Chapter IV
U-boat Restoratives

  1. Course of the war.

    1. Summer 1943. Dispersal and U-boat diversion.

      1. US-Gibraltar lane. Bogue and U.S. Navy radio intelligence.

      2. Weakness of the summer campaign: unavoidable U-boat concentrations in passage areas and at refueling rendezvous.

      3. Results.

    2. Renewal of the North Atlantic Battle.

      1. T-V acoustic torpedo. "Offense of the best defense."

      2. Non-radiating search receivers will restore the balance "in the main theater of battle."

      3. "Thoughts on convoy fighting:" surprise the convoy.

      4. Sink the escorts. ("Thoughts on convoy fighting" continued).

      5. Results.

  2. Major changes underway.

    1. German diagnosis of U-boat ailment.

    2. Plans for new types of submarines.

    3. Schnorchel.

  3. U-boat restoratives before schnorchel.

    1. Against aircraft: GSR, infra-red receivers, radar, deception, anti-aircraft.

    2. Against destroyers.

    3. Tactics.

    4. Against the Allied intercept net.

    5. Failure of all countermeasures.


U-boat Restoratives

The history of the U-boat war from May 1943 to the fall of 1944 is a repetition of German defeat. The U-boat continued to suffer heavy losses without being able to come to attack. Conviction of the U-boats defeat did not come easily. At first it seemed a matter of offsetting a temporary Allied technical advantage by devising suitable countermeasures. After the briefest treatment of the courses of the war down to the invasion of France, this chapter will summarize the background of this period in terms of major changes started for a new U-boat warfare and in terms of various devices which were used in the attempt to keep the old U-boat alive.
  1. Course of the war.

    1. Summer 1943. Dispersal and U-boat diversion.

      The situation could not have seemed hopeless at first, for it was not believed that the Allies could distribute their advantage so as to cover every ocean area or every individual U-boat in the way in which U-boat groups had been covered in the North Atlantic during April and May. Meanwhile German technical genius--undisputed as ever--would produce the needed countermeasures and the U-boat could then return to the major battle.

      In leaving the North Atlantic convoy lanes, it was recognized that the U-boat was withdrawing from the one area where victory was absolutely necessary. In one of his last messages to U-boats before the abandonment of the North Atlantic, Admiral Dönitz had made it plain that there were no longer substitute areas where the U-boat might fulfill its mission. U-boat commanders had been warned on 21 May: "... do not fail to understand that you must answer for your actions." (1900/21). The withdrawal three days later admitted a major and decisive defeat, although, at the time, it was presented as a strategic withdrawal.

      1. US-Gibraltar lane. Bogue and U.S. Navy radio intelligence.

        That the US-Gibraltar lane was covered by carrier air power was quickly demonstrated early in June when Command


        tried to contain group warfare with the U-boats salvaged out of the last North Atlantic operation in May. The USS Bogue attacked group Trutz on the basis of U.S. Navy decryption intelligence before the convoy could arrive which Trutz expected on the basis of German decryption intelligence. The combination of superior intelligence with the expansion of U.S. Navy air power in the Middle Atlantic, which Command did not anticipate, was to cost him dearly.

      1. Weakness of the summer campaign: unavoidable U-boat concentrations in passage areas and at refueling rendezvous.

        The U-boat dispersal which became marked after the Trutz affair was as natural and customary as it was necessary. Once dispersed, however, U-boats could find no place in the Atlantic where they might recoup their losses and reestablished their prestige. Instead, the effort led to an intensification of defeat by an alarming reduction of the operating fleet--and by the loss of the U-tankers. There were still two areas of unavoidable U-boat concentration: first, the passage areas of Biscay and Iceland-Faeroes; and second, the middle Atlantic refueling rendezvous, which were all the more necessary now that the 500-tonners were being employed in American waters. Against the first concentration the British waged a highly successful joint surface-aircraft sweep. Against the refueling rendezvous the U.S. Navy CVE's made a lasting impression on the U-boat navy.

      2. Results.

        In fulfilling the necessary conditions of attack, namely, passage and refueling, was a highly dangerous and nightmarish business, for the U-boat, once in his operational area he had no escape from the problem of survival. Of those which reached American waters, more than half failed to return home.


