboard and, since they need no air to operate, they can generate power to drive the electric motors and propellers whether they are on the surface or deep underwater.

Tower

Submarines are cigar-shaped so they can slip smoothly through the water. But in the very center, there's a tall tower packed with navigation and other equipment. Sometimes known as the conning tower (because, historically, it contained a submarines controls), it's also referred to simply as the tower or the sail.

Planes

Just as sharks have fins on their bodies to help them swim and dive, so submarines have fins called diving planes or hydroplanes. They work a bit like the wings and control surfaces (swivelling flaps) on an airplane. As the sub's propellers push it forward, water rushes over the planes, creating an upward or downward force that helps the sub gradually rise or fall. The fins can be tilted to change the angle at which it climbs or dives through the sea.

Navigation systems

Photo: Periscopes are useful if you're near the surface searching for enemy ships but they're useless underwater. Photo by Jeffery S. Viano courtesy of US Navy.

Light doesn't travel well through water, so it gets darker and darker the deeper down you go. Most of the time, submarine pilots can't even see where they're going! Submarines have periscopes (seeing tubes that can be pushed up through the tower), but they're useful only when subs are on the surface or just beneath it. Submarines navigate using a whole range of electronic equipment. There's GPS satellite navigation, for starters, which uses space satellites to tell the submarine its position. There's also SONAR, a system similar to radar, which sends out pulses of sound into the sea and listens for echoes reflecting off the seabed or other nearby submarines. Another important navigation system onboard a submarine is known as inertial guidance. It's a way of using gyroscopes to keep track of how far the submarine has travelled, and in which direction, without referring to any outside information. Inertial guidance is accurate only for so long (10 days or so) and occasionally needs to be corrected using GPS, radar, or other data.

Photo: The sonar apparatus in a typical submarine. Photo by courtesy of US Navy.

Life-support systems

A large military submarine has dozens of people onboard. How can they eat, sleep, and breathe, buried deep beneath the sea, in freezing cold water, for months at a time? A submarine is a completely sealed environment. The nuclear engine provides warmth and generates electricity—and the electricity powers all the life-support systems that submariners need. It makes oxygen for people to breathe by chemically separating molecules of water (turning H₂O into H₂ and O₂) and it scrubs unwanted carbon dioxide from the air. Subs can even make their own drinking water from seawater using electricity to remove the salt. Trash is compacted into steel cans, which are ejected from an airlock system (a watertight exit in the hull) and dumped on the seabed.

A brief history of submarines

Photo: The USS Holland (Submarine Torpedo Boat # 1) underway, circa 1900. Photo by courtesy of Naval Historical Center.

1620: Englishman Cornelis Drebble (1572–1633) built the first submarine by waterproofing a wooden, egg-shaped boat with leather and coating the whole thing in wax. Scientists are uncertain whether Drebble's boat ever set sail.
1776: During the US revolution, David Bushnell (1742–1824) built a hand-powered one-person submarine called the Turtle to help attack British warships.
1800: American steam engineer Robert Fulton (1765–1815) designed a convertible ship with folding-down sails that could turn itself into a submarine for travelling underwater.
1897: American inventor Simon Lake (1866–1945) launched the Argonaut, the first submarine to operate in the open sea.
1900: The US Navy launched its first ever submarine, the USS Holland, named for its Irish-American inventor John Holland (1840–1914). Although Holland had offered submarines to the Navy for years beforehand, it had originally shown no interest.
1914-18: During World War I, the German navy operated a fleet of highly effective military submarines called U-boats (short for Unterseeboot, which means underwater ship). In the 1930s, the Germans started using snorkel tubes (invented by a Dutch engineer) to supply air to their U-boat's diesel-electric engines, giving them greater range and effectiveness.
1955: The US Navy launched the USS Nautilus, the first nuclear-powered submarine.

Submarines