Chapter VI
Growth, Significance and Decline of the U-boat Refueling Fleet

  1. Significance of refueler sinkings, summer and fall 1943.
    1. Sharp increases in rate of loss beginning June 1943.
    2. Permanent character of losses sustained.
    3. Importance of refueler losses to U-boat war as a whole.

  2. Review of refueling problem in early stage of war.
    1. No U-boat refuelers active before spring 1942; Brief cruises without refueling; Surface refuelers.
    2. Operations off U.S. eastern seaboard before appearance of U-tankers, winter and spring 1942.

  3. Growth of refueling fleet, May 1942 to June 1943.
    1. Operations in Caribbean and South Atlantic made possible by introduction of refuelers, spring and summer 1942.
    2. Refueling at peak; North Atlantic Offensive 1942-1943.
    3. Refueling facilities strained even with tanker fleet at top strength.
    4. Precautions for guarding refuelers.
    5. Up to June 1943, Command's policy on the whole successful in protecting refueling rendezvous.

  4. Decline of refueling fleet, from June 1943.
    1. Offensive resources at disposal of Allies, summer of 1943; radio intelligence and CVE task groups.
    2. Reason for failure to use radio intelligence in attacking rendezvous before June 1943.
    3. Increased number of CVE's available for anti-submarine duties.
    4. Effectiveness of radio intelligence despite lag in decryption.
    5. Relative importance of CVE and land-based aircraft in attack on refuelers.
    6. Summary of elements combining to reduce refueling fleet.
    7. Increased precautions by Command.
    8. Fruitlessness of attempts to save refueling systems.
    9. Effects of destruction.


  1. Significance of refueler sinkings, summer and fall 1943.

    1. Sharp increases in rate of loss beginning June 1943.

      The loss of Czygan on 12 June 1943 marked the beginning of a series of disasters to U-boat Command's refueling fleet whose significance in the history of the U-boat war it is difficult to overstate. A study of Table I will show how sharply the rate of refueler sinkings increase with the assignment in June 1943 of the carrier task groups to offensive operations. Prior to that time, in the thirteen months that had passed since the first refuelers began operating, the Germans had put 12 tankers into the Atlantic, of which three had been lost. During this time, in other words, Command had built up his U-tanker fleet from nothing to an effective total of nine. The contrast with the next seven months is striking. In the period from June to December 1943, while three refuelers were added to the operating fleet, no fewer than ten were sunk, for a net loss of seven. This left only two refuelers at the end of the year.

    2. Permanent character of losses sustained.

      The fleet of refueling boats was never to recover from this reduction. The Germans added two more in May 1944, but in turn lost three in the four months April to July of that year. This left the Atlantic fleet completely stripped, with U-219 in the Far East the sole survivor of seventeen refuelers launched since the spring of 1942.

    3. Importance of refueler losses to U-boat war as a whole.

      A brief glance at the history of the refueling fleet will suggest the decisive importance of these losses in the decline of the U-boats effectiveness. The fleet was entirely the creation of the war years. Its period of growth, from April 1942 to March 1945, marks the most active and successful phase of the U-boat war. Its period of decline, beginning in June 1943, marks the contraction of the U-boats striking power.


Table 1
U-boat Refueling Fleet
Monthly Additions and Losses
Month Total No. Previously Complete No. added to Fleet during Month No. Sunk During Month No. Sunk as of last day of Month
(Running Total)
Total No. Avail
for Ops at
End of Month
April 0 12 0 0 2
May 2 0 0 0 2
June 2 1 0 0 3
July 3 1 0 0 4
August 4 2 1 1 5
September 6 2 0 1 7
October 8 1 0 1 8
November 9 0 0 1 8
December 9 0 21 2 7
January 9 0 0 2 7
February 9 1 0 2 8
March 10 1 0 2 9
April 11 0 0 2 9
May 11 1 1 3 9
June 12 0 2 5 7
July 12 1 4 9 4
August 13 0 2 11 2
September 13 1 0 11 3
October 14 1 2 13 2
November 15 0 0 13 2
December 15 0 0 13 2
April 15 0 1 14 1
May 15 2 0 14 3
June 17 0 1 15 2
July 17 0 1 16 1


      The connection is, of course, partly fortuitous. It was not until the spring of 1943 that the resources of the Allies in the Battle of the Atlantic began to overtake those of the Germans; as more U-boats of all types were sunk, it was inevitable that an increasing number of refuelers should be among them. But there is also a causal connection. In concentrating on refueling rendezvous in the summer of 1943, Allied air power was in truth striking at the heart of Dönitz's system. For without refuelers, neither prolonged independent cruises nor extended convoy operations were possible to the 500 and 740 tonners that made up the majority of the U-boat fleet. By the end of 1943 the U-boat had been in large measure reduced to meager ranges that had limited its effectiveness at the beginning of the war.

