The VIIC Type U-boot

The bridge atop the conning tower.  Ship’s steering devices and surface torpedo attack controlling devices are visible. Visualisation 3D: Waldemar Góralski

Ballast tanks no. 1 and no. 5 located fore and aft of the pressure hull created reserve buoyancy when the ship was on the surface. No. 5 tank’s vent cover was opened outwards, unlike the covers of all remaining tank vents.
Ballast tank no. 3 with crescent-like cross section was inside the hull under the control room. It was divided into port and starboard chambers, both reinforced inside with perforated bulkheads. The Kingston valves were installed in the bottom and operated mechanically by the handwheels in the control room. At the top, on the outside, the chambers had pressure-tight venting ducts going through the chambers of the compensating tank no. 2 and further over the control room to the vent valves. Manually operated levers of both port and starboard chamber valves were in the control room above behind the attack periscope shaft. The tank is flooded during the submersion. When the submarine reached the depth of 15 m the Kingston valves were closed. The tank was pressure-proof as to resist the eternal pressure of the sea water in case of valves’ leak.
Ballast tanks no. 2 and no. 4, starboard and port were on both sides of the pressure hull. The Kingston valves’ covers of these tanks were opened inwards mechanically by handwheels inside the hull. Venting ducts shared by both no. 2 and no. 4 port tanks had one vent, the other one was for the respective starboard tanks. The lever operating both vents were in the control room above, just in front of the levers operating vents of the ballast tank no. 3. The venting duct of each of these tanks had a cut-off valve operated by a handwheel inside the hull. It was closed when the tank was used for fuel storage (opening of the vent would have caused the fuel to run outside the hull). The starboard and port side tanks no. 2 had additional venting ducts in the rear, because when diving, the ship was trimmed by the head, so the air would have remained there. These ducts led to a separate vent valve shared by both port and starboard tank and operated by a handwheel from the control room. When submerging starboard and port tanks no. 2 and no. 4 were completely flooded, so they were not affected by external pressure. Their walls were made of 5 mm thin plates.
Stern and forward watertight compartments were on both ends of the outer hull. They stabilized the ship’s roll. The were flooded through the free-flooding holes in the bottom and had vent valves at the top operated by handwheels in the control room and opened when the ship was diving. When submerged, both compartments were completely flooded, so they were not pressure-tight.
Compensating tanks no. 2, on starboard and port side were pressure-tight and were used to compensate for the changes in the weight of the provisions, ammunition, fuel, number of the crew members and salinity of sea water. Both were connected by pipes with the main bilge pump, as well as the auxiliary bilge pump and trim pump through the flow meter in the control room, where the level of water could be checked on the tubular glass water gauges. It was also possible to pump the water by means of the compressed air.
Compensating tanks no. 1, on starboard and port side were also pressure-tight. They were flooded and blown like the compensating tanks no. 2. Moreover, they were connected to the fuel system – the fuel could be stored there. They were also equipped with tubular glass water gauges that allowed the control of the water or fuel level.
Trim tanks were used to establish and maintain the longitudinal stability of the ship when submerged, thus they were located almost at each end of the pressure hull, 41.6 m apart. There was a pipeline that allowed for the pumping of water between the tanks. It went through the flow meter and the distribution valve in the control room. The flow could be forced by the use of compressed air or the auxiliary bilge and trim pump. The flow meter allowed for the precise control of the amount of pumped water.
Crash-dive tanks located on both sides of the hull in front of the compensating tanks were pressure-tight. On the surface they were flooded to increase the ship’s weight, so it was possible to dive in a shorter period of time. They were flooded by opening the valves, one on each side. They were vented to the inside of the ship. When the depth of 15 m was reached, they were blown, usually with the compressed air, but it was also possible to use the auxiliary bilge or trim pump.
Torpedo compensating tanks were inside the pressure hull, two (starboard and port) in the forward torpedo compartment and one in the stern torpedo compartment. They were flooded to compensate for the weight of the launched torpedoes or mines. They admitted either sea water or the water from the torpedo tubes through the flow meter and the valves that allowed for the precise control of the water flow. They were blow outside the hull or to the inside of the torpedo tubes by compressed air. The tanks were equipped with probes that allowed for the control of the water level.


A 20 mm anti-aircraft cannon ready for action.  Visualisation 3D: Waldemar Góralski

Flooding and blowing of the ballast tanks
The means of blowing the tanks: