Chapter IV
(Torpedoes and Anti-aircraft Guns)

  1. Summary

  2. Information on torpedoes available in ENIGMA traffic

  3. Standard torpedoes

  4. "Curly" torpedoes

  5. The electric-acoustic T-V

  6. Anti-aircraft armament prior to December 1943. Quadruple mounts and flak-boats

  7. The 37mm automatic anti-aircraft gun


German Submarine Torpedoes


1. Summary

ULTRA sources contributed information of value not only in following technical progress in matters of armament, but in showing the potential striking power and duration of cruise of operating U-boats. In the field of torpedoes, the most important developments were the "curly" (FAT) torpedo, which increased the probability of a hit by traversing the immediate area of a convoy several times, and the "GNAT" (T-V or "Zaunkonig"), a torpedo which homed acoustically after an initial straight run. Both types met with considerable success, and the "GNAT" accounted for the sinking of many Allied destroyers before noise-makers were devised to lure it from its intended prey. Developments in anti-aircraft armament were clearly indicated in ENIGMA traffic. Armament was greatly increased on U-boats in the first months of 1943, and during that year there were concurrent experiments with 20mm quadruple mounts and 37mm automatic guns, with inclusion of both types on a special anti-aircraft U-boat. Failure of the Flak-U-boat led to reversion to the principle of greater antiaircraft protection for all boats at a cost in crash-diving speed. Outfitting with 37mm guns was undertaken on a large scale in December 1943, but the weapon never reached the effectiveness hoped for by Command.

2. Information on torpedoes available in ENIGMA traffic

Among developments in the field of weapons which could be followed with great advantage by the reading of enemy traffic, the most important were probably concerned with the U-boats' chief weapon, the torpedo, upon the development of which the technical staffs of the German navy expended considerable efforts and ingenuity. Data available from ULTRA sources included more than one type of information of value. There were, for instance, frequent reports from U-boats at sea giving the number of torpedoes of different types on hand. These reports, along with other pertinent information, were an indication of the potential striking power of the U-boat concerned, and thus aided in determining how long the boat was likely to operate before returning to port. Another type of


information furnished, especially from the reading of administrative traffic which began with the breakdown of German landlines in June 1944, concerned the problem of supply of torpedoes to U-boat bases as well as the departing U-boats. In addition there were extensive orders and reports which gave valuable clues as to the character of new types of torpedoes and torpedo parts and their effectiveness.

3. Standard torpedoes

Torpedoes at the disposal of U-boat commanders were of three basic types: air, electric and electric-acoustic. The capabilities of the air and electric types were further increased by use of improved exploders1 and fuzes as well as by introduction of a "curly" mechanism, which increased the probability of a hit by directing the torpedo back and forth several times over the area being traversed by a convoy or ship. The standard 21-inch air torpedo, generally carrying an impact pistol, had three settings: 44 knots to 6,500 yards, 40 knots to 8,700 yards, and 30 knots to 15,300 yards.2 It make a visible track, so was usually used in night attacks. Because it required less attention than the electric torpedo, it was generally used by U-boats carrying out long patrols. The standard 21-inch electric torpedo had only one setting allowing for a speed of 30 knots to possibly 7,000 yards if the battery was pre-heated. Owing to the fact that it showed no track, it was a more popular type for general use.

4. "Curly" torpedoes

The first FAT (Feder-Apparat-Torpedo) were introduced aboard U-boats late in 1942, as evidenced by a message to Dietrichs in January 1943 (1326/1) in which he was required to "report in detail on FAT and pistol." An order sent the middle of the same month (1902/15) gave a hint as to the nature of FAT when it cautioned U-boats that overestimation was to be preferred underestimation, since as a result of underestimation "the entire FAT falls short and the torpedo


changes course before reaching the target," whereas "an overestimated FAT distance utilizes at least the preliminary run of the torpedo and consequently affords the same chances of success as does the normal straight shot." A flurry of short-signal reports during February and March giving notice of intention to attack with FAT indicated that the device was receiving adequate trial in the field, and there were several reports giving firing data for successful shots. Such reports continued to be sent throughout the course of the war.

