1. Quick Summary
A submarine floats or sinks depending on whether it weighs more or less than the water it displaces. Filling ballast tanks with water makes it heavier; blowing them out with compressed air makes it lighter.
Once it is neutrally buoyant, it changes depth using hydrodynamic forces — diving planes acting like small wings and a slight change in trim.
2. What It Means
Archimedes’ principle says the upward buoyant force equals the weight of the fluid displaced. The submarine’s hull volume is essentially fixed, so the displaced weight barely changes.
That means the only practical way to change whether it rises or sinks is to change its own mass by moving water in and out of ballast tanks.
Neutral buoyancy is the goal underwater: weight and buoyancy cancel, so the boat can hold depth without constant effort.
3. Why It Happens
To surface, high-pressure compressed air is blown into the ballast tanks, forcing water out through bottom vents. The boat gets lighter and rises.
To dive, vents open at the top and let air escape, water floods in, and the boat gets heavier.
Diving planes are horizontal fins that deflect water as the boat moves, pushing it up or down. They only work while the boat has forward speed.
Trim means distributing weight fore and aft so the boat sits level; small errors are corrected by pumping water between trim tanks.
Depth pressure rises by roughly one atmosphere for every ten metres, so the hull must resist crushing, and compressed air must be stored at very high pressure to expel water at depth.
Air independence matters because a diesel engine needs air. Nuclear submarines generate their own power from a reactor, so they can stay submerged for months limited mainly by food and crew endurance.
4. Real Examples
A submerged submarine at neutral buoyancy can be moved up or down simply by angling planes, much as a glider trades speed for altitude.
Emergency surfacing uses a rapid blow of ballast, and in extreme cases dropping solid ballast weights.
Snorkels let diesel-electric boats run engines just below the surface, taking in air while remaining mostly hidden.
Sonar is the primary sense underwater because light and radio waves are absorbed within metres, while sound travels kilometres.
5. How It Affects Us
Submarine design is dominated by the trade-off between hull strength, depth rating and stealth.
Quieting is everything: acoustic signature determines whether a submarine can operate undetected, so machinery is isolated and hulls are coated.
The same buoyancy principles govern underwater robots, deep-sea submersibles and even fish swim bladders.
6. Key Takeaways
- Submarines change buoyancy by changing mass, not volume — pumping water in and out of ballast tanks.
- Neutral buoyancy lets them hold depth without effort.
- Diving planes steer them up and down, but only while moving.
- Pressure grows about one atmosphere per ten metres, which sets hull and air-storage requirements.