How a Submarine Trades Air for Seawater
A submarine’s main ballast tanks are built to hold either air or seawater. At the surface, they contain air, helping the boat displace enough water to float. To dive, the crew opens vents at the tanks’ tops. Seawater enters through openings below, pushing air out. The added water increases the submarine’s mass without greatly changing its overall volume, so it becomes denser than the surrounding seawater and sinks.
To rise, the crew sends compressed air into the tanks. The air forces water out through the lower openings, making the submarine lighter. Because the air must push against water pressure, deeper dives require air at higher pressure; the supply is valuable and must be managed carefully.
Why Sinking Is Only the First Step
Once underwater, a submarine generally aims for neutral buoyancy: a balance in which it neither rises nor sinks on its own. Small adjustments to ballast and trim—the boat’s balance between bow and stern—help maintain that state. Hydroplanes, which act like underwater wings, can then steer the submarine up or down as it moves through the water.
That distinction matters: ballast tanks bring the vessel from the surface into a useful underwater balance, while fine adjustments and forward motion help control its depth. The system is a large-scale exchange of fluids, but its practical purpose is precise control: replacing air with seawater to descend, then using compressed air to reclaim buoyancy and return toward the surface.
