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Chapter V

"B-Dienst" Aboard U-boats


  1. U-boat radio intercept teams, fall 1943.
  2. Previous warning in U-boat traffic.
  3. U-boat D/F equipment.
  4. Warning from traffic of beginning of U-boat D/F on 2410 kcs.
  5. U-664 (Graef).
  6. Renewal of North Atlantic battle, September - November 1943.
  7. Leuthen's success, ONS 18 - ON 202.
  8. Rossbach's failure.
  9. COMSUBs turns to U-boat radio intercept teams.
  10. B-Group performed well in Schlieffen operation, although attack failed.
  11. Examination of U-boat Command's analysis.
  12. Allied preparations.
  13. Allied deception.
  14. Special long range maritime patrol aircraft for convoy location, November 1943.
  15. Subsequent use of B-Dienst on board.
  16. COMSUBs Standing War Order Number 256 (captured document).

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  1. U-boat radio intercept teams, fall 1943.
  2. With the resumption of the North Atlantic convoy offensive in September 1943, U-boats made a serious attempt to exploit convoy and aircraft voice transmissions by means of B-Dienst personnel trained to take bearings on voice traffic (2410 kcs, 124.5 meters) and to correlate the plain language and codewords used in convoy communications. This activity came as no surprise to the Atlantic Section, for German interest in experiments with U-boat interception on 2410 kcs were well advertised in past U-boat traffic. For many details, including shore training and personnel, the Atlantic Section was indebted to OP-16-Z's interrogations of a B-Dienst party captured in August 1943 (ex-U-664)1. During the interrogations, OP-16-Z frequently consulted the Officer in Charge of the Atlantic Section. The results were made available just at the time when the German navy was making its most concerted effort to home on convoys by intercept at sea.

  3. Previous warning in U-boat traffic.
  4. The first known instant of convoy voice interception by a U-boat occurred in some of the earliest current traffic read by the Atlantic Section in December 1942. U-boat Group Panzer was expecting a northeast convoy (HX 217) by 5 December some 300 miles north of Flemish Cap. At about midnight on the 4th Steinaecker (U-524), a member of the group, reported hearing English transmission on 124.5 meters in 51°27'N - 47°35'W.

    Whether or not the shift in the patrol line ordered two hours later was due to Steinaecker's report is not known. Steinaecker made first contact on the convoy at noon on the 6th. Nothing further on voice interception was seen until April 1943, when four U-boats in Group Meise reported voice interceptions and signal strengths. Group Meise was patrolling the area north of the Flemish Cap in anticipation of SC 126 and HX 233. It was evident that COMSUBs sought to overcome the bad fog and weather conditions of 15-17 April and to establish

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    contact by means of voice interceptions. The attempt failed completely.

    Convoys SC 126 and HX 233 were diverted far to the south of Meise, whose members apparently were intercepting transmissions from ONS 3 without realizing that they had the wrong convoy. A study of this operation was submitted to COMINCH on 21 April with the conclusion that U-boats were not yet able to take bearings on frequency as high as 2410 but might be in the near future.2

  1. U-boat D/F equipment.
  2. 2410 kcs is in the medium frequency band (300-3000 kcs). U-boat beacon signals fell in the lower part of the M/F band and it was reasonable to assume that U-boat D/F equipment would be extended to include the upper reaches of M/F, although the reliance on signal strengths for estimating the convoys distance precluded such equipment in the Meise case. There is no indication that U-boats were ever able to take bearings on H/F (above 3000 kcs). The known development of U-boat D/F-ing went in the other direction with the attempt early in 1944 to take bearings on very low frequency transmitters (VLF = below 30 kcs), including those on the U.S. coast, for navigation purposes.

  3. Warning from traffic of beginning of U-boat D/F on 2410 kcs.
  4. Early in August 1943, it was discovered from traffic that eight code groups were being added to the U-boat short signal book for reporting bearings and signal strengths on convoy voice transmissions. A few days later U-664 was sunk, and B-Dienst personnel became available for interrogations.

