Thread: U-Boat Armament

For the first half of the war, the principal armament on most U-boats was the 8.8cm or 10.5cm naval gun, and/or the 2cm flak gun. As the war progressed and Allied anti-submarine measures became far more effective, U-boats tended to remain submerged wherever possible, surfacing only when safe to do so in order to run the main diesel engines to recharge their batteries. Effectively, the deck gun was becoming redundant. Being little used, around April 1943 it was removed from the Type VII in order to save some weight and achieve a modest reduction in drag. At the same time, the danger from air attack having increased so much, U-boat flak defences were significantly enhanced. Despite the fact that several incidents are known where U-boats successfully fought off Allied air attacks, few U-boat commanders would willingly remain on the surface to engage an aircraft in combat unless diving was impossible or unsafe.
The main deck armament therefore was only effectively used in the early part of the war, usually against lone ships or convoy stragglers in waters where there was relatively little chance of encountering enemy warships. The deck gun would have been used most often to ‘finish off a merchantman that had been damaged by torpedo, but had failed to sink. Expenditure of additional torpedoes would be considered wasteful when much cheaper and plentiful artillery shells could be used.

The 8.8cm Deck Gun

The 8.8cm gun used on U-boats was not directly related to the famous 8.8 or Acht-Acht flak gun which ultimately gained fame as an anti-tank weapon. More correctly entitled the 8.8cm Schiffskanone C/35; it was a purely naval weapon, developed from earlier weapons of this type used by the Imperial German Navy in World War I.
The gun was mounted on a low pedestal forward of the conning tower and was traversable through 360 degrees. It could be depressed to -4 degrees and elevated up to 30 degrees. The gun fired a 13.7kg high explosive shell with a muzzle velocity of 700m/sec for a distance of up to 12,350m. It could also fire a 13.9kg armour-piercing shell or an 11.2kg star shell. When submerged, the barrel bore was protected by a waterproof tompion inserted into the muzzle. It was crewed by three men, the Kanonier (gunner), Ladeschütze (loader) and Richtschütze (gun-layer) supported by numerous other crewmen who would bring the ammunition up on deck from its storage under the floor plates of the Zentrale. On the deck, just forward and to port of the gun, was a small watertight ammunition locker giving the gun crew sufficient shells to allow the gun to be brought into action immediately whilst the bulk of the ammunition was retrieved from inside the boat. Two folding, padded, U-shaped supports were provided on both left and right sides of the gun for the gunner and gunlayer to steady themselves against rolling or pitching of the boat. In effect, the gun would be difficult to aim successfully in anything other than calm seas. In rough seas, the crew could strap themselves into position. Quite apart from being a poor gun platform, the narrow slippery deck of a U-boat was not a safe place to be in rough seas and gun crews would always be in danger of being washed overboard. The gun was controlled and directed, usually by the Second Officer (II Wach Offizier or IIWO), from the conning tower.

The 10.5cm Deck Gun

The standard deck gun fitted to the Type IX in the early part of the war was the 10.5cm Schiffskanone C/32 mounted in the identical pedestal fitting as used for the smaller 8.8cm weapon on the Type VII. Able to traverse through 360 degrees, it fired a 23kg projectile up to 15,300m. Alternatively, it could fire a 23.3kg armour-piercing shell or 14.7kg star shell. A crew of three was required to operate the gun, with additional crew members being engaged in keeping the gun supplied with ammunition from the magazine in the boat's interior. As the war progressed, it became clear that any benefits derived from the presence of the deck gun were offset by the increase in the drag imposed on the boat when moving under water. By 1943, most deck guns had been removed from Type VII’s, but for some reason most Type IX boats retained their 10.5cm guns. The principal exception to this was the Type IXD2, particularly those boats of this type operating in the Atlantic.

One crewman operating a 2cm Zwillinge. This shot shows clearly how the gunner braced himself against two large curved, padded shoulder mounts. This weapon had a considerable recoil. The ‘spiderweb’ sight for the gun can also be seen here.

