Chapter III
Winter/Sprint U-boat Offensive
and the U-boat's Defeat, 1943

  1. General considerations on the U-boat's defeat.

    1. Retreat or strategic withdrawal.
    2. U-boat navy's time-table.
    3. Review in terms of merchant shipping and U-boat losses.
    4. World War I compared with World War II.

  2. Pattern of winter/spring offensive.

    1. Independent U-boats in distant areas.
    2. U-tankers.
    3. Group patterns for US-Gibraltar lane. December 1942-April 1943.
    4. Group operations against US-UK convoys.

      1. Pattern of the US-UK group formations.
      2. The simple pattern.
      3. Transition.
      4. March 1943: the pattern works.
      5. Break.
  3. Examination of the U-boat's defeat.

    1. ULTRA intelligence and convoy diversion.
    2. Convoy defenses.
    3. Loss of experienced U-boat commanders and crews.
    4. Surprise attacks from the air. Visibility and Vulnerability.

      1. U-boat Command's refusal to admit defeat.
      2. U-boat Command accounts for the defeat.


Winter/Spring V-boat Offensive and the V-boat's Defeat, 1943

The U-boat was decisively defeated in the spring of 1943. The failure of the German winter/spring offensive can be ascribed in part to Allied communication intelligence. Decryption intelligence assisted materially in successful convoy diversion in January and February, when massed assaults might have produced shipping losses comparable to March, or worse. Delays in decryption intelligence on our side plus timely radio intelligence on the German side played an important part in such convoy successes as the U-boat did achieve, notably in March. In April and May decryption intelligence held its own, but even when U-boats made contact the strengthened surface-aircraft defense attacked with a force the U-boat could not withstand. The war was to last for two more years, but the U-boat navy could not regain the initiative. When it returned to the offensive, it was firmly mirrored in ULTRA intelligence. Unable to find and strike its main target, it moved cautiously, never freed from the constant fear of surprise attack.
  1. General considerations on the U-boat's' defeat.
    1. Retreat or strategic withdrawal.

      Retreat from the major transatlantic convoy lanes was at all times the indication in German U-boat traffic itself of the U-boats defeat. Strategic withdrawal will be the term used by the Germans, for unlike World War I, it was the German U-boat navy that alone remained as a fighting unit at the conclusion of World War II. Its tenacity and resilience, its determination to revolutionize submarine warfare despite disasters at home will be the subject of later comment.

    2. U-boat navy's time-table.

      Without more knowledge than is now at hand on the preceding periods of the U-boat navy's history in World War II, it is a matter for questions that the U-boat was not only so late in starting its all-out offensive against major convoys, but also not really ready even then. The U-boat building program indicates that the spring offensive of 1943 was merely the


      beginning of the all-out offensive, not the real thing. The unexpected invasion of North Africa, the serious situation on the land fronts, and the astonishing productive capacity of the United States apparently forced the U-boat navy to undertake an offensive before it was really ready. The construction of the U-boats reached its peak a year later, in the winter of 1943-44, when U-boats were being commissioned at the rate of 25 per month. But the types being commissioned were the ones that had already shown themselves inadequate for the U-boat mission. In other words, the German Navy's time-table not only showed what was probably a serious underestimate of U.S. productive capacity but also a slowness in active comprehension of what happened in the Atlantic during the spring of 1943. New high-speed underwater types did not reach significant production figures until the winter of 1944-45. Meanwhile the accumulation of old types had continued to rise, in spite of losses at sea, until the actual number of U-boats, including those in training, had never been larger than it was in the winter of 1943-44. Schnorchel was not ready for all these boats until the fall of 1944--after the Allied invasion of France.

      At this point the general effect of decryption intelligence should be probably be taken into account. Part of the German miscalculations, i.e. those continued after spring, 1943, may have been due to the impossible situation in which the German Navy was placed without even, knowing it--namely, that having its high grade operational radio communications wide open to the enemy. Long after the defeat of spring 1943, the German Navy persisted in believing that "invisibility" might be restored to the standard types. With this restoration the U-boat would again be equal to a battle that was recognized as "hard," and "growing harder." The U-boat of spring 1943 had not been prepared for strength and variety of defenses encountered, but the balance could be restored. At least, the Germans saw no reason why old types could not still hold their own.


    1. Review in terms of merchant shipping and U-boat losses.

      The decisive nature of the U=boats defeat in 1943 can be shown by a breakdown of over-all figures for the war. January 1, 1943 is taken as the point of division, thus giving the U-boat the advantage of its all-out offensive.

