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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

During the World War II nearly 700 submarines of that type, with terrible living and sanitary conditions on board, sunk approximately 9 million BRT of Allied shipping. Human casualties are impossible to count.


History of the Type VIIC U-boat development

In August 1936, U-27, the first of the 10 medium-sized type VII U-boats displacing 500 tons and characterized by the placement of the stern (fifth) torpedo tube on the upper, external deck, was commissioned into the newly created U-Bootwaffe (The “Saltzwedel” Flotilla). It was not a very successful design and already in 1938 a new one was worked out, designated Type VIIB. The stern torpedo tube was mounted inside the pressure hull, the submersion system was improved and the diesels were equipped with superchargers that increased their power. The displacement rose to 517 tons. The modernized type of U-boats that had been commissioned into the “Wegener” Flotilla since 1938 met all the requirements and was highly appraised. Also the first combat experiences after the break out of the World War II confirmed its combat effectiveness. By August 1941, 23 units had been built and commissioned. The U-boat designers, however, were required to introduce yet another alterations to the newly built submarines. These were:
- The change of radio communication equipment, which involved extending the radio room by 60 cm (1 frame) and as a result lengthened the entire hull as well as the conning tower by 30 cm,
- The change of the submersion system by adding starboard and port pressure-tight crash-diving tanks, thus increasing stability (especially in heavy seas) and reducing the time needed to submerge,
- Increase in range by the enlargement of the internal fuel tank no. 1 from 32.5 m³ (type VIIB) to 37.5 m³,
- The replacement of one of the two electric-powered Krupp air compressors (starboard) by the crankless diesel driven Junkers air compressor of higher efficiency thus reducing the battery consumption.
- Due to the high consumption of the engine oil by the diesels, the ship was equipped with an oil purifying plant, so the oil could be reused.
- Protection against corrosion by galvanizing and sacrificial zinc anodes of the mechanisms and levers mounted outside of the pressure hull.
That is how a new version of the U-boat, designated VIIC, was created.

 Visualisation 3D: Waldemar Góralski
On December 2, 1940, the day the first U-boat of that type – U 69 was commissioned, its combat effectiveness was highly appraised. It appeared that a quick victory in the tonnage war depended only on considerable acceleration of the U-boat building programme and their rapid operational deployment. On the Allied side, however, there was a gradual development of the anti-submarine units and weapon systems, which in addition to the breaking of the Enigma code as early as 1943, changed the situation in the Atlantic. It was slowly becoming apparent that the U-boats were falling behind due to the rapid technological development of the enemy. The ugre to build the highest number of units did not allow for their significant modernization and the introduction of the new type of U-boat, the “electroboat” dragged on endlessly. The short-term supplement of the equipment with primitive radar detection device FuMB1 “Biscaya Cross”, “Bold” Asdic decoys and “Aphrodite” radar decoys, brought psychological rather than military advantages. Due to the increased danger of air attacks, in the summer of 1943, the platform behind the conning tower (the so called “Wintergarten”) was extended, so the anti-aircraft armament could be strengthened from a single 20 mm cannon to even two twin mounts and a twin 37 mm mount. Fundamentally, however, all type VIIC U-boats right to the type VIIC/41 U 1308, commissioned on January 17, 1945, were very much the same as the U 69. Approximately 660 units of the  type VIIC and VIIC/41 were built and commissioned. Moreover, there were six type VIID minelayers (1940-1942) and four type VIIF torpedo supply vessels (1941-1943).
In July 1940 the type VIIC had the following characteristics:

Armament:

1. Torpedoes and mines:
Fourteen 53.3 cm torpedoes, four in forward tubes, one in the stern tube and 9 spare ones; it was possible to carry two TMA or three TMB mines instead of a one torpedo. The following schemes of torpedo/mine armament were provided for U-Boats on the patrol missions:

Scheme    G 7a* G 7e** TMA TMB
1 5 9
2 3 2 21
3 3 2 14
4 2 1 24
5 3 1 16
* steam powered
** electrically powered



2. Artillery
– 8.8 cm SK C/35 mounted in front of the conning tower (the main deck there was widened to 3.8 m),
– 2.0 cm MG C/30 anti-aircraft gun at the back of the conning tower.
Ammunition supply: 8.8 cm – 205 rounds, 2.0 cm – 1500 rounds stored as follows:
– in front of the officers’ quarters: 177 8.8 cm rounds, 1200 2.0 cm rounds,
– under the upper deck: 28 8.8 cm rounds,
– in the conning tower: 20 2.0 cm ammunition cartridges.
Additionally placed in the watertight containers in the control room were:
– two boxes with four explosive charges with fuse,
– two boxes with twelve unarmed explosive charges,
– two boxes with twelve time fuses.

