A submarine changes depth by controlling its average density compared with seawater. Ballast tanks are the main system that makes this possible, allowing the vessel to dive, hover, or rise without changing its solid structure. When tanks fill with seawater, the submarine becomes heavier for its volume and sinks.
When tanks are filled with air, the submarine becomes more buoyant and rises toward the surface.
Main ballast tanks usually sit along the outer hull and connect to the sea through flood ports near the bottom. To dive, vents at the top are opened so trapped air escapes and seawater floods in from below. To surface, high-pressure compressed air is released into the tanks, forcing seawater out through the flood ports.
Fine depth control also uses trim tanks, diving planes, and careful balancing of weight, buoyant force, and forward motion.
Key Facts
- Buoyant force equals the weight of displaced water: F_b = ρ_water g V_displaced.
- A submarine dives when its average density becomes greater than the density of seawater: ρ_sub > ρ_water.
- Opening ballast tank vents lets air escape, so seawater floods in and increases the submarine's weight.
- Blowing ballast tanks uses compressed air to push seawater out, decreasing weight and increasing buoyancy.
- Neutral buoyancy occurs when weight equals buoyant force: W = F_b.
- Pressure increases with depth according to P = P_0 + ρgh, so deeper water pushes harder on hulls and valves.
Vocabulary
- Ballast tank
- A compartment that can hold seawater or air to change a submarine's buoyancy.
- Buoyant force
- The upward force on an object in a fluid caused by the fluid pressure being greater at the bottom than at the top.
- Vent valve
- A valve at the top of a ballast tank that releases trapped air so seawater can enter.
- Flood port
- An opening near the bottom of a ballast tank that allows seawater to flow into or out of the tank.
- Compressed air
- Air stored at high pressure that can be released to force seawater out of ballast tanks.
Common Mistakes to Avoid
- Thinking submarines dive by pulling themselves downward with engines. This is wrong because diving mainly begins by increasing density as ballast tanks flood with seawater.
- Assuming ballast tanks are sealed when diving. This is wrong because vents must open so air can leave and water can enter through flood ports.
- Forgetting that buoyancy depends on displaced volume. The upward buoyant force is set by the amount of water displaced, while ballast changes the submarine's weight.
- Treating surfacing as simply pumping water out with ordinary pumps. This is incomplete because emergency and normal surfacing often use compressed air to blow water out rapidly.
Practice Questions
- 1 A submarine displaces 2.0 × 10^6 kg of seawater. What is the buoyant force on it? Use g = 9.8 m/s^2.
- 2 A ballast tank contains 50,000 kg of seawater. If compressed air forces out 35,000 kg of that water, by how much does the submarine's weight decrease? Use g = 9.8 m/s^2.
- 3 Explain why opening the top vents of a ballast tank helps the submarine dive even though the flood ports are at the bottom.