Submarine

A submarine is a vessel designed to operate underwater for extended periods while maintaining the ability to surface. It is used for military purposes, marine research, and technical operations such as underwater cable installation and subsea communication. The defining characteristic of a submarine is its ability to adjust its weight balance in order to submerge or surface.

Modern submarines vary in size and capability. Some are small research vessels carrying two to four personnel, while large military submarines can carry hundreds of crew members during deployments lasting many months underwater.

Submerging and surfacing

Submarines have compartments called ballast tanks. When water is admitted to these tanks, the vessel's weight increases and it submerges. When pressurized air is blown into the tanks to force out the water, the weight decreases and the submarine surfaces. During movement, control surfaces called hydroplanes mounted on the hull move up and down to steer the vessel, similar to aircraft wings. Weight distribution and trim control are critical, as a submarine has limited support if its balance is disrupted.

Power and propulsion systems

For a long period, submarines used a dual system: a diesel engine on the surface to charge batteries, and an electric motor underwater, since combustion engines require air. The limitation of this system is that batteries must be recharged, forcing the submarine to approach the surface or use a snorkel. Nuclear-powered submarines, which became common from the mid-20th century onward, can operate underwater for months at a time without surfacing, as the reactor requires no external air. There are also systems called AIP (air-independent propulsion) that extend the underwater endurance of non-nuclear submarines.

Hull and pressure

As depth increases, water pressure rises. Therefore, submarines have a strong inner hull, typically cylindrical and made of steel or steel alloy, to distribute pressure. Some larger vessels have an additional outer hull that streamlines the shape and contains the ballast tanks internally. The maximum depth each vessel can reach is a defined limit determined by design specifications.

Navigation and sensing

Light does not reach far underwater, and radar does not function in water. For this reason, submarines rely on sonar—a system that detects objects using sound waves. Active sonar transmits sound and listens for echoes; passive sonar only listens to other sounds in the sea, keeping the vessel hidden. Navigation is achieved through inertial systems that calculate course and movement, since satellite signals do not penetrate underwater. Underwater communication typically uses long-wavelength signals that can propagate through water, but the data transmission rate is limited.

Applications

In naval forces, submarines serve as platforms for defense, attacks on other vessels, and delivery of long-range weapons. For civilian purposes, small and unmanned submarines (ROVs and AUVs) are used for seafloor surveying, inspection of oil and gas pipelines, salvage of sunken vessels, and research of deep-sea life. There are also small submarines designed for tourism.