    1. Renewal of the North Atlantic battle.

      During the summer the Navy had, or though it had, developed the devices which Admiral Dönitz had promised to the U-boats in May. When U-boat groups formed on the US-UK convoy lanes in September the Admiral apparently believed that the U-boat was once more equal to its task. The hopes and ground for these hopes are best stated in the Admiral's own words. First of all, there was the acoustic torpedo, "Zaunkonig."

      1. T-V acoustic torpedo.

        "Offense is the best defense." As U-boats sailed from France to await the signal for the first convoy operation, all TV equipped submarines were instructed as follows:

        To be deciphered only by Zaunkonig subs:
        Offizier R
        Make use of every opportunity to bring your sharp weapon into play. In Biscay on the way to your operational area, be at all times clear for a defensive or offensive shot against destroyers or corvettes. If the enemy bears down on you, do not blind yourselves by going to great depths, but in the daytime remain at periscope depth and fIre. You still have enough time after firing to dive. The same applies at night, first fire, then dive. Offense is the best defense. Act accordingly.
        (1128/2 September 1943)

      2. Non-radiating search receivers will restore the balance "in the main theater of battle."

        Unaware of the development in Allied centimeter radar, the Germans had become convinced in the course of the summer that Allied location successes were due to radiations from the old style search receivers. This leak had supposedly been stopped with the introduction of a new receiver, covering the old band of 120-180 cm. Hence the following general message to T-V U-boats on 13 September:

        To all Zaunkonig U-Boats:
        Offizier J
        After an interruption of months you will resume the submarine war in the main theater of battle, the North Atlantic. New


        weapons and equipment are at your disposal for that purpose. Experiences in the Bay of Biscay have shown that the situation has changed fundamentally and in your favor as regards the matter of radio locating (funkortung). The prerequisites for a successful fight have been given. I feel certain that you will take up this battle which is likewise decisive for the future of our race and take it up in the old warlike spirit of the men of the submarine corps. The Führer is following every phase of your fight. Attack. Go to it. Sink them. COMINCR and COMSUBs.
        (2056/13 September 1943)

      1. "Thoughts on convoy fighting:" Surprise the convoy.

        Since the U-boats invisibility had been restored by the new search receiver, it was permissible to repeat the doctrine of surprise-attack. On 14 September U-boats were sent these instructions:

        To all Zaunkonig subs:
        Offizier O
        Lines of conduct for first operation on a convoy. 1) Approaching (the attack area). Remain absolutely out of sight when cruising in formation as well as in (attack) strips: However, make the most of favorable opportunities for attack. Contacting the enemy, defense, and successful operation depend on surprising the enemy. During the current offensive in the Mediterranean, convoys in the Atlantic may be weakly guarded. An example of this was the Moroccan campaign. 2) Operating: uniform procedure in the different states in accordance with "Thoughts on Convoy Fighting," is the most important requirement. In the initial attempt make the most of the surprise technique, transition to open battle only after sending the short signal.
        (0837/14 September 1943)[Emphasis added]

      2. Sink the Escorts. ("Thoughts on convoy fighting" continued.)

        With the T-V the U-boat was now specifically armed against surface defense.


        To all Zaunkonig subs:
        Offizier Z
        Thus the main goal is to decimate the escort. The destruction of even a few destroyers will have a considerable effect on morale of enemy and will in addition greatly facilitate attack on ships. When advancing on surface do not groups of more than two subs. The goal must be to surround in even distribution, in order to disperse enemy defense. ... 3) I expect all commanding officers to make use of every opportunity for shots on destroyers. The sub is now the attacker. Shoot first, submerge afterwards. COMINCH.
        (0905/14 September 1943)

        It is interesting that no reference was made in these opening messages to the U-boats anti-aircraft, on which considerable effort had been expended during the summer. With adequate warning of aircraft approach in the search receiver it was more advisable to dive.

      1. Results.

        The renewed campaign against major convoys was, of course, a failure. In November, U-boat groups again evacuated the northwestern Atlantic. Somehow or other the U-boat was still being located by "... location methods which in part we have not yet been able to pick up ..." (1903/13 November 1943). Shipboard intercept and D/F of convoy voice traffic and special very long range aircraft reconnaissance found a few convoys but to no avail. There was no dispersal this time; U-boats were drawn back to the eastern North Atlantic and, concentrated nearer home. The U-boat did not give up. It stayed on and worked hard to develop more counter devices. Insofar as it tied down Allied naval forces, it was told, the U-boat kept just that much Allied strength away from the homeland. In January U-boats took up the task of reporting weather for invasion forecasts.

