i found this
The Fleet Type Submarine
can be found here
http://www.maritime.org/fleetsub/index.htm
The Fleet Type Submarine, Navpers 16160, is the first in a series of submarine training manuals that was completed just after WW II. The series describes the peak of WW II US submarine technology.
it has pretty interesting stuff ...
5B3. Fuel oil tanks. a.
Normal fuel tanks. The normal fuel tanks are used only for the storage of fuel oil. They are usually located toward the extremities of the boat rather than close to amidships. They vary in size, but normally have capacities of from 10,000 to 20,000 gallons each. Most modern submarines have four of these tanks. In a typical installation (
Figure 5-1) they are numbered No. 1, No. 2, No. 6, and No. 7.
b.
Fuel ballast tanks. Fuel ballast tanks are large tanks, amidships, between the pressure hull and the outer hull, which may be used either as fuel storage tanks or as main ballast tanks. They are connected to the fuel oil system in the same manner as the normal fuel oil tanks, but in addition, they have main vents, main flood valves, and high-pressure air and low-pressure blower connections which are necessary when the tank is in use as a main ballast tank. When rigged as a main ballast tank, all connections to the fuel oil system are secured.
Most fleet type submarines have three fuel ballast tanks varying in capacity from about 19,000 to 25,000 gallons. On a typical installation (
Figure 5-1), the fuel ballast tanks are numbered No. 3, No. 4, and No. 5. Current practice is to depart on war patrol with all fuel ballast tanks filled with fuel oil. Fuel is used first from No. 4 fuel ballast tank, and as soon as that tank is empty of fuel (filled with salt water) it is converted to a main ballast tank. Upon conversion, the tank is flushed out several times to insure that all fuel oil is out of the tank. The conversion of No. 4 FBT to a main ballast tank increases the stability of the submarine and decreases the amount of wetter surface of the hull when on the surface.