        U-boats lost
        M/Vs sunk by U-boat by direct
      Allied action
      at sea
      by other
      Sep 1939-Dec 1942 11,422,350 GRT 139 14
      Jan 1943-May 1945 3,493,571 GRT 556 73
      Total 14,915,921 GRT 695 87

      U-boat success had been achieved at a reasonable cost while the U-boat fleet was still adolescent--the greatest success, of course, (over 6,000,000 GRT) off our own coasts in 1942. But before the small-scale weapon which had accomplished so much in 1942 could be really forged as a large-scale weapon, it was already under Allied control.

    2. World War I compared with World War II.

        MV's sunk by U-boats U-boats
        % U-boats
      World War I 12,191,966 GRT 343 178   52%
      World War II 14,914,921 GRT 1,200 782 (all
        695 (enemy action) 58%


      If the above figures for merchant shipping losses are correct, there is a very marked contrast between the effectiveness of the U-boat in World War I and World War II. While the percentages of U-boats lost are in line with one another, the much greater U-boat effort of World War II did not yield anything like a result comparable to World War I. When these figures are examined together with those in paragraph 3 above, one sees that the U-boats failure in World War II lies in the last 2-1/2 years of the war, precisely during the period when the U-boat navy had reached its greatest potential. Had the U-boat been able to continue its earlier success with figures at all proportional to the increased effort the result for the Allies would have been indeed calamitous.

  1. Pattern of the winter/spring offensive.

    There were two principal types of U-boat operations: Group (wolfpack) operations against major convoys, Individual station patrol, primarily in distant coast areas.

    Supplementary to the above, but in itself of unique and great importance was the U-tanker.

    1. Independent U-boats in distant areas.

      The U-boat attempted to harass the ocean edges simultaneously with the convoy offensive, although the expenditure of effort in distant areas which characterized the fall of 1942 could not be maintained. After some 23 U-boats in these areas during December, the number dropped to 7 in January, but thereafter increased month by month to 18 in May. The active fleet of 740 and 1,200 tonners, however, was too small to maintain a constant pressure comparable to that in the North Atlantic. Furthermore, these distant operations demanded a disproportionate share of U-tanker services.

    2. U-tankers.

      In December 1942, the German Navy had seven 1,600 ton supply submarines, upon which the Atlantic offensive was dependent for its full effect. By March, 2 more had sailed on their first cruises. Even though these refuelers spent half their


      time at sea, U-boat supply was working on a slender margin, for it was apparently intended to maintain 3 refueling stations: one for US-UK convoy submarines, one for US-Gibraltar convoy submarines, and one for Trinidad-Caribbean and South American submarines. As a result, US-UK convoy operations suffered during the early part of the winter. Continuous stations could not be maintained. In January, for example, U-459 (Wilamowitz) had to be sent on a long haul into the South Atlantic to service the few Capetown submarines, while U-boats in the North Atlantic were being forced to drop out of groups and come home for lack of supply facilities. Not until February were continuous stations maintained off the US-UK and US-Gibraltar lanes, and the situation was still pinched. In March 3 of the 4 supply submarines that delivered fuel serviced US-UK convoy submarines. This measure of attention to the North Atlantic continued for the remainder of the spring offensive.

      It was of course recognized at the time that these refueling rendezvous constituted a kind of Achilles heel, but it was months before efficient antisubmarine forces were available to act on decryption intelligence of these meetings at sea.

    1. Group pattern for US-Gibraltar lane. December 1942-April 1943.

      U-boats operating on the US-Gibraltar lane formed a circuit independent of group operations farther north. There was frequent exchange with IXc (740 tons) submarines bound to or returning from middle or south Atlantic coastal areas, but not with the 500 tonners operating on US-UK lanes. Proceeding from Biscay to the Azores areas, U-boats would form for eastbound convoys and sweep out sometimes as far as 40°. After a period of waiting, the group would turn and cruise back along the lane for westbound convoys. Having arrived in the area east of the Azores and north of Madeira, the group would break up along the Canaries and in the Gibraltar approaches for mixed group and individual station patrol tactics. As their final gesture before putting back into Biscay, the U-boats that had not been refueled for further operations would form once more off Portugal to block the UK-Gibraltar lane. Meanwhile replacements would have formed in the Azores area. It was not a very lucrative run, yet U-boat Command gave


      approximately one-third as many submarines to it in the peak months of February and March as he gave to the US-UK lanes. The destruction of oil tankers in the convoy TM 1 in January had apparently impressed High Command with what these operations might achieve for the North African front, but in March Admiral Dönitz admitted that US destroyers could put up very stiff resistance. In April U-boats were withdrawn from the lane.