The 2cm Flak Gun

Two basic designs of 2cm flak gun (Flugabwehrkanone) were used. The earlier version, the 2cm Flak 30, was a single-barrelled weapon, with 360 degree traverse and capable of 2 degree depression and 90 degree elevation. It fired a 0.32kg shell with a range up to 12,350m. Maximum cyclic rate of fire was 480 rounds per minute, but effective use was around half this rate. A second, improved model, the 2cm Flak 38, was a very similar model but had an increased rate of fire at 960 rounds per minute. The second version was also produced in twin-barrelled (Zwilling) and four-barrelled (Vierling) versions. It was a direct development of a weapon designed for the army, and simply fitted to a naval pedestal mount (the Lafette C/35).

The 3.7cm Flak Gun

In the second half of the war, many U-boats received the 3.7cm Flak M742. Also an army weapon adapted for naval use, it fired a 0.73kg round up to 15,350m at a maximum rate of fire of 50 rounds per minute.

Other Weapons

As well as the main deck armament and flak defence weapons, a limited amount of small arms were kept on board the U-boat for use by boarding parties, guards when the boat was in dock, etc. These would include the 9mm or smaller 7.62mm pistol, 9mm sub-machine gun, 9mm machine gun and 7.92mm rifle. A small number of 7.92mm machine guns could also be carried to supplement the boat’s anti-aircraft armament. These could be fitted to mounts along the edge of the conning tower.

A side view of the aft deck gun shows the relatively large size of the 3.7cm weapon, not significantly smaller than the 8.8cm forward deck gun on the Type VII. This gun was not specifically intended as a flak weapon, but was also for use against surface targets.

The Torpedo

German torpedo nomenclature can be extremely confusing. There were, however, only two principal types of torpedo used on U-boats, but with several variants in detonating devices (the pistol) and in directional control. These two principal types were in fact developments of torpedoes used in World War I, the G7a and the G7e. By World War II, torpedo sizes had been standardised at 54cm (21in.) so that all torpedoes, whether launched from surface ships or U-boats, were of the same diameter. The standard length was 7.16m and some 280kg of explosive was contained in the warhead.

Torpedo Types

G7a (TI)
The G7a (TI) was a relatively simple weapon, propelled by steam from the burning of alcohol in air, supplied by a small on-board reservoir. The torpedo was driven by a single propeller. The G7a (TI) had a top speed of some 44 knots and a range of up to 6km. Its biggest drawback was its visible ‘bubble’ wake, meaning that it was best suited to night attacks. It featured an impact pistol type detonator which would be set off by the torpedo hitting the ship’s side. Like many weapons from the early part of the war, it was manufactured to very high standards, and was thus expensive to produce.

G7e (TII)
Broadly similar to the G7a model, the G7e was electrically powered, being driven by a small 100 bhp electric motor. In this case two contra-rotating propellers were fitted. The G7e series left no visible wake, and the G7e (TII) had a range of some 5km at 30 knots. The electrically powered drive system meant that the G7e was easier and far cheaper to produce than the G7a.

G7e (TIIIa)
This was a development of the G7e (TII) with greater battery capacity, allowing its effective range to increase to 7.5km. It was used in conjunction with the FaT directional control.

G7Es(TIV) Falke
Introduced in July 1943, this torpedo was a little-used (only around 100 being manufactured) forerunner of the Zaunkonig acoustic torpedo. It carried a 274kg warhead and had a long range capacity of up to 7.5km, but a low speed of just of just 20 knots.

Detonators

The pistols used to detonate the torpedo were a source of great trouble to the U-Bootwaffe, with numerous failures to detonate being recorded in the early part of the war. The basic pistol was a dual-function component that could be activated by contact (Aufschlagzündung) or by the detection of the magnetic field generated by the hull of the ship (Magnetiscberzündung).

Loading torpedoes into the cramped interior of a Type VIIC was an awkward and laborious business requiring the use of special winches and pulleys, and much sweat and effort.