In December 1943 (1058/28 and continuations), in a long Current Order, it was pointed out that the FAT torpedo, as well as the newer electric-acoustic T-V, was valuable for firing under the current difficult conditions which prevented U-boats from approaching their targets to favorable distances and from favorable directions. It was indicated that these torpedoes were especially suitable for "blind" firing by hydrophone data.

Meanwhile, continued experimentation had produced not only improved models of the FAT, but also a new refinement of the same principle, LUT (Lagenunabhangiger Torpedo), a torpedo which could be fired effectively in a "curly" pattern independent of the target angle from its straight run and its mean speed of advance along its "curling" direction could be pre-set at will from 5 to 19 knots. The LUT was not used extensively until the middle of 1944, when COMSUBs stated that a destroyer and two large freighters had been sunk by LUT spread shots. (2333/6 July) Most reports of the period, however, were concerned with tube runners caused by failure of the LUT adjuster to disengage, and this trouble continued at least until the beginning of November, after which little was heard of LUT.

5. The electric-acoustic T-V

The most spectacular torpedo developed by the Germans, because of its appeal to the imagination as well as the impact of its sudden initial success, was the T-V, called the "Zaunkonig" ("Wren") by the Germans and the German Naval Acoustic Torpedo (GNAT) by the British. That COMSUBs was relying heavily upon this new torpedo to restore to the U-boat its former advantage in the Battle of the Atlantic was evidenced by a series of messages during September 1943, all sent in Offizier settings for purposes of security. The first reference (1128/2 September) called upon all U-boats equipped with


Zaunkonig to make use of every opportunity to bring their "sharp weapon" into play and to act upon the principle that "offense is the best defense." Decimation of the convoy's escort was stated to be their main goal.

The great importance of secrecy with regard to the new weapon was emphasized not only by the use of Offizier settings in the first messages concerning it, but by the fact that the contents of the messages, in so far as they could be made known to the crew at all, were withheld until shortly before the beginning of the first operation. (1226/13 September) Moreover, in reports on shots with "Zaunkonig," the target angle was to be disguised by use of an additive and comments on enemy speed were to be given in code-words. It was expressly directed that details of firing and the limits of effectiveness of the torpedo should be kept secret from the crew. (1422/21 September)

Continuation of this secrecy into 1944 was demonstrated by instructions to U-boats enroute to the Far East to sink their "Zaunkonigs" and keep them absolutely secret from the Japanese.3 (1024/4 April 1944) It was not until the end of May that Admiral Dönitz was ready to supply the Japanese with general information and even in June an order was given that while the Japanese at Penang were to be allowed to inspect the torpedo itself, they were under no circumstances to be permitted to see the instructions for its use. (2119/19 June) The implication even then was that the inspection was to be allowed because it could no longer be avoided. Fortunately for the Allies, all the German secrecy was to no avail, for radio intelligence had made the nature of the torpedo available even before it was actually used for the first time by Group Leuthen in mid-September 1943.

The T-V was a 21-inch torpedo which could be fired either on the surface of down to a depth of about 100 feet with a speed of about 24 knots up to 6,000 yards. The torpedo's most important attribute was its ability to home acoustically to the propeller noise of the target after an initial straight run. The immediate widespread use of the T-V can be appreciated form the fact that after the initial report of a tube runner by Mader on 15 September (0029) there followed within ten days fifteen separate reports on the firing of T-V's. Almost half of


the reports claimed indications of success, and it was significant that the successes were generally registered against escorting vessels rather than freighters. In a message of 12 October (1555) COMSUBs enthusiastically claimed further successes, including five destroyers and three freighters sunk and one destroyer probable sunk, all between the second and tenth of October. In message 1847/25 November, while admonishing U-boat commanders to utilize all opportunities for firing T-V's "has fundamentally altered the U-boat-destroyer relationship in favor of the U-boat. In this torpedo the U-boat has not only a good defense, but a strong attacking weapon as well."

