Buoyant Force on a Submerged Submariine

Hi, I suddenly got confused about buoyancy. A question reads as "a submarine is perfectly stationary below the surface in still water. The seabed is several hundred meters below. Briefly explain how using compressed air to force water out of the tanks inside the submarine allows it to rise towards the surface". Now the answer is the buoyant force remains the same whilst the submarine becomes less dense than water. My problem is how would the buoyant force remain the same? If the submarine gets less dense, it will float and hence the volume of displaced fluid gets less and buoyant force will decrease which is in itself confusing because I would think that floating objects experience a higher buoyant force but if the volume submerged is less, wouldn't that mean a smaller buoyant force?
- Doaa (age 40)
Sydney, NSW, Australia

Hello Doaa,

The buoyant, or upward, force for a completely submerged submarine is proportional its total volume irrespective of its density.  The downward force is proportional to its total volume times its average density. Archimedes figured this out a long time ago.   In order for a sub to rise from a neutral position you have to reduce its density.   That you do by pumping water out of the ballast tanks and replacing it with compressed air. 

There is a nice little demo at    that illustrates the idea.

LeeH

(published on 04/16/2015)