How Does a Submarine Sink, Hover, and Rise? Buoyancy, Ballast Tanks, and Controlled Density
A submarine controls depth by balancing weight and buoyancy with ballast tanks, trim systems, compressed air, and hydroplanes.
Short answer: a submarine changes its overall buoyancy by changing how much seawater and air its ballast system contains. On the surface, air-filled main ballast tanks help make the vessel positively buoyant. To dive, the submarine admits seawater and moves toward neutral buoyancy; once underwater, smaller weight adjustments and wing-like control surfaces help it hold or change depth. To return to the surface, compressed air can force water out of the main ballast tanks so the submarine becomes positively buoyant again.
The important idea is that a submarine does not defeat gravity. It manages two competing forces with unusual precision: its downward weight and the upward buoyant force produced by displaced water.
Start with the two forces acting on a submarine
Gravity pulls the submarine downward. Water pressure acts on every part of the hull, and because pressure is greater lower in the water, those forces combine to produce a net upward force called buoyancy.
Archimedes’ principle states that the buoyant force on an immersed object equals the weight of the fluid the object displaces. This gives three basic possibilities:
- Positive buoyancy: the upward buoyant force is greater than the vessel’s weight, so it tends to rise.
- Negative buoyancy: the vessel’s weight is greater than the buoyant force, so it tends to sink.
- Neutral buoyancy: the two forces are approximately balanced, so the vessel can remain at roughly the same depth without steadily rising or sinking.
OpenStax’s explanation of Archimedes’ principle shows that even an object that sinks still experiences buoyancy. Sinking only means that the upward force is not large enough to balance its weight.
How can a heavy steel submarine float?
“Heavy things sink” is an incomplete rule. A solid steel block usually sinks because its mass is packed into a small volume. A hollow steel vessel encloses a large volume of air, so its average density—total mass divided by total external volume—can be lower than the density of the surrounding water.
That is why shape matters. A stone and a hollow steel bowl might contain similar amounts of solid material, yet the bowl displaces much more water. If it displaces enough water for the buoyant force to balance its weight, it floats. A submarine uses the same principle but adds controllable tanks so its effective weight and average density can be changed.
What happens during a controlled dive?
1. The submarine begins with reserve buoyancy
At the surface, the main ballast tanks contain mostly air. The submarine displaces enough water to support its weight and keeps part of its hull above the surface. This is positive buoyancy.
2. Seawater enters the main ballast tanks
To begin diving, vents are opened so air can leave while seawater enters through openings in the ballast system. Replacing low-mass air with much denser water increases the submarine’s total mass without greatly changing the volume of water displaced by its outer shape.
As weight increases, the extra reserve buoyancy disappears. The submarine may briefly become slightly negatively buoyant to start descending, then the crew adjusts the vessel toward a controlled near-neutral condition. The National Museum of the U.S. Navy provides an educational overview of submarine buoyancy and operation.
3. Trim and depth-control tanks make finer adjustments
Main ballast tanks handle the large change between surfaced and submerged conditions. Smaller variable or trim tanks can take in, release, or transfer water for finer control. Moving water between locations also changes trim—the balance between the bow and stern—so the submarine can travel level instead of continually pitching nose-up or nose-down.
The Smithsonian’s submarine exhibit explains how main ballast tanks, variable ballast tanks, and control surfaces work together. The precise arrangement varies by submarine, so it is better to think of ballast, trim, and steering as cooperating systems rather than one universal tank design.
4. Hydroplanes use motion through water
Submarines also have control surfaces often called diving planes or hydroplanes. They act somewhat like underwater wings. When the submarine moves, water flowing around an angled plane produces a force that can guide the vessel upward or downward or change its pitch.
This means ballast and hydroplanes do different jobs. Ballast controls the vessel’s broad buoyancy condition; hydroplanes provide responsive depth control while the vessel is moving. A submarine that is nearly neutrally buoyant needs less continuous control force than one that is strongly trying to rise or sink.
Can a submarine truly hover motionless?
In a simplified diagram, neutral buoyancy looks like a perfect balance: upward force equals downward weight. In real water, maintaining one exact depth is more demanding. Water density can vary with temperature and salinity. The vessel’s mass changes as supplies are used or equipment is moved. Currents act on the hull, and pressure increases with depth.
