The oceans are most turbulent where wind and water meet: on their surface. The waves racing across the ocean are a sign of energy originally transmitted by the sun and whipped up in winds that rush from one side of the planet to the other. Ships struggle and tumble over tough seas where no fish worth its salt would ever swim in. Sailing ships take advantage of the winds and gusts of air to create a very effective form of propulsion. Diesel-powered ships stay on the surface for another reason: their engines need a constant supply of oxygen to burn fuel. In theory, it should be much easier for ships to swim under the waves, where the water is calmer and offers less resistance; in practice this leads to a different series of problems.
If you’ve ever gone snorkeling or diving, you will 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 when the water pressure is high, everything feels uncomfortable and claustrophobic. Submarines are ingenious constructions designed to transport people safely through this very harsh environment. Although originally invented as military machines, and most large submarines are still built for the world’s navies, some smaller submarines also work as scientific research vessels. Most of them are submersibles (generally small, non-motorized, one- or two-person submarines, attached to scientific research vessels during their operation).
Structure and parts
The pressure of the water pushing inward is the biggest problem for anyone who wants to go deep below the ocean surface. Even with scuba tanks, we can only dive that far because the immense pressure soon makes breathing impossible. At a depth of 600 m (2000 ft), the maximum depth you will ever dive to, the water pressure is over 60 times higher than on the surface!
How do subs survive where humans cannot survive? The hull of a standard ship is the metal outside that keeps the water out. Most submarines have two hulls, one inside the other, so that they can survive. The outer hull is waterproof, while the inner hull (called the pressure hull) is much stronger and more resistant to the immense water pressure. The strongest submarines have hulls made of tough steel or titanium.
Just as sharks have fins on their bodies that help them swim and dive, submarines have fins called diving planes or hydroplanes. They work a bit like the wings and control surfaces (pivoting flaps) of an airplane, creating an upward force called lift. Buoyancy is the tendency of something to sink, rise, or float at a certain depth. While underwater, a submarine has negative buoyancy i.e. it tends to sink when it is not moving. But when the submarine’s propellers push it forward, the water rushes over the planes, creating a buoyancy force called buoyancy that helps it stay at a certain depth, creating a state of neutral buoyancy (swimming). The planes can be tilted to change the buoyancy force so the submarine can soar or dive through the sea as needed. The planes take control of the submarine over its depth most of the time. The amount of lift they generate depends on both the angle of inclination and the speed of the submarine (just as the lift generated by the wings depends on the speed and “angle of attack” of the plane).
There are spaces between the two hulls that can be filled with either air or water. These are known as ballast tanks and, together with the diving planes, provide subordinate control over their buoyancy, especially during the first part of a dive or when returning from the depths to the surface. When the ballast tanks are filled with air, the submarine rises to the surface because it has positive buoyancy. When there is water in the tanks, the submarine has negative buoyancy, so it sinks deeper into the ocean. The front tanks (called the front trim tanks) are usually filled with water or air first, so that the front (bow) of the submarine falls or rises before the stern (stern). The ballast tanks can also be used to help a submarine to the surface very quickly in an emergency.
Gasoline and diesel engines in passenger cars and trucks, as well as jet engines in aircraft, require a supply of oxygen from the air in order to function. The situation is different with submarines that work underwater where there is no air. Most of the submarines, with the exception of the nuclear submarines, have diesel-electric motors. The diesel engine works normally when the submarine is near the surface, but it does not drive the submarine’s propellers directly. Instead, it powers a generator that charges huge batteries. These drive an electric motor, which in turn drives the propellers. Once the diesel engine has fully charged the batteries, the submarine can shut down its engine and go underwater, where it will rely entirely on battery power.
Early military submarines used breathing tubes called snorkels to supply oxygen to their engines from the air above the ocean, but that meant they had to work very close to the surface, where they were vulnerable to attack by aircraft. Most large military submarines today are nuclear powered. Like nuclear power plants, they have small nuclear reactors, and since they don’t need air to operate, they can generate electricity to power the electric motors and propellers, whether they’re on the surface or deep underwater.
U-boats are cigar-shaped so they can slide easily through the water, but right in the middle there is a tall tower. On older submarines, the tower was crammed with navigation and other equipment and was sometimes referred to as the command tower (because historically it contained submarine controls). It is also referred to simply as the tower or the sail because in a modern submarine the control and navigation equipment takes up more space and is usually located in the hull.
Light penetrates water rather poorly, so that it gets darker the deeper you sink. Most of the time, submarine pilots can’t even see where they’re diving! Submarines have periscopes (which see tubes that slide up through the tower), but they’re only useful when the submarines are on the surface or directly below. A variety of electronic devices are used to navigate submarines. First of all, there is GPS satellite navigation, which uses space satellites to inform the submarine of its position. There is also SONAR, a radar-like system that sends sound pulses into the ocean and listens for echoes reflected from the ocean floor or other nearby submarines. Another important navigation system on board a submarine is the so-called inertial guidance. Gyroscopes are used to track how far and in which direction the submarine has moved without referring to outside information. Inertial guidance is only accurate for a certain period of time (around 10 days) and occasionally needs to be corrected using GPS, radar or other data.
Life support systems
A large military submarine has dozens of people on board. How can they eat, sleep and breathe, buried deep under the sea, in ice-cold water, for months at a time? A submarine is a completely sealed environment. The nuclear engine provides heat and generates electricity – and the electricity powers all of the life support systems that submarine drivers need. Electrolysis chemically separates water molecules from oxygen (converting H2O into H2 and O2) and removing unwanted carbon dioxide from the air. Submarines can even make their own drinking water from seawater by using electricity to remove the salt. The waste is compacted in steel cans, which are ejected from a lock system (a watertight exit in the hull) and dumped onto the seabed.
Other important information
Most of us are more familiar with submarines from films or books, in some holiday regions special tourist versions are used to make the beauty of the underwater world visible. Submarines allow us to stay under water for longer periods of time and can cover certain distances without surfacing in between. You can also move in all spatial directions underwater.
A submarine has special tanks called ballast tanks (also called diving cells). When these tanks are filled with air, the submarine behaves like a normal ship, it swims due to the buoyancy on the water surface. The ballast tanks are filled with water so that the submarine can dive. The density of the submarine increases, the submarine becomes heavier than water overall – it sinks.
In order to surface again, air is filled into the tanks, which displace the water. The overall density of the submarine will decrease again, the weight will decrease and it will rise. We know this principle in nature with fish with a swim bladder.
The dynamic diving
A submarine can also dive through the oars by gaining speed. Depending on their position, the rudders provide downward or upward lift – it works in a similar way to the wings of an airplane. A submarine is pushed down (or up) as long as it moves forward. If the drive fails, the submarine will rise again on its own. This type of diving is called “dynamic diving”.
What are trim cells?
The trim cells are arranged in the bow and stern of the submarine and ensure that the boat remains in a horizontal position in the water. If the stern protrudes too far, then water is pumped into the trim cells at the stern and the boat becomes lighter at the bow (the center of gravity shifts towards the stern) and the situation changes.
The air for the diving and trim cells is carried as compressed air.