Why Do Submarines Have To Resurface At All? Can’t They Remain Submerged Indefinitely?

Submarines are tremendous maritime machines – they dive underwater and stay there for extended periods of time, conducting military operations or other forms of underwater research. The biggest advantage they offer, especially in the militaristic context, is that they can hide themselves beneath the water, away from the suspecting eyes of the enemy. In fact, that’s the biggest reason why they were first used in the world wars by the German Navy to wreak havoc on the Allied vessels.

german submarine

A German submarine post the second World War. (Photo Credit : German Federal Archive/Wikimedia Commons)

However, the moment a submarine resurfaces, i.e., appears on the surface of water, it becomes a much easier target for the powerful guns and canons of enemy attacker and destroyer ships. That’s why its often said that if you are able to successfully force a submarine to resurface, you’ve already won half the battle.

That being said, if appearing on the water’s surface is so dangerous for a submarine and its crew, why does it have to resurface at all? I mean, what keeps a submarine from staying submerged for an indefinite period of time? Can’t it stay underwater for the entire duration of a mission?

submarine aircraft carrier

Unlike a regular ship, a sub can go underwater whenever it chooses to do so. (Photo Credit : Pixabay)

To understand the answer to this question, it helps if you generally know a thing or two about submarines.

Submarines can be categorized into two types based on the kind of engine they run on: diesel-electric or nuclear.

Submarines that run on diesel electric engines are casually referred to as diesel-engine submarines or simply diesel  subs. Similarly, submarines that use power generated by a nuclear reactor onboard are called nuclear-powered submarines, or simply, nuclear subs.

Irrespective of their type, submarines usually surface periodically, but the reasons for doing so differ in both cases.

Why do diesel submarines have to resurface?

Diesel engines generate power through the process of internal combustion (the word ‘internal’ simply indicates that the diesel is burnt inside the main part of the engine). Note that internal combustion engines are different from external combustion engines.

Now, you might recall from your high school science class that combustion is simply the process of burning something in the presence of oxygen. The ‘presence of oxygen’ is very important here, especially in the case of large metallic vessels operating underwater, away from a direct and fresh supply of oxygen.

Recharging the batteries of diesel submarines

As the name suggests, a diesel submarine runs on a diesel engine, which means that it must come up to the surface (or at least periscope depth). Periscopes on submarines may be as tall as 18 meters (around 60 feet). When a sub is submerged to a depth that is equal to the height of the periscope, the sub is said to be at periscope depth.


The tall, thin tube erected at the front is the sub’s periscope. It’s aptly called the ‘eye of the submarine’. It lets the sub’s crew have a view of the surrounding horizon while the sub remains submerged.

A submarine comes to the surface once every few days (or even more frequently than that), not only to get a fresh supply of atmospheric oxygen from above the water’s surface, but also to dispose of the waste gases that it produces onboard.


There’s a device known as the snorkel (the British call it the ‘snort’), which lets subs operate submerged while still taking in air from above the surface. Once the sub comes to the surface, its diesel engines run and produce power, which is used to recharge the batteries that ultimately run the sub.

Submarine snorkel

USS U-3008 (ex-German submarine U-3008) View of the submarine’s conning tower, with her snorkel raised. Taken at Key West Naval Station, Florida Note her insignia painted on the front of the conning tower. (Photo Credit : USN/Wikimedia Commons)

Nuclear subs, on the other hand, rely on the power generated by the nuclear reactor onboard. The reactor generates enough power to run all electronic and electrical systems onboard, in addition to the life support systems of the crew. Therefore, unlike diesel subs, nuclear subs can go for days, or even weeks on end without surfacing a single time. In fact, theoretically, a nuclear reactor onboard a submarine produces enough power to run the sub for a couple decades!


Radio signals don’t travel well deep underwater, particularly at the depths at which submarines usually operate during a mission. Therefore, both nuclear and diesel submarines must resurface in order to communicate with their home base, and receive orders and/or convey vital information.

Rations and supplies

A nuclear submarine can stay and operate underwater for a couple decades, so long as it’s adequately stocked with enough supplies and ration for its onboard crew to survive that long.


Obviously, that’s nowhere near the realm of possibility, so submarines must resurface to load fresh supplies (from another vessel) and carry on, especially if it’s a long mission.


Howsoever sturdy and robust it may be, a submarine, at the end of the day, is still just a machine. It has a number of systems of different types – electrical, mechanical, electronic, hydraulic etc. – that make it work underwater. At times, some of these systems have problems that the onboard crew cannot fix on the go. If any of these problems are critical, the captain usually orders the resurfacing of the sub so that essential repairs can be made.

submarine marine army boating

A submarine heads to a port to fix critical problems. (Photo Credit : Pixabay)

The ‘human’ angle

Let’s not forget that a submarine is a manned vessel, i.e., it’s run by human beings. Sometimes a member of the crew gets injured or falls ill, and requires proper medical attention. There could also be some other sort of human emergency that calls for an unplanned resurfacing of the sub.

In addition to that, human beings are social animals. They have to have a ‘social life’, i.e., they have to talk to friends, meet people, visit places, eat different kinds of food, binge watch TV shows, play video games, go out for a walk, take a long drive and do plenty of other stuff to maintain their sanity.


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To make sure that the crew doesn’t go crazy as a result of staying inside a metal tube for weeks on end, completely detached from family, friends and the rest of the world, submarine missions are planned in such a way that the crew keeps ‘rotating’, meaning that no single crew must bear the burden of staying underwater for ‘too long’. To make that happen, submarines have to resurface so that the existing crew can debark as a fresh crew boards the sub for the next leg of the ongoing mission.

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About the Author

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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