Type VII Variants
704 Type VIIs were built by Germany before and during WWII, the majority of which were of the C variant. The numbers of each variant built are as follows:
- VIIA: 10
- VIIB: 24
- VIIC: 660 (572 standard VIICs and 88 VIIC/41)
- VIID: 6
- VIIF: 4
It was soon discovered that the only real drawback of the first variant, the VIIA, was the small fuel storage for the role intended. This was mostly fixed in the VIIB, which had their saddle tanks increased in length and cross-section. A fuel tank and a compensating tank were added on each side between the ballast tanks. This gave the B model an additional 33 tons of fuel in external saddle tanks. The hull was also lengthened by two metres, which increased the size of the internal fuel bunkerage. Together this gave them an additional range of about 2500 nautical miles at 10 knots. The B models enjoyed a 20% increase in power over the VIIA models by using superchargers on the diesels. This increased the surface speed from 16 knots to 17.2 knots.
NB: The original variants of the Type VII, which are nowadays known as the VIIA, were simply referred to as VIIs when they were built. When other variants were introduced, these original VIIs (U-27 to U-36) were then designated as VIIAs so as not to cause confusion. It is now acceptable to use both designations - VII or VIIA - to refer to U-boats of this batch. The latter is preferable, since the former could easily be misinterpreted as being a VII of any variant.
The B models, and all following the design, had two rudders instead of the one found on the VIIA, and this gave them even more agility. Another major difference that was incorporated into the B models and subsequent variants was that the rear torpedo tube, which had been mounted externally on the VIIA models, was brought within the pressure hull and fired between the rudders (U 83 was the only type VIIB without the stern torpedo tube). Three additional torpedoes could now be stored for a total of 14.
In VIIAs and VIIBs, the supply of air to the diesels was poor. The main trunk providing air to the diesels was routed forward under the rear deckcasing and up inside the after end of the tower under the Flak platform. Ventilation holes to let air into the intake trunk were located on the after sides of the tower wall, primarily on the starboard side, and these were susceptible to interruption in high seas. The air trunking was therefore modified. Large external trunks were built up both sides of the tower, starting under the Flak platform and extending up the after edge of the bridge. This effectively solved the intake problem, and on model Cs and later marks the air trunking was built into the tower structure.
The VIIAs and the early VIIBs had their 20mm Flak gun mounted on the aft deck. This site had the following disadvantages - the bridge created a dead zone, it took time to get the gun into action, and it took time for the crew to get inside the U-boat in an emergency. For these reasons, it was decided to move the Flak gun to the aft end of the conning tower.
The change from B to C model was not motivated by any dissatisfaction with the B model, but by the planned availability of a new sensor technology. The B model lacked the space for the installation of the S-Gert's electronics, so the design was modified by adding a full frame section of 60cm length into the centre of the control room. This increased the conning tower by 30cm in length and 6cm in width. The internal fuel tank was also increased by 60cm, giving a volume increase of 5.4 cubic metres. Externally, a small extra buoyancy tank was added, and this could be partially flooded to allow faster dives. In the C model, there were other minor upgrades to the mechanical and electrical systems of its predecessor. These changes resulted in a boat that was marginally slower than the B model, because although it had an increased size, the power output of its same machinery was not increased accordingly. For the same reason, the slight increase in fuel storage brought about no increase in range.
The VIIC/41 model had a weight saving of 11.5 tons over the C model, and this was used to increase the thickness of the pressure hull from 18.5mm to 21mm. This increased the normal depth rating from 150 to 180 metres and the failure depth from 250 to 300 metres.
The VIIC/42 was never produced. This planned variant was to have extra power and a top speed of 18.6 knots. It was to have a heavier displacement - 999 tons surfaced/1,099 tons submerged - and a thicker pressure hull. Normal depth rating would have been 300 metres, and the failure depth would have been 500 metres. The VIIC/43 was to have been even more capable, but it too was never produced.
The D variant was a minelayer with an additional hull length of 9.8 metres, which was added just aft of the control room. This increased the volume of the saddle tanks, thus increasing the range.
The E variant was a design project intended to test the lightweight 2-stroke V-12 Deutz diesels, but the abandonment of the diesel's development caused this variant to be cancelled.
Finally, the F model was a torpedo re-supply U-boat. The 10.5 metre section that was added aft of the control room allowed 24 extra torpedoes to be carried.