    1. Group operations against US-UK convoys.

      It has already been suggested that the U-boat navy was not really ready to launch its convoy offensive in the winter of 1942-1943. The shift of emphasis to US-UK lanes was not noticeable in December, when independent operations sank more ships than were sunk by North Atlantic groups. From January through May 1943 the number of U-boats at sea in the Atlantic averaged 166 per month, 179 being the top monthly figure-reached in both March and May. The average number actually operating was 108. The remaining 58 U-boats were either outbound or returning, and, although potentially operative, could not be relied upon for the operating plans at any given moment. Thus only 65% of the submarines at sea might be called immediately available. This figure was again cut down by the demands of other areas, with the result that an average of only 72, or 66% of the operating submarines, was devoted to US-UK lanes in January and again in February, but the number stalled in March, fell off in April, and then reached its highest point (81) in May, when the battle was practically over.

      1. Pattern of the US-UK group formations.

        The major convoy offensive might be described as a pulsation or rhythm: first the convergence of U-boats from port (both Biscay and the Baltic or Norway) as they made for their waiting areas, then the formation of the patrol line followed by the reconnaissance sweep, and lastly contact and the vortex of operation. In practice, of course, the pulsation was irregular, the rhythm broken; at any given time there might be several distinct beats in differing degrees of completion. After the operation came disentanglement and division: some heading directly for port, others to refueling stations whence they


        joined up again with another group or formed the nucleus of a new group or returned home. Forming and reforming, they traced individual patterns and group patterns in a whole whose periodicity was determined by the convoy cycle.

        The group formations had to fulfill two basic requirements: maximum reconnaissance and maximum striking power once contacts had been made. Disposition for maximum reconnaissance reflected the work of German naval intelligence (largely communications intelligence) on convoy movements, but trial and error method was also used against a background of elementary deductions concerning convoy routes, such as preference for the northerly route which could be covered to a greater extent by land based aircraft. With a sufficient number of U-boats lines could be set up near the beginning of the ocean route which would cover the major arc of possible diversions. It was practically impossible for a convoy to pass through a line undetected. To obtain maximum striking power the groups had to be arranged in a pattern that would make encirclement possible--without sacrificing other favorable opportunities.

        Certain of the more important conditioning factors should be mentioned:

        1. Considerations of emphasis as between eastbound and westbound convoys.

        2. Intelligence: specific convoy intelligence, such as that gained from decryption which recommended or necessitated changes in plans. Also, of course, the failures of intelligence represented in successful convoy diversions.

        3. Convoy contacts not planned for, with the resulting changes made for immediate exploitation.

        4. Allied defenses and location devices

        5. Availability of U-boats, their condition, etc.

        6. Availability of U-tankers at right time and place.

        7. Weather: the phenomenally bad winter of 1942-43.


      1. The simple pattern:

        December 1942 to mid-January 1943, consisted of two basic groups, one at either end of the convoy lane, shifting up and down in accordance with anticipated outbound convoy routes. Attention was divided more or less equally between east and west bound convoys by this arrangement, although, as things worked out, more attention was actually given to westbound convoys. An effective modification of this simple pattern was the planned encirclement involving a contact group backed up by a support group, an arrangement which led to the successful operation against convoy ON 154 at the end of December.

      2. Transition:

        Late January. ULTRA intelligence was sufficiently current during the first part of January to be of material assistance in the successful diversion of convoys. U-boat failure to contact expected convoys at either end of the lane forced the initial pattern into a natural evolution. With its accumulation of fresh U-boats the eastern group was lengthened in an attempt to close outbound routes. When the convoys continued to evade the patrol lines, the groups were compelled to go in search of the convoys. By the end of January the long line originally intended for westbound convoys had turned into a westerly fan sweep, beginning about 30W and covering the area from 55° to 60°N. A few days later, U-boats began to sweep the area between 50°N and 55°N in the same manner.

      3. Revised pattern:

        For destruction of eastbound convoys. The transition just described was to end, of course, in the massing of U-boats in the northwestern Atlantic. The revised pattern might be reduced to four primary constituent groups:

          Contact group off Newfoundland.

          Westerly sweep group north of 55°N.

          Westerly sweep group between 50° and 55°N.


          Replacement group, the fresh boats from port heading for and forming up in the area of 55°N - 27°W.