The large number of sinkings which continued to be reported bore out this opinion, and the Japanese Naval Attache in Berlin reported on 29 January 1945 (JNA #914) that the rate of hits of acoustic torpedoes since they were first put into use was 43 percent. When the French U-boat bases were evacuated in the middle of 1944, the "Zaunkonig" was so highly valued that the removal of T-V torpedoes and testing facilities from the bases was considered a paramount task. The latter half of 1944, however, showed a diminishing effectiveness of the T-V owing to success of noise-makers used by Allied ships as well as the generally more difficult operating conditions for U-boats, both clearly indicated in ENIGMA traffic.

6. Anti-aircraft armament prior to December 1943. Quadruple mounts and flak-boats

As the struggle between the U-boat and its enemy, the airplane, progressed, it became necessary to provide the U-boats not only with means of avoiding aircraft but of waging battle with them when they could not be avoided. In the first half of 1943 anti-aircraft armament aboard U-boats was increased so that the typical 500-tonner was carrying an 33mm gun forward, a 20mm on the bandstand, and tow machine guns on the bridge, with one or two additional 20mm guns on a temporary platform below and abaft the bandstand. When the effect of this increased in armament proved negligible with existing tactics, the next step was a change in tactics whereby U-boats remained on the surface and fought it out when surprised by aircraft.

This change in procedure caught the airplanes unprepared and achieved a temporary success reflected in traffic by eighteen occasions during the month of July alone in


which U-boat commanders reported that the enemy had been "warded off." Nonetheless, a large number of U-boats were sunk by aircraft in July, and the success of the new policy dwindled in August and September when planes were reported driven off only six times altogether. A few boats had been equipped between May and September with 20mm quadruple guns, for accommodation of which the bridge was permanently enlarged. On some of the 500-ton boats these guns were mounted on the lower platform, abaft the bridge, with one or two smaller 20mm installations on the bandstand. The 33mm gun forward was generally removed from these boats.

Manseck (U-758) was among the first U-boats to be equipped with a 20mm quadruple mount, and his first engagement with it occurred on 8 and 9 June 1943 when he was attacked by four aircraft from the USS Bogue. The attacking aircraft, confused by the enlarged bridge, believed they were dealing with a 1600-ton supply submarine. Manseck, though badly damaged, managed to drive off his four assailants. One of them was hit in the engine but all returned to the USS Bogue. Manseck's report on the engagement stated:

Eight carrier planes warded off; one shot down, four damaged. (0527/9 June 1943)

COMSUBs was delighted. He replied: "Well done. Long live your quadruple." (1257/9), and announced a few days later that other U-boats were to be equipped with the new weapon.

There was little indication of the effectiveness of the 20mm quadruple mount from ENIGMA traffic, but there was only a moderate increase in the number of planes hit or brought down in the last months of 1943. Enigma traffic did reveal the fragility of the weapon, however, with no less than seventeen U-boats reporting their quadruple mounts out of order in October alone.

Of great importance was the fact that the increased armament had also increased the crash-diving time of the U-boats by probable a quarter of a minute. Such an increase was quite unacceptable, and experiments with a special antiaircraft U-boat, or Flak-U-boat, were instituted in May 1943, when one such boat was sent out. By November there were five of them actively operating. Conversion of a 500-tonner to a Flak-U-boat consisted of enlarging the superstructure and increasing the fire power. The 88mm gun forward was


replaced by a 20mm quadruple mount, with a second quadruple mount on the rear of the greatly extended bridge, and below it a 37mm gun on a platform. Conversion took from 11 to 13 weeks in most cases.

The Flak-U-boat was apparently envisioned as a floating fortress, capable of defending itself and other U-boats on the surface. Indeed, these boats were assigned as escorts in the Bay of Biscay and as protectors for refueling operations. A case in point was that of Brauel (U-256), a Flak-U-boat assigned to protect Barber (U-220) in a refueling operation at the end of October 1943. Both boats were attacked by aircraft on the 28th. Barber was sunk, but Brauel must have submerged even before ascertaining that fact, for his only report on the incident three days later merely stated that he had not seen Barber since the attack.

In protecting either themselves or other vessels, the boats were conspicuously unsuccessful, and the whole record pointed to the conclusion that the Flak-U-boat was a failure, with its one positive contribution being its service as a preliminary testing ground for the automatic 37mm gun. Even that service turned out to be of limited value in view of the necessity for long-continued experimentation even after the 37mm was fitted on other boats.