  1. Major changes underway.

    Whatever confidence may have been placed in the U-boats restoration, in August 1943 German High Command decided to build a new fleet.


    1. German diagnosis of U-boat ailment.

      German diagnosis concentrated on the surface U-boat as the occasion for its major weakness. The existing U-boat had depended largely on surface cruising for its mobility. To maintain its ability to maneuver as an invisible attacker it had to surface for battery charging. Since the U-boat was a visible object on the surface and since there seemed to be observers everywhere--day and night, the only final solution to the problem of restoring the U-boats power, its invisibility, was to eliminate the necessity for surfacing.

    2. Plans for new types of submarines.

      By September 1943 designs were underway for a U-boat that would really be what the name says: an "underwater boat," not visible to the human eye either directly or indirectly by the aid of electronics. It was first hoped to build a turbine propelled submarine combining both underwater endurance and speed. Later a compromise of diesel-electric propulsion with short burst of high underwater speed was accepted. The new type was to depend on schnorchel. It is not known to what extent the simultaneous development of Schnorchel may have determined the acceptance of the compromise type. Plans for the new types could not be fulfilled before the fall of 1944 at the earliest, even with the use of mass-production technique.

    3. Schnorchel.

      The diesel air trunk may have been under consideration for some time, though the revival of the earlier Dutch experiments was not noted in UTRA sources (British) until December 1943, when schnorchel trials were made in the Baltic. The first schnorchel-fitted U-boat in the Atlantic appeared in January 1944 (U-539, Lauterbach-Emden). Experiments at sea were not completed until after the Allied invasion of France.

  1. U-boat restoratives before schnorchel.

    Thus the continuation of the U-boat war--from the spring--of 1943 to the fall of 1944--depended largely on shifts and


    expedients, whose provisional character was at all times apparent. The history of the U-boat war during this period is written in the trails and hopes of these contrivances. The U-boat war had become a technical war for advantage in counterdevices. How effective in general these devices might have been, under the circumstances, cannot here be judged. It is sufficient to say that decryption intelligence furnished advance and accurate information on these devices--as well as the U-boats locations and plans.

    1. Against aircraft: GSR, infra-red receivers, radar, deception, anti-aircraft.

      GSR: Probably the most important of these restoratives, both in terms of time and energy expended, were the radar search receivers, which were to give adequate warning for submergence before aircraft could actually locate the U-boat. But the Germans were always a step behind, having the disadvantage of being on the defensive, i.e., trying to match an active instrument with a passive instrument after the fact. If they installed a receiver which covered the basic band of Allied radar frequencies, then the Allies changed their transmission tactics or their frequencies, leaving the U-boat in confusion. The Germans could never be sure of what was happening. Matters were not helped by the breakdowns of the receivers and aerials. There was never a permanent aerial for the centimeter bands. Reports from the sea were incomplete and inconsistent. Again and again U-boats were attacked without warning. They did not know how they had been spotted and U-boat Command could only guess at the means; visual, phosphorescence, infra-red search lights, U-boat radiations, new radar frequencies. U-boat Command probed and quizzed, exhorted and reprimanded. What exactly had been the state of visibility and the conditions of the sea? If any lights had been seen, what was their color? Had the U-boat been charging its batteries? Had it been tuning its radio transmitter? Was the receiver on? Was the search receiver in working order? Had nothing peculiar been heard or seen? No crackling noises in the headphones?

      The state of affairs in the winter of 1943-44 is well illustrated by the following incident in which an experienced U-boat commander apparently thought he had been flown at by a planet in her lawful orbit.


      From Bugs (0-629):
      Radar and light location continuously from 0400 to 0800. Thus (location of patrol) line continues to be known.
      (2131/9 December 1943)

      From Control:
      Bugs: before the night is over give more detailed account of light location.
      (0425/10 December 1943)

      From Bugs:
      White, yellow to red disc then out.
      Approached abeam flying from horizon preceded by rad. Diving was possible during and after light location.
      (0540/10 December 1943)

      From Blauert (0-734):
      Reference light location reported by Bugs.
      Similar observations. On the 8th aircraft alarm three times. The disc, however, always turned out to be the planet Venus.
      (2043/10 December 1943)

      In addition to receivers for possible Allied transmissions, both radar and infra-red, and many experiments conducted at sea in an effort to identify the locating beam deception measures become standard procedures, particularly in the form of "Aphrodite."

