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

– by compressed air coming through the distribution valve on the right side of the control room and pipelines going inside the pressure hull. There were cut-off valves installed where the pipelines were piercing the pressure hull on the way to the ballast tanks. The pressure in the system was 25 atm and ensured buoyant force necessary to surface the boat and safe opening of the bridge access hatch.
For one blowing of the tanks the following amount out of the 3900 m³ compressed air supply was used:
– 155 m³ to completely blow all the ballast tanks at the depth of 40 m.
– 105 m³ to blow only the tanks no. 1 and no. 5 at the depth of 65 m.
– 71 m³ at the depth of 100 m.

– by diesel exhaust fumes directed through the distribution valve in the control room not to the exhaust silencers, but instead to the ballast tanks, in order to expel the remaining water out of them (the pressure of exhaust fumes was 0.5 atm). It was only possible when the conning tower was on the surface and the diesels could be started.

Venting and securing of the ballast tanks
All the ballast tanks as well as the stern and forward watertight compartments were equipped with vent valves. They were manually operated by means of handwheels and levers. The vent outlets were protected against external fouling by the steel wire netting. The handwheel of the stern watertight compartment vent valve was in the stern torpedo compartment, the handwheels and levers of the remaining ballast tanks were in the control room. The handwheel of the ballast tank no. 1 was on the stern bulkhead of the control room.
The levers controlling vents of both chambers of the ballast tank no. 3 – starboard and port were above in the control room.
Venting ducts of the ballast tanks no. 2, 3 and 4, starboard and port and remaining ducts of the tanks no. 2 were equipped with safety valves. The mechanisms of these valves were in the control room and officers’ quarters. The safety valves were sealed with lead when the tanks were filled with fuel. The handwheel of the ballast tank no. 5 vent valve was on the forward bulkhead of the control room, next to the one controlling the forward watertight compartment.

Pumps of the diving (and trimming) system:
a)    the main bilge pump – two-stage rotodynamic self-suction pump: both stages could work in series connection (low efficiency when the pressure was high outside the hull at the depth of 105 m – 500 litres per minute at 2600 r.p.m.) or in parallel connection (high efficiency when the pressure was low outside the hull at the depth of 15 m – 1300 litres per minute). The pump was electric powered and was used to pump out a high volume of water out of the bilge, compensating tanks, torpedo compensating tanks or when there was a substantial hull leakage.
b)    auxiliary bilge and trim pump – twin electric powered twin piston pump had a work output of 358 litres per minute at 115 r.p.m. and maximum pressure of 10 atm. It was used to pump out the smaller volume of water out of the bilge, compensating tanks, torpedo compensating tanks and also for trimming.
Both pumps could also pump water in or out of the ballast tank no. 3.
Compressed air system
It was used for the blowing of the ballast tanks, starting the diesel engines, launching torpedoes and operating the periscope hoisting mechanisms.
The air from the interior of the ship was compressed by:
– four-stage twin piston electric powered air compressor with the work output of 6.1 litres per minute at the compression pressure of 205 kg/cm², water-cooled by the ship’s diesel engines and other mechanisms cooling system. In its housing there was a two-stage water pump and the oil lubricating pump. The compressor was powered by a watertight, ventilated DC electric motor with variable magnetic field that enabled the power control.
– four-stage Junkers air compressor, a two-stroke, opposed-piston diesel two-step had a work output of 8.5 litres at the compression pressure of 205 kg/cm². It was water-cooled by the ship’s cooling system. There was a twin vane pump in compressor’s housing as well as oil pump started by the compressed air.

Visualisation 3D: Waldemar Góralski

Compressed air tanks
Compressed air at 205 atm was stored in the 12 high-pressure tanks. Each one had a capacity of 325 litres, making it 3900 m³ total. The tanks were in six groups, two tanks in each.
group 1 – at the stern under the upper deck,
group 2 – above the starboard and port diesels assembly openings,
group 3 – in the officers’ quarters on each side,
group 4 – in the forward torpedo compartment on the port side,
group 5 – in the forward torpedo compartment on the starboard side,
group 6 – one tank in the forward watertight compartment and one the starboard next to the reserve torpedo container.
Compressed air was used in three separate and closed systems:
– high pressure,
– medium pressure,
– low pressure.
Individual mechanisms were supplied by the high or low pressure distributors and by a tank blowing distributor; the pressure for blowing the compensating and crash-diving tanks was provided by a separate distributor.

Torpedo tubes
The 533.6 mm torpedo tubes (four at the bow and one at the stern) were 7552 mm long and mounted in such a way, that 2/3 of their length was outside the pressure hull. The torpedoes were launched by compressed air. On the outside, the tubes had covers opened towards the ships axis of symmetry along with the outer casing doors in the outer hull. They were operated by transmission gear and cranks. The inside caps mechanism had a safety feature that prevented their opening when the outer casing doors were opened. The torpedo tubes are equipped with devices that allowed for the input of depth, heading angle and speed in the torpedoes that were already loaded. The launching device had a 220 litre high-pressure compressed air tank. The air was fed into the tube and a torpedo launched when the lever of the compressed air valve was mechanically or electrically released. The air pushed the piston that launched the torpedo and stopped at the end of tube to prevent the air from escaping and forming an air bubble that would give away the ship’s position and the fact of the torpedo launch. Along with the torpedo launching a side valve of the torpedo compensating tank was opened. Immediately after the launch the tube covers were closed and vents were opened releasing the high pressure air inside the hull. The piston was moved back to its initial position by low pressure compressed air. The tube was drained and the inside cap was opened. The piston was then removed and another torpedo was loaded by means of an overhead crane.
Torpedo launching device
Apart from mechanical torpedo launching it also allowed for an electrical launch from the conning tower or the bridge. When a “fan” torpedo salvo was launched, the individual torpedoes were launched at time intervals. If the electrical launch had failed (which was indicated by a flashing of a lamp), the mechanical launch was possible. The device was made of the following components:
– at the bridge – a switch-key in the pressure-tight casing,
– in the conning tower – a switch-key and a control board with lights for five tubes,
– in the control room – a control switch with bride or conning tower position, a single shot switch-key, “fan” salvo switch-key, time-delay switch (Zeitschalter) and the fuse box,
– in the forward torpedo compartment – electric distributor box, control board with standby lights, automatic magnetic switch-key for each tube,
– in the stern torpedo compartment – electric power box.
The torpedo command device consisted of:
– at the bridge – optical and voice command transmitter,
– in the control room – voice command transmitter,
– forward torpedo compartment – optical and voice command transmitter,
– stern torpedo compartment – optical and voice command transmitter.

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