  5. U-664 (Graef).
  6. According to prisoners of war, an operator named Dobberstein left St. Andries, the communications intelligence training station, in April 1943 for Brest, where he was assigned

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    to U-664. While there is no trace of intercept activity in the traffic covering U-664's penultimate cruise, the period (May 1943) would fit in nicely with the known progression of German experiments on convoy voice interception. U-664 left Brest late in April and took part in Group Donau's unsuccessful attempt in May on convoys SC 130 and HX 239. For U-664's final cruise there is again no record in traffic read of a functioning B-Dienst unit. There is, however, the interesting message (1936/16 August) which ordered U-664 not to use her SADIR receiver under any circumstances. Flotillas were also instructed to remove the equipment from U-boats being outfitted with it.

    The interrogation reports established that SADIR was part of the B-Dienst equipment aboard U-664, as well as standard equipment for intercepting aircraft signals by shore B-Dienst stations. Special operators with such receivers as SADIR were probably to assist the U-boat in its unequal struggle with the carrier task groups. While there is no certainty as to Command's plans for U-664's last cruise, it is interesting to note that Graef received orders (1909/25 July) to cruise out with Wilamowitz (U-459) a supply U-boat, sunk while outbound. Their destination was apparently the general refueling station west of the Azores. Graef's association with Wilamowitz may have been accidental and is subject to other explanations, but it does suggest that Command was trying a new method for protecting his tankers.

    In both June and July refuelers (U-118 and U-487) had been sunk in their rendezvous areas by aircraft from carrier task groups, and at the time of U-459's departure Command warned that a carrier force was in the outer Biscay. In view of the recognized inadequacy of German search receivers during the summer of 1943 and the lack of any replacement at that time, it is reasonable to suppose that Command would have taken some extra precaution with his dwindling supply submarines and have drawn on B-Dienst personnel and equipment to intercept aircraft traffic for warning purposes.3 The Germans had temporarily abandoned convoy operations

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    and hence had no opportunity to carry on with plans for intercepting convoy traffic for group attacks.

  1. Renewal of North Atlantic battle, September - November 1943.
  2. From the reopening of the North Atlantic convoy battle in September to the withdrawal of the last patrol line from the Newfoundland area in November, seven large groups operated. The first three, Leuthen, Rossbach, and Schlieffen, were each provided with two B-Dienst U-boats suitable disposed in line for taking cross bearings. The last four, Siegfried, Korner-Jahn, Tirpitz and Eisenhart, had only one, Poel (U-413).

  3. Leuthen's success, ONS 18 - ON 202.
  4. When COMSUBs hailed Group Leuthen on 23 September for its vindication of the U-boat arm and its "proof of the new weapons" (2056/23 September), the B-Dienst groups were presumably congratulated along with everyone else. No preceding convoy operation had combined so many aids for the U-boat commander. In addition to the acoustic torpedo, improved GSR, radar counter devices, and new tactics, B-Service not only followed the operation against ONS 18 and ON 202 very closely from the shore, but also furnished information on the spot.

  5. Rossbach's failure.
  6. Enthusiasm was short lived, for the next three westbound convoys, ON 203, 204, and ONS 19, all slipped around the north end of the Rossbach line. Contacts had been made but the risks involved in transmitting to Control had discouraged U-boat commanders. COMSUBs reaction came quickly in the form of a Current War Order (#4, 1035 and 1137/6/10). The location of the convoy was the "chief problem of the U-boat warfare" and the responsibility rested on the U-boats themselves. Short signals for sighting reports were to be held ready for immediate transmission. Not even the slightest hint of the convoy's whereabouts might be withheld from Control.

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  1. COMSUBs turns to U-boat radio intercept teams.
  2. Despite assistance of German long range reconnaissance aircraft and shore D/F, Rossbach failed again with SC 143. On 11 October the B-Group in U-584 (Deecke) was requested to report its experiences, presumably as part of Command's attempt to analyze the failures and determine whether or not full use was being made of shipboard intercept possibilities. U-584's report was substantial and fairly detailed. Voice frequency had been heard over a radius of 200 miles. Convoy diversions had been detected. Information on the number of aircraft, the period of their coverage, and their radio frequency, as well as the general disposition of escort groups was submitted.

