How Do Submarines Work? Explained Simply
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A submarine is the only vehicle that can disappear beneath the sea, travel for months without surfacing, and return with its crew alive, and it does all of that in an environment that would crush a human being in seconds. The pressure at a depth of three hundred metres is about thirty times the pressure at sea level, the water is cold and dark, and there is no air to breathe. Yet a modern nuclear submarine can stay submerged for more than three months, limited only by the food on board, while its crew eat, sleep, work and breathe in a steel tube that is sealed against the ocean. The secret is not one invention but a combination: ballast tanks that control whether the vessel floats or sinks, a pressure hull that resists the crushing force of the water, a power plant that needs no air from the surface, and a system that makes oxygen and fresh water from seawater. This guide explains each of those systems in plain words, starting with the simplest question of all, why a steel tube does not sink like a stone.
Buoyancy: Why a Steel Tube Floats
A submarine floats for the same reason a ship floats, and the reason is buoyancy, the upward push of water on any object placed in it. The Greek scientist Archimedes described the rule more than two thousand years ago: an object in water is pushed up by a force equal to the weight of the water it displaces, which is the water it pushes aside. A submarine is mostly hollow, filled with air, so the water it displaces weighs more than the submarine itself, and the upward push wins, so it floats. When the crew want to dive, they let water into tanks called ballast tanks, which makes the submarine heavier until the weight of the vessel equals the weight of the displaced water, and then it neither rises nor sinks. Let in a little more water and the submarine becomes heavier than the water it displaces, so it sinks, slowly and under control.
The trick is that the submarine must be able to change its weight quickly and precisely. The ballast tanks sit between the outer hull, which gives the vessel its streamlined shape, and the inner pressure hull, where the crew live. When the submarine is on the surface, the ballast tanks are full of air, so the vessel floats high with its tower, called the sail, above the water. To dive, the crew open vents at the top of the tanks, air escapes, and seawater floods in from below. To surface, they blow compressed air into the tanks, which forces the water out and makes the vessel light again. Small adjustments are made with trim tanks, which move water between the front and the back to keep the submarine level, and with the diving planes, the small wings on the sail and the stern that work like an aeroplane’s wings but in water. A submarine that is perfectly balanced, called neutrally buoyant, can hover at a chosen depth with almost no effort, which is why a skilled crew can hold a submarine steady for hours.
The Pressure Hull: A Steel Egg That Resists the Sea
The crew live inside the pressure hull, a thick steel cylinder with rounded ends that is built to resist the pressure of deep water. The shape matters: a cylinder spreads pressure evenly, while a flat wall would bend and fail, which is why submarines are round in cross-section and why the ends are domed rather than flat. Modern attack submarines are built from high-strength steel that is several centimetres thick, and they can dive to depths of three hundred to four hundred metres, while some research submarines with titanium hulls have gone deeper. The hull is tested in stages during construction, with the whole vessel lowered into the water and taken to its maximum depth in careful steps, while engineers listen for any sign of weakness. If the hull failed at depth, the water would rush in with such force that the vessel would be destroyed in a fraction of a second, so the design margin is large and the inspections are relentless.
Inside the pressure hull, the air is kept at roughly normal surface pressure, which is why the crew do not need diving suits or special breathing equipment. The hull is divided into compartments by watertight bulkheads, so that if one compartment floods, the doors can be closed and the rest of the vessel can survive. The hull also holds the control room, the engine room, the living quarters and the weapons, all packed into a space that is smaller than most people imagine: a large nuclear submarine is about one hundred and seventy metres long, but much of that length is taken up by machinery, and the crew of more than one hundred people share bunks, narrow corridors and a small mess hall. Life inside is a lesson in using every centimetre, with pipes, cables and valves running along the walls and ceilings, and with every crew member knowing exactly where their bunk, their station and their emergency equipment are.