  1. Review of refueling problem in early stage of war.

    1. No U-boat refuelers active before spring 1942; Brief cruises without refueling; Surface refuelers.

      The construction of the refueling fleet probably signals the increasing ascendancy of Admiral Dönitz, with his grandiose ideas for the role of the U-boat, in the war councils of Nazi Germany. At the beginning of the war the active U-boats consisted mostly of 250 and 300 tonners. These operated in relatively near waters, chiefly the North Sea and Western approaches to the British Isles. As late as September 1940, even after the conquest of France had made the Bay of Biscay ports available to U-boat Command, it was considered a new departure that U-boats were operating as far west as 20°W. This restriction of operating areas permitted brief war cruises, which could be accomplished even by small U-boats without refueling. By the early part of 1941, the standard 500 ton type VIIc submarine had begun to appear in some numbers, bringing with it an increase in cruising range. In the spring of that year, as the area invaded by the U-boat was extended west of 40°W, the Germans began to use surface supply ships for refueling. This must, however, have been viewed as a temporary expedient, even at this period, when British naval power in the Atlantic was inadequate to its tasks. Surface refueling in Atlantic waters was apparently abandoned


      sometime early in 1942,4 after several supply ships had been sunk. (Accurate information is not available because of the failure to read traffic) Meantime the first underwater refuelers, U-116 and U-459, had been laid down (probably in the fall of 1940) and commissioned in the fall of 1941. They were ready for their first war cruise in the spring of 1942, and a new phase of the U-boat war began.

    1. Operations off U.S. eastern seaboard before appearance of U-tankers, winter and spring 1942.

      It is true that as early as January 1942 - several months before the first U-tanker was ready for active service - German U-boats had been operating in considerable numbers and with great success off the eastern seaboard of the U.S. Since U-boat radio traffic for that period was not read, details are lacking as to the manner in which the fuel problem was handled during these operations. Possibly surface refuelers were still being used to some extent. Prisoner of War reports give some instances of fuel picked up in the mid-Atlantic by American bound boats from other combat U-boats, the latter presumably assigned to less distant areas. Even this assistance was probably unnecessary in most cases, in the special conditions then prevailing. The feeble state of American anti-submarine defenses at the time, which permitted U-boats to lie at their ease on coastal shipping lanes waiting for traffic, made fuel consumption vastly less than in later periods when stronger defense required of the U-boats extensive detours, maneuvers of evasion and costly pursuit of traffic sighted from a distance. The plentiful and easy targets available off the U.S. coast in early 1942 would often allow a U-boat Commander to expend his torpedoes with profit in an extremely short operating period, where as later, held off by aircraft and surface forces, he might have to search for weeks before even making contact with a possible target. Thus for roughly the first half of 1942, it was possible for 740 and even 500 ton U-boats to operate successfully on the farther side of the Atlantic without any regular provision for refueling.


  1. Growth of refueling fleet, May 1942 to June 1943.

    1. Operations in Caribbean and South Atlantic made possible by introduction of refuelers, spring and summer 1942.

      As American coastal defenses began to tighten, U-boat Command fenced for weak spots and stretched Allied resources by extending the operation of his boats to yet more distant waters. This could not be accomplished without refueling. It is no accident that May 1942 is the first month in which U-boats operated to an important extent in the Caribbean and entered the Gulf of Mexico; for (according to prisoner of war reports) the task assigned to U-459 on her first cruise, March to May 1942, was to refuel U-boats on the outskirts of this area. Further evidence of the work of the refuelers that now began to be added with some regularity to the operating fleet, is seen in the increased number of U-boats active in the South Atlantic and the increase in tonnage sunk there through the summer and fall of 1942. By the end of 1942, the U-boat, once confined to a small area around the British Isles and Norway, had taken the entire Atlantic as its hunting ground.