7. The 37mm automatic anti-aircraft gun

The failure of the Flak-U-boat led to reversion to the original approach, namely an increased in the armament of all U-boats at the expense of quick crash-diving by fitting them with the 37mm gun. A memorandum sent to COMINCH in December 1943 summarizing the above facts concerning antiaircraft armament pointed out that this reversion on to the original approach showed the same trend as other developments of the period, namely, a transformation of the U-boat from an offensive weapon to a defensive one. In support of this statement it was possible to bring to witness a statement made by COMSUBs in a long message of 13 November (1903/13 and continuations) in which he said as consolation to his commanders that enemy forces engaged by U-boats "cannot be put into action against the homeland. Therefore, you are protecting your home even in battles which seem to you at the time unsuccessful." In the same message, COMSUBs hailed the new hope of the U-boat, stating:


In the fight against the main enemy, the opponents' air arm, the automatic 37mm gun will bring further easing . . . It is expected that form the beginning of December on, all U-boats departing from Western France will be equipped with this excellent weapon and that from the middle of December on, all home U-boats will be so equipped. In addition to the 37mm gun, U-boats will be equipped with the most modern ammunition, the effect of which amounts to that of the 20mm many times multiplied, and one hit of which generally will bring a plane down.

Unfortunately for the Germans, "this excellent weapon" was constructed and installed with such rapidity that the usual thorough test were omitted, as acknowledged in 1034/5 January 1944. Even so, there was hopeful confidence in the weapon form the first, even to the extent of ordering U-boats equipped with 37mm guns to charge their batteries in the daytime in the Bay of Biscay. (2238/6 January) This order was rescinded about a week later owing to decreased aircraft activity at night rather than to any success of failure of the 37mm.

The continued confidence of COMSUBs was demonstrated when, in planning an approaching convoy operation by a group of U-boats, he ordered that all U-boats whose 37mm guns were in order should remain on the surface if the operation extended into the daylight hours so that the mass of U-boats could "shatter and scatter" the enemy's defenses. (2236/17 February) This test of the effectiveness of the 37mm did not take place as the group operation failed completely and offered no opportunity for attack. In recapitulating the failure, COMSUBs betrayed some misgivings about his weapon when he said:

A continuation of the operation by day was not possible, since, with only a few U-boats in the vicinity of the convoy, the battle with the aircraft defenses could not be taken up: especially since there were carrier-borne aircraft with the convoy. This attempt will be made as soon as more U-boats are with a convoy and we can rely better on the 37mm. (0207/20 February 1944)


In the actual fact, the reports of mechanical trouble with the 37mm gun far outweighed reports of successes in the firs months of 1944, but the equipping of U-boats, including those operating in the Arctic area, continued. The continuing confidence of the U-boat command was further expressed in an Arctic Order (0420/17 May) which stated:

With the equipment of U-boats with the 37mm a first-rate weapon is given to the boats. It will pierce any armor of carrier or land based aircraft or flying boats, and often makes it possible for U-boats to remain on the surface and to advance. In all situations, take advantage of the power of this weapon.

Yet, in contrast to this expression of enthusiasm, COMSUBs was impelled to warn Atlantic U-boats in massage 2201/10 June not to become overconfident in their antiaircraft and search receivers and thus jeopardize the safety of the ship.

The anti-aircraft armament of U-boats continued to be the subject of many reports throughout the remainder of the war. After the Allied invasion of France, there was the usual preoccupation with difficult supply for U-boats based in France. Most of the other messages dealt with mechanical difficulties, with a few indicating definitely that the 37mm was still an object of experiment. In this connection, Raabe (U-246) was told in November (0911/7) that experiences with the new 37mm gun were very important, and messages similar in content were sent as late as January 1945.


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1. Exploder is the American term for the detonating mechanism of the warhead. Throughout the war the British term, pistol, was more commonly used.

2. The 44 knot setting was for Schnell boats only and not used by U-boats.

3. The Japanese Naval Attaché in Berlin had already gotten wind of the weapon.

Transcribed and formatted for HTML by Ian Williams for the HyperWar Foundation