      U-boat radar: The use of U-boat radar offers a significant commentary on the U-boats predicament. For many months U-boat Commanders were reluctant to use it at all, for they were naturally very receiver-conscious. Knowing that a receiver's range was greater than radar range, they were afraid of giving themselves away by transmitting. After Command became reasonably sure that antisubmarine aircraft did not carry receivers, he had to wage a campaign to get his men to use their radar--not as a target locating device, but as an additional aircraft detection measure.

      Anti-aircraft guns: Emphasis on the policy of staying up and fighting aircraft tended to fluctuate with the rise and fall of confidence in search receivers. Tower alterations for increased anti-aircraft were noted in U-boat traffic in the winter of 1942-43, but orders to stay up and fight did not


      become pronounced until the inadequacy of the Metox GSR had been underscored by U-boat losses. None of the attempts to give the U-boat effective anti-aircraft fire succeeded. The quadruple anti-aircraft mount did not solve this problem. Flak-U-boats failed. Much was expected of the 37 mm automatic late in 1944, but it too failed. For one thing, the Allied practice of strafing before bombing tended to nullify the anti-aircraft crew. For another, the permanently installed guns (e.g., the 37 mm) were subject to too many breakdowns as a result of long immersion. Furthermore, tower alterations for extra gun mountings increased crash diving time and added one more worry for the Commander in his problem of deciding when to dive and when not to dive. Commanding advice was not to dive if the Commander felt himself "surprised."

    1. Against destroyers.

      T-V: The search receivers and deception devices mentioned above were employed against surface forces as well as against aircraft, but the principal counterdevice against surface forces was, in a sense, the acoustic torpedo. It is noteworthy that the only new addition to the U-boats proper armament after the spring of 1943, the T-V, should have been in effect a counterdevice. If the U-boat could not get at its real target, the merchant shipping, without being first discovered and held down or hunted down, then it had to have a torpedo designed for destroyers. Thus T-V was to Allied surface defenses what increased anti-aircraft was to aircraft defense. It was more effective probably than anti-aircraft, for aircraft did not move in the U-boats medium.

      Sound: There was an interesting parallel between the use of passive and active instruments against aircraft and against surface forces. U-boats did not use Asdic but relied on hydrophones in their preference for stealth and listening. The sound and bubble device (Bold) offered a further parallel to anti-radar devices such as "Aphrodite" and "Thetis."

    2. Tactics.

      Against both aircraft and destroyers, but mainly against aircraft, the U-boat was compelled to adopt every conceivable tactic as a countermeasure. It is difficult to imagine changes


      which might have been tried and were not. For example, there were countless suggestions as to how surface charging of batteries could be done without an attack. Everything from phosphorescence and weather to habits of antisubmarine patrols was taken into minute consideration.

    1. Against the Allied intercept net.

      Radio silence on the part of the unit was offset by the German operational system. If the U-boat kept still, Command did not, but continued to inform the Allies where the U-boat was going, what it was planning to do, and where he thought the U-boat should be. A U-boat could not really observe complete radio silence for Command's personal direction of the war required passage /reports, return reports, and situation reports. In the closing weeks of the war, Command demanded such transmissions from reluctant Commanders because they were of the utmost importance for decisions concerning the future disposition of U-boats.

      To enable U-boats to transmit with out fear of interception and D/F, elaborate off-frequency systems were set up in the fall of 1943. Without foreknowledge by decryption intelligence, these systems would have presented a serious obstacle to the Allies.

    2. Failure of all countermeasures.

      The countermeasures could not correct the fundamental difficulties. First, they did not touch decryption intelligence as a locating device. The reading of U-boat traffic, in addition to its negating tendency on German efforts, offered an absorbing study in unsuccessful attempts to work from known effects to unknown causes. Second, once the U-boat was deprived of its invisibility, its weakness in a defensive role could not be compensated. A resort to its ability to submerge, its one strength in defense, was as dangerous as staying up when surprised by aircraft. The U-boat had to be told in the summer of 1943 that "diving is death." There was no way of protecting the vessel at the moment of submerging, when it passed from one medium to another. The dual life complicated the countermeasures and interfered with their effectiveness.


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