How Do Submarines Sink?

An American Ohio-class ballistic submarine weighs over 18,000 tons—nearly twice the mass of the Eiffel Tower. It is constructed from thick plates of specialized high-yield steel designed to withstand immense crushing pressures.

If you dropped a solid chunk of that steel into a harbor, it would plunge straight to the muddy bottom like a stone. Yet this colossal leviathan can float comfortably on the ocean surface, dive silently 800 feet into the pitch-black abyss on command, hover motionless in mid-water, and rocket back to the sunlit surface whenever the captain orders.

How can a machine made of thousands of tons of metal defy gravity underwater? The answer lies in mastering one of the most fundamental laws of classical physics: buoyancy.

The Ancient Secret of Archimedes

Over 2,200 years ago in ancient Syracuse, Greek mathematician Archimedes stepped into a full bathtub and noticed water spilling over the edges. He famously shouted "Eureka!" as he realized a revolutionary rule of nature:

Archimedes' Principle: Any object placed in a fluid experiences an upward buoyant force equal to the exact weight of the fluid it displaces.

Whether an object floats or sinks is a tug-of-war between two opposing forces:

• Gravity pulling the object down (determined by its total mass).
• Buoyant Force pushing the object up (determined by how much volume of water the object pushes aside).

If an object weighs less than the water it displaces, it enjoys positive buoyancy and floats like a cork. If it weighs more, it suffers negative buoyancy and sinks.

The holy grail of underwater navigation is neutral buoyancy—when an object's overall weight exactly equals the weight of displaced water. When neutral, a submarine neither sinks nor floats; it hovers effortlessly weightless in the water column like a spacecraft in zero gravity.

The Plumbing of Ballast Tanks

A modern submarine achieves this delicate balance using a double-hull architecture. The crew lives inside a cylindrical, airtight pressure hull built of super-strong steel or titanium.

The space between this inner pressure hull and the outer streamlined skin contains enormous hollow chambers called main ballast tanks.

Here is how a submarine controls its density:

1. Cruising on the Surface: When on the surface, the ballast tanks are filled with lightweight air. Even though the steel hull is heavy, the vast volume of trapped air makes the submarine’s average density lower than seawater. The ship floats with positive buoyancy.
2. Executing the Dive: When the order to dive is given, hydraulic vent valves open at the very top of the ballast tanks. Seawater rushes in through open flood ports at the bottom, violently forcing the air out the top. As heavy seawater replaces the air, the submarine’s average density increases until it exceeds the density of surrounding water. The sub develops negative buoyancy and sinks beneath the waves.
3. Surfacing (Blowing the Ballast): To return to the surface, the top vent valves are sealed tight. High-pressure air flasks—storing compressed air at over 3,000 pounds per square inch (psi)—blast air directly into the tops of the ballast tanks. This immense air pressure forcibly shoots the heavy seawater out through the bottom flood grates. Lightweight air reclaims the tanks, making the submarine buoyant once more and carrying it swiftly to the surface.

Flying Underwater: Hydroplanes

Ballast tanks control whether a submarine is heavy or light, but they are too slow for fine maneuvering. To change depth smoothly while moving, submarines use hydroplanes—movable underwater wings mounted on the conning tower (sail) and stern.

Just like an airplane banks and climbs through the sky, a submarine uses its propeller thrust to push water over these angled hydroplanes. Tilting the bow planes downward angles the nose down, allowing the sub to literally fly downward through the water at speeds over 30 knots!

Fast Facts About Submarines

• Crushing Pressure: At a depth of 1,000 feet, water exerts over 440 pounds of pressure on every single square inch of the hull! The steel hull of a submarine actually compresses and shrinks by several inches when deep underwater.
• Making Oxygen from Seawater: Nuclear submarines do not need to surface for air for months at a time. They use electricity from their nuclear reactor to run electrolysis machines, chemically splitting ocean water ($H_2O$) into hydrogen gas (pumped safely overboard) and pure breathable oxygen ($O_2$) for the crew!
• Deepest Dive in History: In 1960, the bathyscaphe Trieste carried Jacques Piccard and Don Walsh down to the Challenger Deep in the Mariana Trench—nearly 36,000 feet (11,000 meters) down—surviving over 16,000 psi of crushing hydrostatic pressure!