Diving and Surfacing: The Controlled Sink
Diving begins long before the submarine goes under, with the crew checking that every hatch is shut, every vent is ready, and the diesel engines, if the submarine has them, are stopped. The officer of the watch orders the vents opened, the ballast tanks flood, and the submarine settles lower in the water until the waves wash over the deck and the sail disappears. The dive takes a few minutes, and the crew watch the depth gauge and the angle of the vessel carefully, because a submarine that dives too steeply can go deeper than intended, while one that is too light will bob back to the surface. Once at the ordered depth, the crew adjust the trim, set the speed, and the diving planes take over, holding the vessel level as it moves forward. The submarine can change depth by using the planes like a lift, or by adjusting the ballast, and a skilled crew can make the vessel rise or sink by a few metres with a small change in speed or angle.
Surfacing is the reverse, but it is done more carefully, because the submarine must check that the surface is clear before it breaks through. The crew listen with sonar for ships overhead, raise a periscope to look, and only then blow the ballast tanks with high-pressure air, which forces the water out and makes the submarine buoyant again. In an emergency, the crew can blow all the ballast tanks at once, which sends the submarine to the surface very quickly, but that is a violent manoeuvre used only when the vessel is in danger. A normal surfacing is gentle, with the sail appearing first, then the deck, and then the whole hull riding on the waves. Submarines spend most of their time submerged, because that is where they are safe from detection, and they surface only to enter port, to recharge batteries in a diesel submarine, or to communicate with the outside world.
Power: Diesel, Batteries and Nuclear Reactors
A submarine needs power to move, to make electricity and to run its systems, and the type of power plant decides how long it can stay underwater. A diesel-electric submarine, the type used by many navies, runs on diesel engines when it is on the surface or at periscope depth, using a snorkel to draw in air, and it charges large batteries that power an electric motor when submerged. The batteries last for a few days at slow speed, or for a few hours at high speed, so a diesel submarine must come near the surface regularly to recharge, which is when it is most vulnerable to detection. Diesel submarines are quieter than nuclear submarines when running on batteries, because there are no pumps or turbines running, and they are cheaper to build, which is why many countries choose them for coastal defence.
A nuclear submarine carries a small nuclear reactor that heats water, makes steam, and drives a turbine, which turns the propeller and generates electricity, and it needs no air from the surface at all. A single load of nuclear fuel can last for twenty years or more, and the submarine can stay submerged for months, limited only by food and by the crew’s endurance. Nuclear submarines are larger, faster and able to stay deep for longer, but they are more expensive, they need a large trained crew, and the reactor makes a constant low level of noise from pumps and cooling systems. The first nuclear submarine, USS Nautilus, launched in 1954, travelled under the North Pole in 1958, proving that a submarine could cross the Arctic without surfacing. Today, only a handful of countries operate nuclear submarines, while many more operate diesel-electric boats, and the choice between the two depends on the size of the navy, the distances involved, and the budget available.
Breathing Underwater: Air, Water and Food
A submarine crew cannot open a window, so the vessel must make its own air, and it does so with a system that would have seemed like science fiction a century ago. Oxygen is made by electrolysis, which splits seawater into hydrogen and oxygen using electricity, and the oxygen is released into the submarine while the hydrogen is vented overboard. Carbon dioxide, which the crew breathe out, is removed by chemical scrubbers that absorb it from the air, and the air is constantly monitored for oxygen, carbon dioxide and other gases. Fresh water is made by distilling seawater, heating it and collecting the steam, which gives the crew water to drink, to cook with and to wash in, though showers are short and water is rationed on long patrols. The air inside a submarine smells of diesel, cooking and recycled air, and after a few weeks at sea, the crew stop noticing it, but visitors notice it immediately.
Food is the real limit on how long a nuclear submarine can stay submerged, because the vessel carries enough provisions for about three months, stored in every available space including the torpedo room and the corridors. The cooks work in a tiny galley, serving four meals a day to a crew that works in shifts around the clock, and the quality of the food is taken seriously, because morale depends on it. Fresh fruit and vegetables run out after the first few weeks, and the menu moves to frozen and tinned food, with bread baked on board to keep something fresh. The crew sleep in bunks stacked three high, often sharing a bunk with someone on the opposite shift, a system called hot bunking that is still used on some smaller submarines. Exercise is limited to a small gym, films and books fill the free time, and contact with families is limited to short messages, because radio silence is part of staying hidden.