    2. Refueling at peak; North Atlantic Offensive 1942-1943.

      During the great winter and spring offensive of 1942-1943, the refueling fleet reached its peak in numbers and played its most important role. In addition to the heavy assault on the North Atlantic convoy lanes, the Germans kept U-boat groups active in the Gibraltar lane and continued to send independent U-boats to operate in distant areas. This ambitious program was only possible because of the rapid expansion and slight losses of the refueler fleet since the spring of 1943. By February 1943, the first two series of refuelers had been completed, and all units in the series, except for the two that had been sunk, were in operation. (see Table II) The first series completed, U-459 to U-464, consisted of six 1600 ton type XIV supply boats; the second, U-116 to U-119, of four 1600 ton type Xb combined minelayers and supply boats. In March 1943, the first boat of a second series of type XIV (U-487 to U-490) began operating bringing the total of active


      refuelers to nine, a number that was to be maintained (with one loss and one replacement in May), until a precipitous downward curve began with the sinking of Czygan (U-118) in June.

    1. Refueling facilities strained even with tanker fleet at top strength.

      Even with the supply fleet at its top strength, however, Command found his refueling resources strained to the utmost in the extensive operations of December 1942 to May 1943. A brief analysis of the distribution of available U-tankers over various areas of operations has already been given. It has been shown that as the scale of North Atlantic operations was stepped up in the late winter and spring of 1943 the proportion of refuelers devoted to the North Atlantic had to be increased at the expense of the other areas. During this period three or four refuelers were normally in active operation at a given time, the others being either in port for servicing or enroute to or from their assigned stations at sea. In December 1942, only one of the four tankers that delivered fuel served the North Atlantic groups, the others being assigned to U-boats operating in the South Atlantic, the Trinidad-Brazil area, and the Gibraltar lane. Roughly the same proportion continued in January and February 1943. During these months there were instances of U-boat groups being disbanded and being called back to port because of inadequate facilities. In March (up to then the month of heaviest action in the North Atlantic), three of the four refuelers that delivered fuel operated in the North Atlantic, though obviously demanded by the strategic situation, necessarily meant a further reduction in the numbers and duration of independent cruises to other areas. Even so, the margin in the North Atlantic itself remained too close for comfort. Quite apart from other sources of strain, the scale of operations undertaken by Command could hardly be maintained with the refueling facilities available to him, favorable as these were compared to what they had been a year before and to be a year later.


Table 2
U-boat Refuels by Series
Series U-boat Commander Month Commissioned Month of
First Op
Month Sunk
Series 116-119
Type Xb
116 Schmidt Oct 41 Spring 42 Dec 42
117 Neumann Sep 42 Oct 42 Aug 43
118 Czygan Dec 41 Sep 42 Jun 43
119 Zech, Kamecke5 Apr 42 Feb 43 Jun 43
Series 219-220 Type Xb 219 Burghagen Sep 42 May 43 Active
220 Barber Apr 43 Sep 43 Oct 43
Series 233-234
Type Xb
233 Steen Sep 43 May 44 Jul 44
234 Fehler May 44 Apr 44 Active
Series 459-464
Type XIV
459 Wilamowitz-Moellendorf Nov 41 Apr 42 Jul 43
460 Schnoor Nov 41 Jul 42 Oct 43
461 Stiebler Jan 42 Jun 42 Jul 43
462 Vowe Mar 42 Sep 42 Jul 43
463 Wolfbauer Mar 42 Aug 42 May 43
464 Harms Apr 42 Aug 42 Aug 42
Series 487-490
Type XIV
487 Metz Dec 42 Mar 43 Jul 43
488 Bartke, Studt5 Oct 41 May 43 Apr 44
489 Schmandt Mar 43 Jul 43 Aug 43
490 Gerlach Mar 43 May 44 Jun 44

Note on U-boat types:

Type Xb Combined minelayer and supply boat.
2 torpedo tubes. Fuel capacity approximately 426 cbm.
Type XIV For supply purposes only.
No torpedo tubes and no mines carried.
Fuel capacity approximately 740 cbm.