        This drive against the west was designed for eastbound convoys. From the end of January to the conclusion of the spring offensive U-boat group formations were made with eastbound convoys in mind. Westbound convoys were attacked only as intelligence made these targets irresistible or as opportunity supposedly permitted within the framework of the offensive against eastbound convoys. Whether or not this shift of emphasis can be attributed to the aggressive Admiral Dönitz, who became Commander in Chief at this time, it seems evident that the German Navy could no longer afford to be primarily interested in the sinking of ships as such. It was the cargo of war supplies that had to be destroyed. By January 1943, Allied ship construction had passed total losses by U-boats.

        The new pattern was highly flexible and was arranged not only to block every lane north of 50°N by the staggered sweep groups but also to make certain that once contact had been made every U-boat could be brought to bear. An increased number of U-boats was required. As against 12 new U-boats released from the north for Atlantic operations in January, 23 were released in February. U-boat losses lagged behind the accessions. To keep the requisite number at sea, continuous refueling stations were necessary, for which U-boats might leave their place in line only when down to the very minimum of fuel. As COMSUBs explained to group Haudegen on 3 February:

        Seizing of the convoys running at this time on very scattered routes is possible only with a large number of submarines.
        (1733/3 February 1943)

      1. March 1943: the pattern works.

        As a result of unexpected operations in February on westbound convoys, the revised pattern for eastbound convoys was not restored until March, when it proved its worth. More ships were sunk in convoy during March than in any other


        month of the entire war. The difference did not lie in the number of U-boats, for as many had been there in February, but rather in the deployment of U-boats. First they were in their positions according to the pattern described above. Second, contacts were made on eastbound convoys as planned. The result was total encirclement of the convoys. The groups closed in from every direction; the convoy defense was hopelessly outnumbered. Even so, U-boat successes in March--or rather in the first three weeks of March--must be qualified. More than twice as many ships were convoyed in March as in February.

      1. Break.

        The policy of massing U-boat groups in the northwestern Atlantic continued through April and May, but something had gone wrong. The precision of March could not be attained. There was evidence of increasing haste and uncertainty on the part of U-boat Command. Operations were confused by attempts to exploit contacts on westbound convoys near the end of their run. At the same time U-boats were ordered to pursue all contacts, even the most unfavorable, on eastbound convoys. Command could not concentrate his U-boats where and when needed. The arrangement of the group lines became more complex and less effective. In short, the pattern was breaking up.

  1. Examination of the U-boat's defeat.

    1. ULTRA intelligence and convoy diversions.

      During the winter and spring offensive of 1942-43 Allied radio intelligence was an imperfect instrument. It was of little or no immediate use in December 1942, but by the first part of January decryption was holding its own. By a combination of breaking into the present from time to time and the continual filling in of the past gaps one began to see how the U-boats worked, how their lines were set up, and what patterns were employed. Whether running even with them, a little ahead, or lagging behind, the process of piecing the whole thing together never stopped. It was like trying to watch the U-boats through


      a condition of changing visibility. At intervals the clouds would open and one could see the present situation, but not-completely, for much of the present U-boat situation could be seen only in the immediate past. Unlike the analogy of visibility and cloudiness, however, the record of the immediate past was there, waiting to be read. The present might fade out but the past would open up and complete the picture of what had just been seen. And even though the present was blocked from view the past record would contain many elements known to form a part of the new situation. And thus, working back and forth, combining indirect and direct views, even at worst one had some kind of picture more accurate than any other in Allied possession, particularly when supplemented by D/F and traffic analysis. Since ULTRA intelligence was not available for current use in December, it is of interest to note that every planned group operation met with at least the first condition of success, contact on the expected convoy. On the other hand, the scale of the German effort was small and the weather was bad. The largest number of submarines brought to bear on anyone convoy was 22. (Groups Ungestüm and Spitz; ON 154).

      Despite the enemy's increased effort in January and February U-boat successes on the convoy lanes did not measure up. The winter offensive was not succeeding. Ten U-boat groups or wolfpacks operated against US-UK convoys during January and February. Of the 16 convoy "expectations" for which the line were formed only 4 led to contact, and one of these 4 was probably a case of last minute German communication intelligence on the movement of a westbound convoy, ON 166. It was also the heaviest single convoy loss during this period. There were 6 additional contacts, not "expected" in U-boat traffic. Of these 6, 4 were to some extent accidental and it is not clear that radio intelligence could have prevented them. The remaining two are cases of radio intelligence failure, in which attempted evasion of one group sent the convoys to other U-boats whose movements were not currently known.

