6. HOW HEAVY IS A SUBMARINE AND HOW IS THE WEIGHT MEASURED?
The weight of a submarine is measured by the tonnage of water she displaces (pushes out of the way). When you climb into a bath and the water rises, the weight of the amount of water that has risen is the displacement.
When a submarine is submerged, the buoyancy of the submarine is very nearly equal to the weight of the submarine. It must carry the crew, fuel, food, fresh water, weapons and some essential spares, and it must take seawater into its tanks to make up (compensate) for consumables used (fuel, food, torpedoes etc) and keep the submarine balanced (in trim).
The submarine changes depth using lift from its hydroplanes and so 'flies' through the water like an aeroplane. When on the surface, special buoyancy tanks are filled with air. This additional buoyancy allows the submarine to float on the surface of the sea.