Directional Control

Three principal types of directional control were developed in World War II, all of which were used with some success after teething problems were eliminated.

The FaT (Federapparat Torpedo)

The original FaT design was first used on the G7a (TI). It was an excellent anti-convoy concept in that the torpedo ran in a straight line until reaching the target area and then changed to an ‘S’ configuration through the convoy until finding a target. The required launch position was alongside the convoy. A further development, the FaTII, was based on the G7e (TII).

The LuT (Lagenunabhangiger Torpedo)

This torpedo, similar in concept to the FaT, allowed the U-boat to attack the convoy from any angle rather than having to attain the ideal launch position alongside the convoy.

Zaunkonig (TVb)

This torpedo, based on the G7e, had acoustic detectors, which homed in on the sounds of the cavitations in the water caused by the propellers of the target. It was, however, prone to premature detonation when passing through turbulent waters, such as the wake of a ship. In addition it appears that it was only capable of detecting cavitations caused by ships moving at between 10 knots and 18 knots. This torpedo had a range of 5.75km at 24.5 knots. Zaunkonig II (TXI)
This was a development of the basic Zaunkonig, which had the acoustic detectors tuned to specific frequencies of ships’ propellers to avoid premature detonation, and was to be used with some success as an anti-escort weapon, fired from the stern torpedo tube against pursuing escort vessels.

The Mine

There were a number of developments in submarine-launched mines during
World War II, of which the four most significant were the TMA, TMB, TMC and SMA.

TM (Torpedomine) A

This mine was for use in depths of up to 270m and carried an explosive charge of some 215kg. Launched through the torpedo tube, it was of the same diameter as the standard torpedo, but shorter at 3.4m, so that two could be launched from each tube at the same time. This was effectively a ‘moored’ mine, being attached to a heavy base plate by a long cable.

TMB

Designed for use in shallow waters of up to just 20m, the TMB was shorter again, at just 2.3m, but carried a 580kg charge. Three could be carried in and launched from each tube. Rather than being ‘moored’, it was designed to lie on the sea-bed in shallower waters and be detonated either by magnetic or acoustic sensors.

TMC

This was a development of the TMB, larger at 3.3m in length, but with a 1,000kg charge. Two could be carried in and launched from each tube.

SM (Schachtmine) A

This mine was designed for dropping from a vertical mine shaft in specially designed mine laying boats rather than launching from the torpedo tubes. It was 2.15m in length and carried a 350kg charge. It could be used in waters up to 250m in depth.

EMS (Einheitsmine Sehrohr Triebmine)

This was a small free-floating mine, designed to float on the surface, un-tethered, drifting in the currents. It carried a small charge of just 14kg. Obviously, free-floating mines could not be tracked and could thus endanger all ships, not just the enemy vessels for which they were intended. They were therefore designed to be laid in enemy waters and become active a few minutes after laying, but then to sink after 72 hours, effectively making them safe before they could drift out of enemy waters and endanger other shipping.

MTA (Minentorpedo A)

This device was basically a torpedo with a mine in place of the regular warhead.
It was for use in relatively shallow waters and, being attached to a torpedo body, could be fired into the waters in which it was to be active, from a range of up to 7km away. At the end of its pre-programmed run, the motor would switch off and the torpedo-mine would sink to the sea-bed, thereafter to be detonated by acoustic or magnetic sensors in the same way as the TMB.

Other Weapons

Germany carried out a number of tests on the use of rockets fired from submarines. U-511 was used as a test bed for the employment of Wurfkorper 42 rockets fired from a wooden frame on her deck. Test firings both on the surface and from a submerged depth of 12m were successful but the project was never developed further.
A project was also planned for the use by U-boats of a towed missile container. This large towed vessel would contain a V2 ballistic missile. On reaching its launch point, the container, towed horizontally, would have ballast chambers flooded to turn it to a vertical position ready for remotely controlled launching from the U-boat. Development was under way and one container vessel completed and ready for testing before the war ended but the system was never used in anger.