The levers at the top of the control room, are for a fast opening of the upper tank valves = rapid flooding. No time to turn a wheel, just pull those levers and tear open the upper flooding valves for a crash dive.
The VIIC boats had 5 ballast tanks. They were located inside and outside of the pressure hull. The tanks located in the saddles have two parts: port and starboard! The yare therefore doubly listed in the table below.
According to the plans for type VIIc boats (from the Admiralty Technical Report on U-570), order of description:
Tank number
Between Frames
Capacity (tons sea water)
Fitted with
No. 1 (single) Main Ballast tank (rear end of the boat)
-10 to 0, external, aft of pressure hull above aft torpedo tube
31
Hand-worked vent, direct blow, exhaust blow - no Kingston valve, the bottom is open to the sea
No. 2 (double) Main Ballast and Reserve Fuel Oil tank
17-34, external, in saddle tank
11.4
Two kingston operating positions working four kingstons, one T-wrench operated emergency vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 port and No. 4 port fuel tanks, one after-end auxiliary vent leading into a duct to a common auxiliary vent for port and starboard tanks, direct blow, exhaust blow, compensating and blow-out connections. Saddle tank fuel tanks are tested to only 15 lbs./sq. in.
No. 2 Main Ballast and Reserve Fuel Oil tank
17-34, external, in saddle tank
11.4
As for No. 2 port, one T-wrench operated emergency vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 starboard and No. 4 starboard fuel tanks, one after-end auxiliary vent leading into a duct to a common auxiliary vent for port and starboard tanks.
No. 3 (single) Main Ballast. tank, internal
40-49, internal, below control room
47.75*
Four operating positions working six kingstons, direct blows, exhaust blows, two T-wrench operated emergency vent valves to port and starboard at the pressure hull to a duct which leads through the No. 2 Regulating tank at the end of which are two quick-opening lever-operated vents to the sea - tank tested to full diving depth so Kingstons could remain open at depth.
No. 4 (double) Main Ballast and Reserve Fuel Oil tank
45 1/2-62, external, in saddle tank
13.5
Two operating positions working four kingstons, one T-wrench operated emergency vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 port and No. 4 port fuel tanks, direct blow, exhaust blow, compensating and blow-out connections.
No. 4 Main Ballast and Reserve Fuel Oil tank
45 1/2-62, external, in saddle tank
13.5
As for No. 4 port, one emergency T-wrench operated vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 starboard and No. 4 starboard fuel tanks.
No. 5 (single) Main Ballast tank (at the bow of the boat)
80-106, external, forward of pressure hull surrounding forward torpedo tubes
25.3
One hand worked vent, direct blow, exhaust blow - there is no Kingston valve in this tank the bottom is open to the sea
Apart from those 5 'variable' flooding tanks, there were 'fixed' tight buoyancy tanks, 'variable' trimming tanks, fuel tanks being open to the sea at the bottom (Diesel swims on water) and fuel tanks being used for trimming - either by flooding them with seawater after being spent, or to be pumped around for trimming purposes.
I guess it is easy if you look at one system at a time
Generally the ballast tanks were blown out by compressed air stored in pressure bottles, not pumped out. Or, after surfacing or using a "Schnorchel" they could be fully blown out by the Diesels, having the advantage of the fatty exhaust fumes protecting the inner tanks from seawater.
During diving, tanks could only be blown out by air pressure. The bilge and auxiliary pumps were installed to empty the bilge and for trimming, not for emptying ballast tanks. Air pressure in the bottles were replenished by the Junkers compressor.
Ah whatever