# Submarine buoyancy

Maybe a silly question but..

A submarine has ballast tanks to control it's buoyancy. To submerge, water is allowed to fill the ballast tanks which displaces the air inside.

When a submarine is submerged it contains a fixed amount of air compressed and uncompressed. By filling the ballast tanks with compressed air the submarine rises to the surface.

How is this possible? The submerged submarine has $$x$$ quantity of air. Filling the ballast the sub still has $$x$$ quantity of air, it's just being redistributed.

• The amount of water contained though is not the same.
– nasu
Jan 5, 2021 at 18:43
• At a depth of q, the water in the ballast tanks would have the same density as the surrounding water giving it neutral buoyancy. Right? So it can't be the water itself that produces the rise or fall of the sub, but rather the air that takes the place of the water in the ballast tanks Jan 6, 2021 at 14:12
• The buoyant force on the sub does not change as the volume does not change. What changes is the weight of the sub. You create neutral buoyancy by adjusting the weight of the sub to be equal to the buoyant force.
– nasu
Jan 6, 2021 at 14:37