      The sinkings of the summer of 1943 take on added significance when one realizes on how slim a margin Command had been operating before that time even with nine out of eleven refuelers launched still in operation. It seems clear that U-tankers still under construction or in training were regarded primarily as needed additions to the fleet, not as replacements to meet an anticipated increase in losses.

    1. Precautions for guarding refuelers.

      Refuelers were presumably much more costly to construct than combat U-boats, and in 1943, with building resources and facilities strained, the rate of new construction and commissioning for refuelers did not rise with that for combat fleet as a whole. Command had evidently committed himself to the idea of a relatively small refueling fleet which would be protected with infinite care. The strictest precaution surrounded the operation of the refuelers. References in radio traffic to refueling rendezvous positions were made as secret as possible by the use of Offizier messages or enciphered grid squares or both. The positions themselves were carefully chosen, being placed in mid-ocean away from known convoy routes6 and beyond the range of shore-based aircraft. Radio silence was enjoined on all U-boats approaching or participating in a refueling rendezvous. Combat U-boats were to regard themselves as expendable in protecting the refueler and covering his dive in event of surprise attack.

    2. Up to June 1943, Command's policy on the whole successful in protecting refueling rendezvous.

      These precautions must have seemed to Command, at the end of the first year of refueler activity, to have achieved their desired effect. During the heavy convoy operations of the winter and spring of 1942-1943, numerous refueling rendezvous had been successfully executed without the loss of a single supply U-boat. Before the sinking of Czygan (U-118), in fact, there is no evidence that a supply boat had been lost at a rendezvous in any area. The manner of U-116's sinking is not known; both U-464 (in August 1942) and U-463 (in May


      1943) had been found and sunk by aircraft while on passage from port to their area of operations. Command must therefore have begun his extensive program of refueling rendezvous in the Middle Atlantic in the summer of 1943 with some confidence.

  1. Decline of refueling fleet, from June 1943.

    1. Offensive resources at disposal of Allies, summer of 1943; radio intelligence and CVE task groups.

      Command's confidence was in fact baseless because of two offensive resources at the disposal of the Allies of which the first, decryption intelligence, was evidently not even suspected by Command, and of which the second, the increasing activity of Allied air power in general and of the CVE task groups in particular, he grossly underestimated. These two resources were in fact interdependent in operation, for the first guided the second, the second implemented the first. Circumstances combined to make their joint effectiveness in the summer of 1943 far greater than it could have been at any previous period.

    2. Reason for failure to use radio intelligence in attacking rendezvous before June 1943.

      1. Inadequate forces.

        It is obvious that radio intelligence is without offensive value unless there are adequate forces available to operate on its findings. From the point of view of refueler sinkings, it seems at first sight a paradox that current reading of U-boat traffic was actually less consistent during the summer than during the winter and spring of 1943. In the earlier period, the times and positions of a large number of refueling rendezvous' were known well in advance by Anglo-American radio intelligence. Yet, as far as it is known, no rendezvous was ever attacked during this period on the basis of the intelligence available.7 The main reason is not far to seek. All available


        U.S.-British forces were required for essentially defensive purposes. The pressure of the U-boat offensive on convoy routes was constant and heavy, and the naval forces of the Allies were barely sufficient to bring the necessary tonnage through. In addition, blockade runners were trying to get through the Atlantic in the early months of 1943. The few carriers in service in the Atlantic were needed for escort duty or surface blockade runner patrol.8

      1. Possible fear of compromising Allied radio intelligence.

        It is possible that fear of compromising the fact of Allied decryption of U-boat traffic, at a period when the most valuable use decryption intelligence was in the diversion of convoys away from known U-boat concentrations, played some part in the decision not to attack refueling rendezvous before June 1943. But this consideration, if it had certain influence, was probably secondary to the simple fact of inadequate forces.

