It looks like a giant steel pill. It sinks on purpose. Honestly, when you think about the thousands of tons of high-grade steel and nuclear hardware sitting at the bottom of the Mariana Trench or prowling the Atlantic, the whole concept seems a bit like a dare against physics. But the answer to submarine how does it work isn't actually about "sinking." It’s about a very delicate, very grumpy balance between two invisible forces that want to crush or eject the vessel.
Archimedes figured this out in a bathtub a long time ago. He realized that any object, whether it’s a rubber ducky or a Virginia-class fast-attack sub, is pushed upward by a force equal to the weight of the water it displaces. To stay on the surface, you need to be light. To go down, you need to get heavy. Fast.
The Magic of Ballast and Why Subs Don't Just Sink Like Stones
Most people think a submarine just has a big "down" button. It's more of a plumbing masterclass. Between the inner pressure hull—where the sailors eat their Salisbury steak—and the outer hydrodynamic hull, there are these massive gaps called ballast tanks.
When the sub is cruising on the surface, these tanks are full of air. This makes the overall density of the ship less than the water around it. You're buoyant. You're a boat. But the second the captain wants to disappear, they open "vents" at the top of these tanks. The air rushes out with a roar you can hear throughout the ship, and seawater rushes in through flood holes at the bottom. The sub doesn't just "sink"—it loses its ability to float. This is called "negative buoyancy."
But here is the trick: they don't just fill them up and hope for the best.
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Naval architects use something called "trim." Even after the main ballast tanks are full, the sub might be nose-heavy or tail-heavy. There are smaller internal trim tanks. Sailors pump water back and forth between the front and back of the ship to keep it perfectly level. If you don't get the trim right, the sub will "porpoise," angling up or down wildly, which is a great way to make a crew of 130 people throw up their lunch.
Moving Through the Deep: It’s All About the Planes
Once the sub is underwater, it stops acting like a boat and starts acting like an airplane.
It has "wings." They’re called hydroplanes or fairwater planes. Usually, you’ll see a pair on the sail (the big tower on top) or near the bow, and another pair at the stern near the propeller. By angling these planes, the water rushing past the sub creates lift or downforce.
Want to dive deeper? Angle the bow planes down.
The water pushes the nose toward the seafloor.
The engine—usually a nuclear reactor or a diesel-electric system—provides the forward thrust to make those planes work. If the sub stops moving, the planes stop working. At that point, the sub relies entirely on its "static" buoyancy. If you're too heavy and you stop moving, you're going to hit the bottom. Hard.
Living in a Pressure Cooker
The ocean is heavy. Really heavy. For every 33 feet (10 meters) you go down, the pressure increases by one atmosphere. If you’re at 1,500 feet, the water is trying to crush that hull with thousands of pounds of pressure per square inch.
This is why the "pressure hull" is the most expensive part of the machine. It’s usually a perfect cylinder or sphere because those shapes distribute stress evenly. Modern US Navy subs use HY-80 or HY-100 steel—high-yield alloys designed to flex slightly under pressure and then pop back into shape when the sub surfaces. If the steel was too rigid, it would just crack.
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Think about that for a second. The walls of the room you’re sitting in are literally shrinking by fractions of an inch as you go deeper. Sailors have been known to tie a string across the width of the hull before a deep dive; by the time they reach "test depth," that string will be hanging slack because the ship has physically compressed.
How Do They Breathe? (The Chemistry of Life)
You can't just crack a window.
To understand submarine how does it work for the humans inside, you have to look at life support. Oxygen is actually the easy part. Most nuclear submarines use "electrolysis." They take the surrounding seawater, kick the salt out of it, and then run an electric current through the water molecules ($H_2O$). This splits the water into hydrogen and oxygen. They pump the oxygen into the ship and discharge the hydrogen overboard.
The real killer isn't a lack of oxygen; it's the buildup of carbon dioxide.
Humans exhale $CO_2$. In a sealed metal tube, that gas becomes toxic pretty quickly. To fix this, subs use "scrubbers." These machines use a chemical called monoethanolamine (MEA) to soak up the $CO_2$ from the air. When the chemical gets "full," they heat it up to release the gas and pump it outside. It's a continuous, mechanical lung.
Then there's the "atmosphere burner." This gets rid of trace gases like hydrogen (from the batteries) or even the fumes from the galley's deep fryer. If you didn't burn off those oils and gases, the air would eventually become flammable or just plain disgusting.
Seeing Without Eyes: The Reality of Sonar
Forget the movies. You don't look through a periscope 24/7. In fact, most of the time, the periscope is tucked away. And you certainly don't have windows. Light doesn't travel well through water, so "looking" is useless.
Instead, they listen.
Passive sonar is just a bunch of incredibly sensitive microphones (hydrophones) mounted on the hull. A skilled sonar technician can listen to a ship five miles away and tell you not only that it's a freighter, but how many blades its propeller has and if one of those blades has a tiny nick in it.
Active sonar—the famous "ping"—is rarely used. Why? Because sending out a loud sound wave is like screaming in a dark room to find your friends. Sure, you'll hear the echo when the sound hits something, but everyone in the room now knows exactly where you are. In a stealth game, active sonar is a "last resort" or used for navigating through narrow, rocky canyons.
The Nuclear Advantage
Why are some subs small and only stay down for a few days, while others are the size of two football fields and stay down for six months? It comes down to the "fire."
- Diesel-Electric: These subs use diesel engines on the surface to charge massive banks of batteries. When they submerge, they run off the batteries. Eventually, the batteries die. The sub has to surface or "snorkel" (poke a tube up for air) to run the engines and recharge. They are incredibly quiet because batteries don't make noise, but they are limited by time.
- Nuclear: A nuclear reactor is just a fancy way to boil water. The heat from the reactor creates steam, which spins a turbine, which spins the propeller. Since nuclear fuel lasts for 20+ years, these subs are limited only by how much food they can carry. The US Navy's submarine fleet is entirely nuclear for this reason.
Practical Insights for the Aspiring Naval Enthusiast
Understanding the mechanics of a submarine is one thing, but seeing the physics in action requires a bit of perspective. If you're interested in diving deeper (pun intended), here are the real-world ways to observe these principles:
- Visit a Museum Ship: You cannot understand the "cramped" nature of the pressure hull until you stand in one. The USS Blueback in Portland or the USS Nautilus in Groton are the gold standards. Notice the thickness of the hatches—that's the "submarine how does it work" reality in physical form.
- Study Fluid Dynamics: If you're a student, focus on Bernoulli's principle. The way water moves over a hydroplane is identical to how air moves over a Cessna wing.
- Watch the "Trim" in Non-Fiction Docs: Look for footage of a submarine's "Command Center" during a dive. You'll see two sailors (the helmsman and the planesman) fighting to keep the "bubble" centered. That bubble is a literal level that shows the ship's angle.
The submarine remains one of the most complex machines ever built, rivaling the Space Shuttle. It is a fragile bubble of life maintained by high-pressure plumbing and nuclear physics, operating in an environment that is actively trying to destroy it.
To stay informed on modern maritime engineering, track the development of "AIP" (Air-Independent Propulsion) systems, which are currently allowing non-nuclear submarines to stay submerged for weeks at a time, bridging the gap between old-school diesel and modern nuclear tech.