Submarines therefore monitor depth, attitude, and motion and make small corrections. “Hovering” is best understood as maintaining depth within a controlled range, not as becoming frozen at one mathematical point forever. PhET’s free buoyancy simulation lets learners explore how density, displaced liquid, weight, and balanced forces affect whether an object rises, sinks, or remains suspended.
How does the submarine return to the surface?
To regain positive buoyancy, compressed air can be released into the main ballast tanks. The air pushes seawater out, reducing the submarine’s mass while its external displacement remains broadly similar. Once the upward buoyant force exceeds the vessel’s weight, it tends to rise.
Hydroplanes and propulsion can help control the ascent while the submarine is moving. Near the surface, the ballast system restores enough reserve buoyancy for the vessel to float securely. The phrase blowing the ballast tanks refers to using compressed air to expel water; it does not mean the tank explodes.
Ballast tanks, trim tanks, and hydroplanes are not interchangeable
| System | Main job | What it changes |
|---|---|---|
| Main ballast tanks | Move between surfaced and submerged buoyancy conditions | The amount of air and seawater carried in the main ballast spaces |
| Trim or variable tanks | Fine-tune weight distribution and balance | Small amounts or locations of water within the control system |
| Hydroplanes | Guide depth and pitch while water flows past the vessel | The direction of hydrodynamic force |
A stone, a steel bowl, and a fish
The stone
A compact stone usually has an average density greater than water. It displaces only its own small volume of water, so the resulting buoyant force is not enough to balance its weight.
The hollow steel bowl
The bowl contains air and has a much larger external volume than the same steel crushed into a solid lump. It therefore displaces more water and may float. If water floods the bowl, its average density rises and it may sink.
The fish’s swim bladder
Many bony fish use a gas-filled swim bladder to help regulate buoyancy. Increasing or decreasing the gas volume changes how much water the fish’s body displaces relative to its mass. The comparison is useful, but it is not exact: not every fish has a swim bladder, and submarines control tanks, water, motion, and trim through engineered systems.
Four common submarine misconceptions
“A submarine dives because its engine points downward”
Propulsion and hydroplanes matter, but ballast is what removes surface reserve buoyancy and establishes the vessel’s underwater buoyancy condition. The systems work together.
“Filling a tank with water reduces displacement”
For a submerged submarine whose external shape remains nearly unchanged, the amount of surrounding water displaced is tied mainly to that external volume. Admitting water increases the vessel’s mass; it does not need to shrink the whole submarine.
“Neutral buoyancy means there are no forces”
Gravity and buoyancy are still present. They are approximately equal and opposite, producing little net vertical force.
“Pressure is the same thing as buoyancy”
They are related but not identical. Buoyancy results from the pressure difference across an immersed object: deeper portions experience greater fluid pressure than shallower portions. Pressure also creates a major engineering challenge because the hull must protect its interior as outside pressure increases with depth.
A simple way to reason through any floating problem
- Identify the object’s total mass and external volume.
- Ask how much fluid the object displaces.
- Compare the object’s weight with the weight of that displaced fluid.
- Decide whether the net vertical force points upward, downward, or is approximately zero.
- Check whether motion or a control surface is adding another force.
Younger students can use arrows labelled “weight” and “buoyancy.” Older students can connect the same diagram to density, pressure with depth, Archimedes’ principle, and the forces produced by hydroplanes.
What students, families, and educators should remember
- Students: Do not decide whether something floats from mass alone. Shape, displaced water, and average density all matter.
- Families: A foil boat, modelling-clay boat, or sealed Cartesian-diver activity can demonstrate the concept, but use only safe containers and adult supervision around water.
- Educators: Separate static buoyancy from motion-based control. Asking learners to draw force arrows before discussing tanks often prevents the misconception that water simply “pulls” a submarine downward.
Keep exploring everyday physics
- Compare average density with another material-property puzzle in Why Does Metal Feel Colder Than Wood?
- Explore how water changes what we see in Why Does a Straw Look Bent in Water?
- Connect fluid pressure and phase changes with aviation in Why Do Planes Leave White Trails in the Sky?
The big idea
A submarine sinks, hovers, and rises by managing force and density—not by becoming mysteriously heavier than gravity or immune to it. Ballast tanks change the amount of water carried, trim systems refine balance, and hydroplanes steer the moving vessel through the water. Together, those systems turn Archimedes’ ancient principle into precise modern engineering.