    Before the next operation began, Group Schlieffen's attention was especially called to the importance of voice interception. U-413 (Poel) and U-631 (Kruger) were singled out by name, for there were two B-Group men on board. (1150/15 and 1852/16) The above sequence suggests that U-boat operational Command was slower than Allied communications intelligence in attributing major importance to German intercept teams, for, in addition to emphasizing the need for caution in convoy and carrier voice transmissions, the Atlantic Section was at this time calling COMINCH's attention to the possibility of deception which could be practiced by B-parties with voice transmitters.

  3. B-Group performed well in Schlieffen operation, although attack failed.
  4. Group Schlieffen's operation against ONS 20 and ON 206, 16-18 October, marked the climax of German effort with B-Dienst in the Atlantic. A premature sighting of ON 206 confused the beginning of the attack, for COMSUBs apparently assumed the convoy to be ONS 20, the one he seemed to know more about. The B-Dienst U-boats were called upon to clarify the situation, but COMSUBs' initial confusion continued to show-up in his misinterpretation of their data. The sinking of Kruger on the night of the 16th placed the full burden of B-Service on Poel's U-boat.

    A careful analysis of the operation indicated, however, that Poel's group performed well and that the operation's failure was due in part to faulty correlation of information at

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    home. Command did make use of Poel's bearings in directing the attack but could not form a clear picture from the various types of reports which all U-boats submitted. At a critical moment, for example, the was considerable hesitation in trying to decide between a radio bearing reported by Poel and a conflicting hydrophone bearing sent in by Hungershausen (U-91).

    After the operation had been discontinued, Poel's B-men were requested to report. Somewhat on the defensive, they explained that through lack of experience they could not always distinguish immediately between convoy and remote escort groups. On the 20th COMSUBs summarized the problem as follows:

    "Wrong data concerning location of convoy have originated in at least some cases from the circumstances that during a convoy operation submarines have taken medium frequency bearings on aircraft or escort groups which are far from the convoy, a bearing on escort groups is a valuable clue to the convoy's position (for example, noon on the 16th). During sweep and pursuit, on the other hand (noon of the 17th), bearings on remote groups are misleading. In the future, therefore, when reporting bearings, report also the object on which the bearings are taken, insofar as this is known. Procedure regulations will follow." (0716/20 October 1943)

  1. Examination of U-boat Command's analysis.
  2. Poel's bearing, noon 16th, had been an accurate one on ON 206, then approximately 190 miles north-northeast of U-413 and on a diversion course to the northwest. It was disregarded in disposition orders, apparently because it did not agree with preceding sighting reports which had been correlated with ONS 20's plot, and it was thus probably interpreted as coming from a remote escort group. About noon of the 17th, Poel reported a radio bearing of approximately 10°T and, from the same position, Hungershausen reported hydrophone bearings of 170-250°T. COMSUBs did not attempt a clarification for five hours.

    It is believed here that both U-boats were probably right but that Poel's bearing was on ONS 20, then about to alter from

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    a northwest to a southwest course, and that Hungershausen had heard stragglers or rear escort of ON 206, then clearing the infested area on a southwest course. When COMSUBs clarification finally came, it rejected Hungerhausen's evidence altogether. Using Poel's bearings Command drew the wrong conclusion and sent his boats in a sweep to the northwest. Meanwhile convoy ONS 20 was cutting in behind the U-boats on a southwest course. COMSUBs' consistent exclusion of ON 206 from consideration, despite his general awareness that the convoy would be in that area, was probably a major factor in the failure.

  1. Allied preparations.
  2. Although the U-boat arm failed to reestablish itself in the open Atlantic during the group operations of October 1943, the threat of radio intercept parties was appreciated and efforts were increased to deprive the enemy of any support he might derive from this source in the struggle for advantage. CINCLANT disseminated warnings to Atlantic forces and CINCWA sent instructions and defined states or degrees of radio silence to be enjoined according to the situation. Early in November CINCWA reported two cases in which U-boats "used radio in apparent effort to obtain D/F bearing of reply. Correct convoy call sign and phonetic alphabet employed." (COMNAVEU Secret 051651Z November 1943) Operators were warned "against answering transmissions made with incorrect procedure or with foreign accents." Other cases of presumed deception by U-boat intercept parties were reported, but no one of them was ever confirmed by Atlantic U-boat traffic.