A view of a Type VII A U-boat’s conning tower with an anti-aircraft cannon mounted behind it. Visualisation 3D: Waldemar Góralski


3. Small arms
– one MG 34,
– two sub-machine guns with 3000 rounds of ammunition,
– six Mauser 7.65 mm pistols with 860 rounds of ammunition,
– three bayonets,
– one single-barrel and one double-barrel signal pistol with 180 flares and 200 signal rockets.
The small arms ammunition was stored in the officers’ quarters.

 

Construction

The Pressure hull was cylinder-shaped with truncated cones on both ends. It was built with electrically-welded steel plates placed on 82 steel frames. The quality of the weld was examined in the shipyard using X-ray photography. The minimum of 25% of all welds were being examined. The walls of the conning tower, the area were the torpedo tubes were mounted, side diesel exhaust valves and the diesels assembly openings were reinforced. The hull plates were 18.5 mm thick in the cylinder-shaped parts and 16 mm in the conical ones, whereas the reinforcements and the conning tower were 35 mm thick. Two round bulkheads separated the interior of the hull into three sections and three additional ones divided it into six compartments altogether:
Section I:
Compartment 1. – stern torpedo and motor room – the stern torpedo tube, electric motors, transformer, electric motors’ control panels, air compressor, the rudder and stern diving planes engine, torpedo compensation tank, aft trim tank, one compressed air tank and the extra fresh water tank were located there.
Compartment 2. – engine room - two diesel engines with their auxiliary equipment were mounted there. There were also two compressed air tanks used to start the engines, two compressed oxygen tanks, engine oil tanks - auxiliary, header and used oil ones as well as the diesel fuel oil header tank (under the floor plates).
Compartment 3. – non-commissioned officers’ room, which contained the toilet, galley, provisions storage, battery no. 1 circuit breaker, refrigerator, high-pressure compressed air tanks. Below were the fresh and dirty water tanks, internal fuel tank no. 1 and battery room no. 1.
Section II:
Compartment 4. – control room, located there were the main bilge pump, auxiliary bilge and trim pumps, both periscopes’ hoisting mechanisms, rudder and diving planes control mechanisms, electric compass/gyrocompass system, sea water distiller unit, course indicator, fresh water tank, four oxygen tanks. Below was the ballast tank no. 3.
Section III:
Compartment 5. – officers’ and senior non-commissioned officers’ quarters, which contained radio and listening rooms, battery no. 2 circuit breaker, small provisions magazine, toilet and the fresh water tank. Below were internal fuel tank no. 2, watertight ammunition container, wash and dirty water tanks and the battery room no. 2.
Compartment 6. – forward torpedo room with four torpedo tubes, reserve torpedo stowage (2 torpedoes), high-pressure compressed air tanks and three oxygen tanks. Below were reserve torpedo or mine stowage (4 torpedoes), port and starboard torpedo compensation tanks and the forward trim tank.
Conning tower, ellipse-shaped in horizontal cross section to reduce the water resistance when submerged, was placed above the control room. It had a thin metal casing. Inside the tower were the attack periscope with its hydraulic control motor, the rudder control station and torpedo fire control equipment.
The openings in the pressure hull necessary for propeller shafts, pipelines, side valves closing mechanisms, radio wiring and measuring equipment were inspected at the pressure of 15 atm.
Outer hull
In the after section there were the stern watertight compartment and ballast tank no. 1. Both were equipped with vents at the top and had free-flooding holes at the bottom; when submerging the water was allowed by opening the vents.
The fore or bow section contained the forward watertight compartment and ballast tank no. 5. They were also equipped with vents at the top had free-flooding holes at the bottom.
On both sides of the hull were the ballast, compensating and crash-diving tanks. The compensating and crash-diving tanks, just like the pressure hull, were pressure-tight, as they were only partially flooded with the sea water depending on the consumption of the provisions, fresh water and U-boat’s maintenance materials. The crash-diving tanks were flooded when the submarine was on the surface increasing her weight and thus reducing the time needed for submersion. They were blown when the ship was at the periscope depth.
The upper deck (external) was made of metal frames welded to the pressure hull and planks mounted on them. There were gaps between the them that prevented the creation of air bubbles during the dive.
The bridge encased with thin metal sheets houses the ventilation shafts, radio aerial shaft, radio direction-finder, attack periscope, watertight container for ready 2 cm ammunition rounds, and the cooling water tank. When on surface the bridge was the U-boat commander’s post and 2 cm anti-aircraft gun platform.

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

Submersion system

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:

– 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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