    1. Increased number of CVE's available for antisubmarine duties.

      By June 1943, the pressure on the convoy routes had been greatly reduced, blockade runners had disappeared from the Atlantic until late December, and the number of escort carriers available for anti-submarine operations had substantially increased. As against one CV and one CVE, both presumably inexperienced, assigned to anti-submarine duties in the Atlantic in the spring of 1943, no less than five CVE's, growing constantly more expert, were in action during the summer and fall. For the first time it was possible to deploy CVE task groups not merely to take advantage of current intelligence, when available, on the U-boat rendezvous',


    1. Effectiveness of radio intelligence despite lag in decryption.

      The fact that the reading of U-boat traffic was irregular during the summer of 1943, though on certain occasions it prevented valuable targets from being located, did not have as great an effect in reducing the value of available intelligence as might be expected. In the first place, refueling rendezvous' were normally announced in traffic some days - sometimes as much as two weeks - ahead of time. Thus decryption of traffic could be far from current and still give precise information on a rendezvous in time to organize an attack against it. In the second place, even where precise information was not available, more general knowledge based primarily on radio intelligence - the identification of certain U-boats as refuelers, for example, and there determination of favorite areas for refueling rendezvous' - permitted imperfect data (such as a refueler's heading point) to be intercepted with sufficient accuracy to warrant the assignment of a task group. In the following sections the contribution of radio intelligence to various sinkings of refuelers by CVE groups will be examined. It will be seen that while luck and above all the - initiative of the task group played a great part, the influence of radio intelligence, direct and indirect, had a decisive effect on the great majority of the operations.

    2. Relative importance of CVE and land-based aircraft in attack on refuelers.

      To the combination of radio intelligence and CVE task groups must be added the increasing number and effectiveness of shore-based aircraft assigned to anti-submarine operations in the summer of 1943. In July and August no fewer than four refuelers were sunk by land-based aircraft, three (U-459, 461, and 462) in the Biscay area and one (U-489) midway between Scotland and Iceland. All were on passage from or into port. These sinkings, like that of U-119 by surface units on 24 June, were the result of intensified patrols, part of the Biscay Offensive, instituted in the early summer of 1943. Land-based aircraft, however, could not reach the mid-ocean locations of refueling rendezvous'. For this mobility of the carrier task groups were required. In the twelve months beginning in June 1943, CVE groups sank seven, land-based


      aircraft four, and surface ships unassisted by aircraft two, of the total thirteen refuelers destroyed. The relative importance of the CVE's contribution, however, is not adequately represented by these figures, for most sinkings by CVE groups were accomplished at or near the rendezvous', and were accompanied by the sinking or damaging of other U-boats beside the refueler.

    1. Summary of elements combining to reduce refueling fleet.

      In the effectiveness of the attacks made by both land based and carrier aircraft, the improved Allied radar of which Admiral Dönitz was so, painfully conscious doubtless played a decisive part. Further, the tactics and techniques of bombing and strafing attacks on U-boats by one or more planes were being constantly studied and improved. It must be observed, however, that the success of the CVE task groups did not depend solely on aircraft. The task group remained a naval unit. The destroyers that accompanied the carrier had an important role, making underwater searches, and in several instances finishing off the kills of U-boats that had first been sighted or damaged from the air. When the history of the antisubmarine tactics of carrier task groups comes to written by those who participated in them, a story of increasingly expert cooperation between the surface and the air will undoubtedly be revealed. It was, then, the offensive use of radio intelligence, the increased numbers and the perfected technique and teamwork of carrier task groups, and the greater effectiveness through the improved radar and extended ranges of land-based aircraft that accounted for the destruction of the German refueling fleet in the year beginning in June 1943.

    2. Increased precautions by Command.

      Surprised by the force and skill put into action against him, German Command was never able to devise effective countermeasures. U-boat radio traffic for the summer and fall of 1943 is full of supplements and revisions to the precautionary rules already in effect for the protection of refuelers. For example, prior to 5 June, though radio silence had been required of U-boats participating in a refueling


      rendezvous, combat U-boats had as a matter of routine been guided to the refueler by the latter's beacon signals. In 2306/5 June this procedure is prohibited, combat boats being ordered not to request beacon signals until after two days unsuccessful search for the supplier. In September and October 1943, after the disastrous losses of the summer, Command broadcast orders intended to increase the protection afforded the refueler by fighting boats in the event of air attack (1202/16 September 1943, 1640/7 October 1943); and on 11 October issued elaborate instructions designed to keep rendezvous' from being discovered (supply boats and boats to be supplied to proceed submerged on day of rendezvous, to make contact two hours before sunset, to carry out refueling in general only at night, taking particular care with lights. Current Order Number 24, 1035/11 October 1943).