  3. Allied deception.
  4. The Allies, on the other hand, did practice deception as a countermeasure. As early as 2 November CINCWA drew up a proposal for detaching a group or single ship to simulate convoy voice radio traffic. In mid-December, an attempt was made to carry out this countermeasure for the confusion of Group Coronel, then in the area of 55°N - 25°W.

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  1. Special long range maritime patrol aircraft for convoy location, November 1943.
  2. Following the failures of October and early November, COMSUBs announced on 13 November 1943, that, effective at once, reliance would be placed on special long range maritime patrol aircraft (Ju-290) for the location of convoys and the homing of U-boats. U-boat groups were drawn in from the mid-Atlantic and concentrated in the western approaches within the range of GAF reconnaissance. No blame could attach itself to B-Dienst on board, for the difficulties of U-boat operations were too fundamental to be overcome by a device which was tried on a limited scale and which at best presupposed some degree of mobility for the U-boat. On the occasion of this general shift in tactics, COMSUBs gave the standard explanation to his men:

    "Founded assumption is that enemy air reconnaissance, using location methods which in part we have not yet been able to pick up, has contacted our line arrangements and has gone around us." (1903/13 November 1943)

  3. Subsequent use of B-Dienst on board.
  4. The U-boat radio intercept project seems gradually to have been abandoned, at least in the Atlantic, for as U-boat groups came in closer to the convoy gathering and dispersing points radio bearings were more often confusing than not. Certainly COMSUBs paid more attention to the reconnaissance aircraft. A few U-boats on inshore operations around the British Isles continued to carry B-Groups. One of these, Albrecht (U-386), claimed that radio interception warned him of an impending search for his boat during an operation off North Ireland in January 1944.4 During 1944, U-boat radio interception was confined largely to Arctic waters, where in fact it is still being used to a considerable extent.

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  1. COMSUBs Standing War Order Number 256 (captured document).
  2. Standing War Order No. 256, "Observation of Enemy Convoy Voice Traffic," was issued in October 1943. It warned U-boat operators that they must distinguish between ground and sky waves, disregarding the latter, and that they must make certain if the bearings sense by checking with several bearings before reporting. (Poel's group did report one reciprocal bearing in the Schlieffen operation but it came after the operation had already fizzled). German equipment was said to D/F ground waves accurately over a range of 80 to 100 km.

    The digest on convoy communications procedure included types of traffic originators, together with methods for identifying the convoy and originators and methods for determining the sea area, direction, and speed of the convoy. 82 codewords were listed with convoy designators and call letters of the radio guard ships. Of the 101 convoys identified with codewords there were three GUS, four UGS, four FT, four TF, one TH, and two HT convoys.

    Although Standing War Order No. 256 contained nothing of a startling nature it bore adequate witness to the care with which German communications people had worked on voice interception. Had the German navy been able to master the overall problems of a mid-Atlantic U-boat offensive, it is probable that Standing War Order Number 256 would have been a modest introduction to U-boat radio interception at sea rather than an unintended, final summary.


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1. U-664 was sunk on 9 August 1943 by aircraft from USS Card (CVE 11) at 40°12'N 37°29'W.

2. This conclusion did not wholly agree with British findings, which interpreted the above operation as "indication of direction finding being used on this frequency wave (2410)."

3. It is presumed that SADIR was withdrawn because of re-radiation, the same trouble which afflicted GSR and led to so many changes in GSR policy and apparatus. It is worth mentioning here that in October 1943, the Germans introduced anti-aircraft U-boats as protection for U-tankers.

4. Some radio interception by U-Boats could have been on VHF (above 30 mcs) since this equipment (LO 10 UK 39) was reintroduced for operational cruises during the winter of 1943-1944. During the remainder of the war, shipboard interception was largely confined to Arctic waters.

Transcribed and formatted for HTML by Rick Pitz, HyperWar Foundation