How does a submarine stay underwater for so long?

This question actually has three parts, so let’s take them one at a time. First, how does a submarine dive and come back up to the surface? Submarines have what are called ballast tanks that are filled with either air or water, or a combination of the two, and which control the level of buoyancy. To float towards the surface, they fill them with air. To dive below the surface, they fill them with water. They can also achieve neutral buoyancy by partially filling the tanks with a certain proportion of water and air. Often, submarines will have neutral buoyancy and use diving planes, similar to airplane wings, which can pitch the bow or stern up or down, helping to control its depth.

Second, how is the submarine powered when it’s underwater? Before the development of nuclear-powered submarines, the most common type of engine was diesel-electric, which required the intake of oxygen. So when the submarine was at the surface, the diesel engine would simultaneously propel it forward and charge electric batteries. When it dove underwater for extended periods, it would run on the battery power alone. But periodically it would have to surface to recharge the batteries. These types of submarines could stay submerged for only one to two days at most. On the other hand, modern nuclear-powered submarines use a nuclear reactor inside the sub to generate both propulsion and electricity. These submarines could theoretically stay underwater for years or even decades, but crews usually only bring enough food and supplies for about 90 days.

Third, where does the crew get its water, air, and heating to survive underwater? Most submarines have desalination machines that convert seawater into fresh water for drinking, bathing, and cooking. In terms of the air, oxygen is released either through tanks or through an oxygen generator that converts seawater into oxygen. Scrubbers remove carbon dioxide from the air to prevent it from accumulating and becoming toxic. Meanwhile, dehumidifiers absorb the moisture the crew exhales to prevent it from condensing on the interior of the sub and causing damage. And finally, heaters keep the temperature at a comfortable level. And that’s how a submarine stays underwater.

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