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.
Understanding Ships and Submarines: Submarine Ballast Tanks
A submarine must control more than its total mass. It must control where that mass is placed. Water stored near the bow makes the front heavier.
Water stored near the stern makes the rear heavier. Trim tanks move water between different parts of the vessel so it stays close to level. This matters because a badly trimmed submarine can point upward or downward even when its overall buoyancy is correct.
Engineers pay close attention to the centre of gravity and the centre of buoyancy. A stable submarine has these effects arranged so that small tilts tend to be corrected rather than increased.
Depth control is not always a simple sink or rise. Once a submarine is nearly neutrally buoyant, it can use diving planes, which are underwater control surfaces similar to aircraft wings. As the submarine moves forward, water flowing over the planes produces an upward or downward force.
This lets the crew make gentle depth changes without making large changes to tank water. At very low speed, the planes produce little force.
The crew then depends more heavily on trim tanks and pumps. Maintaining one chosen depth takes constant adjustment because seawater conditions change from place to place.
Temperature and salt content affect seawater density. Cold, salty water is denser than warm, less salty water. A submarine that is balanced in one water layer may become slightly too heavy or too light after entering another layer.
Pressure has an effect as well. Greater depth compresses the hull by a tiny amount, reducing its volume slightly. That reduction decreases buoyancy.
The crew must therefore monitor depth, water density, tank levels, and the vessel's angle. People, food, fuel, equipment, and weapons can change the mass balance during a long journey, so trim planning is part of normal operation.
Compressed air is especially important during surfacing. The air used to clear water from tanks must have a pressure greater than the surrounding seawater pressure. This becomes harder at greater depth because water pressure rises with depth.
Air is stored in strong cylinders, but it is not unlimited. A rapid emergency blow can make a submarine rise quickly, though a fast ascent creates control challenges and can expose the hull to rough surface conditions. In class, it helps to separate the ideas of weight, mass, density, volume, and pressure.
A floating object does not need to be light. It needs to displace enough water for the upward buoyant force to match its weight. The same principle explains cargo ships, life jackets, fish swim bladders, and simple bottle experiments.
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.