How A Sub Dives

How A Sub Dives

Conventional submarines have two hulls. The circular inner hull is the water-tight "pressure" hull, where the crew lives and works. Ballast tanks outside the pressure hull form the outer hull. When emptied of seawater, the outer hull increases the amount of water the boat displaces to be more than it weighs. It therefore floats on the surface. Flooding the tanks of the outer hull decreases the displacement, making the boat heavier than the water it displaces causing it to sink. Bow and stern diving planes assist in submerging, using dynamic sea pressure from forward motion provided by the propulsion system. But without a change in displacement, the boat could not dive.

Diving

The Ballast Control Station in a submarine’s Control Room. Above is the ’Christmas Tree’ so named for its red and green lights that indicate open and shut hatches, air supply and engine exhaust valves. An all-’green board’ means the boat is safe to dive. Below are the ballast tank vent and flood control valves that the Chief of the Watch pulls to fill the ballast tanks with water and begin the dive.The Ballast Control Station in a submarine’s Control Room. Above is the ’Christmas Tree’ so named for its red and green lights that indicate open and shut hatches, air supply and engine exhaust valves. An all-’green board’ means the boat is safe to dive. Below are the ballast tank vent and flood control valves that the Chief of the Watch pulls to fill the ballast tanks with water and begin the dive. At the tops of the ballast tanks, vent riser tubes lead to a central vent for each pair of tanks. Normally the vents are shut on the surface. On the "Dive! Dive!" command, opening vents allows sea pressure to flood the tanks through flood ports in the bottoms of the tanks, near the keel. On main ballast tanks, these floods are always open. As sea water enters the ballast tanks, the sub displaces less sea water, becomes heavier than the lesser amount of water it displaces, and ceases to float. It dives. After diving, the vents are again shut.

Surfacing

When the "Surface! Surface! Surface!" command is given, a seaman opens a valve (red in the video below) that allows air from high pressure air banks installed within the ballast tanks to enter the top of the tanks. Since the vents are shut, the air pressure forces the sea water down and 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. To increase cruising speed, the last few feet of water left in the tanks after surfacing is usually expelled by a low pressure blower system, to conserve 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 more detailed diagram is available below.


Detailed drawing of Guppy tanks, from Rik’s qualification book. Ballast tanks are vented in sequence, bow to stern to dive. This gives the boat a slight down-angle. They are blown dry also in bow-to-stern order to surface with a slight up-angle. Angles and depth are further controlled by use of the diving planes.Detailed drawing of Guppy tanks, from Rik’s qualification book. Ballast tanks are vented in sequence, bow to stern to dive. This gives the boat a slight down-angle. They are blown dry also in bow-to-stern order to surface with a slight up-angle. Angles and depth are further controlled by use of the diving planes.


A complete set of manuals for the U.S. Fleet Submarine is available online here.


 

Last modified: 13Oct2019