Sonar, Periscopes and Finding the Way
A submarine cannot see through water, so it navigates and hunts by sound, using sonar, which stands for sound navigation and ranging. Passive sonar listens for the noise made by other ships and submarines, such as propeller sounds and machinery hum, and a trained operator can identify a vessel by its sound signature, sometimes from tens of kilometres away. Active sonar sends out a pulse of sound and listens for the echo, which gives the distance and direction of an object, but it also reveals the submarine’s own position, so it is used sparingly, like turning on a torch in a dark room. The submarine’s own noise is its greatest enemy, so the crew run quietly, the machinery is mounted on rubber to reduce vibration, and the propeller is designed to make as little noise as possible. In the ocean, sound travels much further than light, and a submarine that hears first usually wins.
For navigation, the submarine uses an inertial system that tracks its position from a known starting point, plus satellite navigation when it is near the surface and can raise an antenna. The periscope, which is now often a camera mast rather than a simple optical tube, lets the crew look at the surface without exposing the whole vessel, and it is used to check for ships before surfacing and to identify targets. Communication is difficult underwater, because radio waves do not travel through seawater, so submarines receive messages on very low frequency radio when they are near the surface, or they trail a long wire antenna behind them. That silence is part of the submarine’s power: a vessel that cannot be found cannot be attacked, and the crew’s job is to stay hidden while listening to everything around them.
Life on Board and Why Submarines Matter
Life on a submarine is unlike any other job, because the crew live and work in a sealed tube for months, with no sunlight, no fresh air and no privacy. The day is divided into watches, usually six hours on and six hours off, and the routine is strict: meals, maintenance, training and sleep follow a timetable that never changes, because the submarine runs twenty-four hours a day. The crew are volunteers, they are paid extra for submarine service, and they form a close bond, because they depend on each other for survival in a way that few other jobs require. The psychological pressure is real, and navies screen submariners carefully, train them for confinement, and provide support when they return to shore. The reward is a sense of purpose: a submarine can gather intelligence, protect a fleet, deter an enemy, and do all of it without being seen, which makes it one of the most powerful tools a navy owns.
Submarines also serve peaceful purposes, including ocean research, undersea cable inspection and rescue work, and the technology they pioneered, from air recycling to nuclear power, has found its way into space stations and hospitals. The cost of a modern nuclear submarine runs into billions of dollars, and the debate about whether that money is well spent is a political one, but the engineering itself is admired even by critics. A vessel that can dive to three hundred metres, stay there for three months, make its own air and water, and return safely is a remarkable achievement, and it rests on a simple idea that Archimedes understood: control the weight, control the water, and the sea will carry you. The next time you see a submarine on the surface, remember that the visible part is only a small fraction of the machine, and that the real work happens in the dark, silent world below.
My Take:
I think submarines are the most underrated machines humans have built, because they do something that should be impossible, and they do it quietly, without applause. An aeroplane is visible, a rocket is loud, but a submarine disappears, and disappearance is a strange kind of achievement. It takes a steel tube, fills it with people, seals it against an ocean that wants to crush it, and keeps those people alive for months on air it makes itself. I find that more impressive than speed or height, because it is not about going further, it is about surviving somewhere that does not want you, and survival is a harder engineering problem than speed.
My strongest feeling is about the crew, because they volunteer for a life that most of us would find unbearable. No sunlight for weeks, no phone call home, a bunk shared with a stranger, and a job where a single mistake can kill everyone on board. They do it for pay, for pride, and for a sense of belonging that is hard to find in an ordinary office, and I respect that choice even though I would not make it myself. The submarine service selects for a particular kind of person, calm under pressure, comfortable in small spaces, and willing to trust the people around them completely, and that trust is the real pressure hull. Steel keeps the water out, but trust keeps the crew together.
I also believe the ballast tank is a beautiful idea, because it is so simple. You want to sink, you let water in. You want to float, you push water out with air. There is no rocket, no jet, no complicated mechanism, just the weight of water against the weight of air, and a valve that decides which one wins. I think good engineering often looks like that: a simple principle, applied with great care, repeated thousands of times without failure. The submarine is full of complicated systems, from the reactor to the sonar, but the diving itself is a child could understand it, and that simplicity is what makes it reliable when everything else fails.
