How A Sub Works - An Interactive Demonstration


Conventional submarines have two hulls. The circular inner hull is the "pressure" hull, where the crew lives and works. Ballast tanks outside the pressure hull form the outer hull. Bow and stern diving planes assist in submerging, using dynamic seapressure from forward motion provided by the propulsion system. But without a change in displacement, the boat could not dive. The air-filled ballast tanks make the sub lighter than the water it displaces.

Diving or Submerging

At the tops of the ballast tanks, vent riser tubes lead to a central vent for each pair of tanks. Normally the vents are closed on the surface. Opening a vent allows sea pressure to flood the tanks through flood ports in the bottoms of the tanks, near the keel. As sea water enters the ballast tanks, the sub displaces less sea water and ceases to float. It dives. After diving, the vents are closed.


When the "Surface!" command is given, a seaman opens a valve (red) that allows air from high pressure air banks in the ballast tanks to enter the top of the tanks. Since the vents are closed, the air pressure forces the sea water out through the open floods. When enough water is replaced by air, the sub becomes lighter than the water it displaces and begins to rise. The more water that is expelled, the higher the sub floats. The last few feet of water left in the tanks after surfacing is usually expelled by a low pressure blower system, to conserve use of the high pressure air.

This is a simplification of a very complex system of tanks used for ballast, fuel oil storage, trimming (balancing) the boat while submerged, and special tanks for fresh water, lubrication oil, and safety. A complete diagram in Acrobat PDF is available below.

Balao-Class Tanks

(64kb PDF) or (92kb JPEG)

Tank systems of the Balao-class submarine:
(from a qualification training handout)

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Rik Nilsson |