    1. Fruitlessness of attempts to save refueling systems.

      These belated attempts to salvage the refueling system were ineffectual. Not only could they not bring back the eight refuelers already sunk between June and September 1943, they could not prevent further reduction of the dwindling fleet. In October 1943, the month of two newly issued orders, two supply U-boats (U-220 and U-460) were sunk by carrier aircraft at refueling rendezvous'. After this, Command's refueling fleet was in effect finished. It remained only to sweep up the fragments.

    2. Effects of destruction.

      1. Shrinking of independent operations from June 1943 on.

        The whittling down of the refueling fleet is, of course, only one aspect of the reduction of the U-boat menace in general during the period June 1943 to June 1944. Its particular effects can none the less be observed. Throughout the German reverses of 1944-45 COMSUBs tried valiantly to keep the U-boat arm in effective operation, introducing new types and new devices, and actually increasing the number of active U-boats in the months from January to April 1945. In spite of this stubbornness, the areas threatened by U-boat attack continued to shrink. During the summer of 1944 (after


        D-Day, June 6), a total of only five U-boats - apart from the 1200 ton cruisers enroute to the Far East - were sent out to distant operational areas. All were 750 ton IXc boats, four operated off the American coast, and one in the Caribbean. This is to be contrasted with the forty-four independent cruises to distant areas in the South Atlantic in the summer of 1943. In the winter of 1944-45, an occasional 750 tonner continued to operate off Nova Scotia, Newfoundland, and the Northeastern States. But the scale of these operations was too slight to have more than nuisance value; and there was no U-boat activity whatever south of 40°N. The total absence of refuelers was undoubtedly decisive not only in restricting these operations to 750 ton U-boats but in limiting the range and duration of the distant cruises that were attempted.

      1. Virtual disappearance of group operations after March 1944.

        Further, though by end of 1943 the state of Allied antisubmarine defense had doubtless made the resumption of group warfare against convoys unprofitable, the lack of refuelers made it all but impossible after the spring of 1944 (Group Preussen, March 1944, was the last to operate).9 As the winter of 1942-43 had demonstrated, vigorous and sustained convoy operations were absolutely dependent on refueling at sea.

      2. Disruption of Command's operational plans caused by loss of individual refuelers.

        From the long term point of view, the physical destruction of the supply fleet is the most important single result of the anti-submarine operations of June to October 1943. From the point of view of week to week U-boat activity, however, the havoc wrought in Command's plans by the loss of individual refuelers - requiring the curtailment of planned cruises to distant areas, the conversion of combat U-boats to emergency refuelers, the reorganization of orders for whole groups of boats - had an equally depressing effect on the


        conduct of the U-boat war. In the sections that follow the immediate effects of certain refueler sinkings on U-boat situation as a whole will be studied in some detail.


Table of Contents ** Previous Chapter (5) * Next Chapter (7)


1. For the sake of convenience, U-116 and U-464, the two first refuelers completed, are considered to have begun operations at approximately the same time. The actual date of U-116's first war cruise is not known, but must have been during the spring of 1942.

2. The loss of U-116 is arbitrarily assigned to December on the basis of unconfirmed statements by a prisoner of war from another U-boat, though it is not referred to in traffic read for November and December.

3. In 1944 all sinkings and all additions to the refueling fleet happened to occur in the 4 months listed. Therefore, only figures for these months are given. In April, one supply U-boat (U-234) the last known to have been completed, began its first war cruise, destined for Batavia. On the cruise, however, U-234 was to transport cargo and personnel to the Far East. U-234 has thus never acted as a refueler, and from the point of view of operations does not belong in this table.

4. In the Indian Ocean surface tankers were used for refueling U-boats until the sinking of Brake and Schiemann in February and March 1944.

5. Commander at time of sinking.

6. In some instances, owing to carelessness or ignorance, the Germans failed to follow this condition.

7. There is a possible case in January 1943. At least, Offizier 1108/7 January 1943 states that "an English unit was detailed on 3 January to patrol Neumann's last provisioning area."

8. The USS Santee (CVE 29), occupied with blockade runner patrol in the winter and spring, was assigned to antisubmarine operations in July 1943.

9. An eleventh hour attempt to reintroduce convoy warfare in a limited form was made in April 1945 with Group Seewolf, consisting of six 750 ton U-boats.

Transcribed by Chuck Roberts for the Hyperwar foundation