Another thing I care about is the silence of a submarine, because it is both its weapon and its burden. A submarine that makes noise is a submarine that can be found, so the crew learn to move quietly, to speak softly, and to run their machines at the lowest possible setting. That silence is a discipline, and it changes the people who live in it. I imagine the first breath of fresh air after three months submerged must feel like a gift, and I think we, who breathe fresh air without thinking, could learn something from that. We treat air as free and unlimited, and for most of us it is, but for a submariner, air is something you make, measure and protect, and that changes how you value it.
Finally, I think submarines teach a lesson about limits that applies far beyond the navy. A nuclear submarine can stay underwater for months, but it cannot stay forever, because the food runs out and the crew get tired. Every system has a limit, and the wise operator knows where that limit is before they reach it. That is true of a submarine, a business, a body and a life, and the people who ignore their limits are the ones who break. The submarine service knows its limits, plans around them, and returns safely because of it. I would rather live like that, knowing my limits and respecting them, than pretend they do not exist and discover them at the worst possible moment, three hundred metres down, in the dark.
I also think the submarine is a lesson in how much we depend on things we cannot see, because the crew live on air they make, water they distil, and sound they listen to, and none of those things are visible. We live on the surface, where air is free, water comes from a tap, and we can see where we are going, and we take all of that for granted. A submariner cannot take any of it for granted, and that changes how they think about resources. They measure oxygen, they ration water, and they listen for every sound, because a waste of air or a missed sound can be fatal. I think we could learn from that discipline, because we waste water, we pollute air, and we ignore sounds that matter, and we do it because we think those resources are unlimited. They are not, and the submarine is a reminder of that.
Another belief I hold is about the ocean itself, because the submarine travels through a world that most of us never see, and that world is vast, dark, and full of life we barely understand. The ocean covers more than seventy percent of the Earth’s surface, and we have explored less of it than we have explored the Moon. A submarine crew passes through that world in silence, listening to whales, to ships, and to the sound of their own vessel, and they see almost none of it, because there are no windows. I find that both sad and wonderful, because they travel through the greatest wilderness on Earth and they cannot look at it. I would like to see more of that world explored, studied, and protected, because it is the largest part of our planet, and we know so little about it, and a submarine is one of the few machines that can take us there.
Finally, I think the submarine service is a reminder that some jobs are done in secret, and that secrecy is not the same as unimportance. A submarine crew spends months underwater, no one sees them, no one thanks them, and no one knows where they are, and yet they protect their country, they gather information, and they keep the peace by being a deterrent that no one can find. That is a strange kind of service, because it is invisible, and invisible work is often undervalued. I think we should remember that the world is kept safe not only by the people we see, the soldiers, the police, and the politicians, but also by the people we never see, working in the dark, in silence, far from home, and I am grateful to them, even though I will never meet them.
Frequently Asked Questions
How do submarines dive and surface?
A submarine dives by letting seawater into ballast tanks, which makes it heavier until it sinks, and it surfaces by blowing compressed air into the tanks to push the water out and make it light again.
How do submarines breathe underwater?
They make oxygen by splitting seawater with electricity, remove carbon dioxide with chemical scrubbers, and make fresh water by distilling seawater, so the crew can breathe and drink without surfacing.
How long can a submarine stay underwater?
A diesel submarine can stay submerged for a few days on batteries, while a nuclear submarine can stay down for more than three months, limited mainly by food and crew endurance.
How deep can a submarine go?
Most modern military submarines operate at three hundred to four hundred metres, while research submarines with special hulls have gone much deeper.
How do submarines see underwater?
They use sonar, which listens for sounds and sometimes sends out sound pulses to find objects, because light does not travel far in water and radio waves do not pass through seawater.
Why are submarines so quiet?
Quietness is survival, because a noisy submarine can be heard by enemy sonar. Machinery is mounted on rubber, propellers are specially shaped, and the crew run systems at low power to reduce sound.
