In submarine design, a ballast tank controls whether the vessel rises or dives. Main ballast tanks are flooded with seawater to increase displacement and make the submarine descend, while compressed air expels water to make it surface.
Submarines normally have reserve buoyancy when surfaced. Their ballast tanks are located between the pressure hull and the outer hull or in external structures. When a submarine dives, vents at the top allow air to escape as water enters through flood ports.
Ballast tanks differ from trim tanks. Main ballast tanks make the major change between surfaced and submerged conditions, while smaller trim tanks adjust the submarine’s balance and angle. Confusing those systems is a common mistake.
The system lets a submarine change depth without relying only on propulsion. Once submerged, hydroplanes and carefully managed ballast and trim help maintain depth efficiently.