      In March, the successful U-boat month, there were only 4 planned operations. Two of these expectations accounted for the greater part of the losses. Both are cases of radio intelligence failure, although it is not certain that one of them could have been prevented under the circumstances. The combined attack on SC 122 and HX 229, like the case of ON 166,


      was made possible by last minute shifts in U-boat dispositions, presumably the result of German communications intelligence.

    1. Convoy defenses.

      Effective convoy diversion continued in April and May, but even with contact U-boats could not mass for assault or maneuver into attack positions. The increase in convoy defenses which followed the shipping losses of March was the first step in the offensive sweep against U-boats. Joint surface aircraft escort deprived the U-boat of his mobility. If he came up on the surface to look around or get ahead--aircraft put him down. Once down, the water rumbled with explosions, which U-boat Command interpreted as bluff depth charges, part of the Allied propaganda--campaign against U-boat morale. U-boat crews and Commanders became frightened and lost their "healthy hunter instincts," despite Admiral Dönitz's exhortations and threats.

    2. Loss of experienced U-boat commanders and crews.

      The U-boat arm had already lost many of its "ace" commanders. The effort to step up the North Atlantic offensive in the spring of 1943 had necessarily bought new and inexperienced men into the most difficult battle areas. It is not possible to weigh the personnel factor in the U-boat defeat but its importance can be indicated by the following figures:

      During 1943, 10% of the operating U-boats accounted for 50% of the merchant shipping sunk; 40% of the operating U-boats accounted for all the merchant shipping sunk.

      But even the skilled commanders were subject to surprise attack from the air.


    1. Surprise attacks from the air. Visibility and vulnerability.

      As is pointed out in more detail in Chapter IV of Volume II, two things stand out in the course of the U-boats defeat both of them plainly marked in U-boat traffic:

        Inability of the U-boat to cope with surprise attack from the air;

        Increasing intimidation of U-boat men as they began to appreciate the visibility, and vulnerability of their weapon.

      1. U-boat Command's refusal to admit defeat.

        U-boat Command did not anticipate the rapid expansion of Allied convoy defenses, hence he did not readily adjust himself to the facts reported from sea. A review of his reactions during the course of the winter/spring offensive shows that his concentration on the one end--attack--relegated the difficulties encountered by the U-boats to a subordinate place where they could be handled by expedients or by psychology. In December 1942 U-boats were told to pay no attention to their own search receivers. Allied radar was ineffective. U-boats were to rely upon themselves and think only of attack. In January convoy evasion raised the fear of Allied location devices. It was not a combat of fear, but a fear that the Allies were escaping combat by means of radar. In February the SC 118 operation gave Command cause for worry. Results were not up to expectations and it was clear that very long range aircraft had been effective. His reaction, however, had been "further reports about aircraft are superfluous." If the U-boat persisted it could wear the escort out and then sink the fat merchantmen. Worries were put to one side as U-boats reported their successes against ON 166. The operation was called a "run of the mill success," but that was not true. Contact in the first place had been a matter of good fortune and the convoy had been pursued and attacked for days because it was beyond the range of land based aircraft. The appearance of aircraft carriers with convoys in March seems to have made little impression on U-boat Command. By April, however, the


        combined effect of increased defenses was evident in Command's renewal of the doctrine that the U-boat must surprise if it is to succeed. Chances against convoys were narrowed down to the first night's attack. Claiming that the "aircraft situation is known here," Command saw no reason why the U-boats could not maneuver outside the range of the close aircraft escort (30-40 miles). If they were caught coming in, they could attack submerged. Encouraged by the brief success against ONS 5 in May, U-boat Command tried to dismiss aircraft by telling U-boats to stay on the surface and fire their inadequate anti-aircraft guns, "...then the plane will soon stop attacking." But trapped and defenseless, subject to surprise visitations in the difficult task of maneuvering, U-boats were less bold in pressing an attack which was sure to reveal their presence.

      1. U-boat Command accounts for the defeat.

        U-boat losses did not become alarming until May and then they went up with suddenness for which the Germans were totally unprepared. When U-boats began to disappear at the rate of 1-1/3 per day, Command yielded the area in which he had already lost the initiative. He correctly judged (19 May) that "the enemy has once more gained a few lengths on us in his effort to deprive the U-boat of its most important attribute, invisibility." Allied successes were attributed to the surprise attacks from the air which radar made possible on the outbound and returning U-boat lanes as well as on the convoy lanes. The time was to be passed in "cunning caution" until the U-boat regained its invisibility.


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