Chapter II
The U-boat and Allied Naval Communication Intelligence

  1. The U-boats Mission. Nature of the U-boat war.
  2. The U-boat.

    1. Its secret strength: "Invisibility"

    2. The U-boats weakness.
  3. Communication Intelligence and Perspective on U-boat war from beginning of 1943 to end of war.

  4. The contribution of decryption intelligence to the U-boats loss of invisibility.

    1. Record of decryption
    2. Weakness of U-boat operating system which increased value of decryption intelligence: Operational bookkeeping by radio.
  5. Uses of decryption intelligence in the Battle of the Atlantic.

    1. Accumulation of information on U-boat navy.
    2. The protection of Allied convoys and shipping.
    3. Offensive use.
    4. Allied decryption intelligence as countermeasure.
    5. Technical intelligence.


1. The U-boats mission. Nature of the U-boat war.

The German Navy defined the U-boat as a "torpedo attack underwater boat"; its target, enemy merchant shipping. The continuous and successful use of the U-boat against this target is ". . . in the long run of strategically decisive significance for the outcome of the war, since the destruction of her sea communications means the loss of the war for a nation dependent upon sea-trade." In theory the U-boats peculiar properties enabled it to carry out this mission despite the enemy's control of the seas. Once well launched under Admiral Dönitz, the U-boat navy was never permitted to forget her basic offensive mission nor its significance for the outcome of the war.

The peculiar nature of the U-boat war, or the Battle of the Atlantic, does not lend itself readily to the stirring descriptions which may mark the history of a series of major fleet engagements. It is not an impressive succession of majors but an attrition of minors. To the Allies this battle may have been inglorious, for it was a battle to deliver supplies. When seen as a whole, however, these minor engagements add up to a major battle upon whose outcome week by week the future prosecution of the war abroad depended. Whether striking or hiding, however the U-boat turned to or from attack it always threatened and exacted countless hours of watch and patrol.

An engagement which goes on so long, is so devoid of spectators and correspondents, and is so far to the rear of the battle lines is apt to recede in memory with the passage of time, for it lacks the classical unities of the drama, being neither one in place nor in time nor in action. To the German Navy, in contrast, it may have borne a rather different, more romantic face. The Battle of the Atlantic was a front, "The Uboat front," and was recognized as the first line of battle, at first carrying the offensive to the Allies and then in defending the continental fronts. The U-boat man was a hero, in name at least, and the conduct of the war by U-boat Command made full use of colorful imagination, as in naming U-boat groups "Jaguar," "Seawolf," "Searobber," "Eagle", "Lionheart," . . . etc.


Mr. Churchill managed to convey something of this in reverse to the Allied public in his description of the Nazi beast of the sea stealing out from its caves on the Biscay coast or with drawing to lick its wounds.

2.The U-boat.

  1. Its secret strength: "Invisibility."

    In stressing the strategic significance of the U-boat weapon itself, namely, to destroy the sea communications of an enemy dependent on merchant shipping, the U-boat Commander's Handbook stressed the following basic principles:

    1. "...the characteristic property and greatest strength of the U-boat is its invisibility." This comes from its ability to submerge. "Invisibility" gives the U-boat its own advantage in combat: surprise. "invisibility" determines the entire theory of U-boat tactics, even in defense, which depends on escape underwater.

    2. To keep its secret strength, the U-boat must "not be seen, not heard, not be located (geortet)" before or during an attack. If the U-boat is located by the enemy, " loses almost all prospect of success."

    The ULTRA history of the U-boat war is a commentary on this section of the Commander's Handbook, as part of the U-boats strength in surprise, was the intelligence of convoys which guided the wolfpacks to the right place. Insofar as the U-boat anticipated the possibility of location before attack it began to add various counter-location devices which characterized the German U-boat in World War II. As the possibility of location turned into a probability, the morale of the Commander was affected and the entire U-boat navy became engaged in developing one counter-device after another--either to restore "invisibility" and "surprise" or to enable the U-boat to defend itself.


  1. The U-boats weakness.

    The U-boat lost its strength upon being located because it was an offensive weapon incapable of surviving a defensive role in combat. Extremely vulnerable, its only defense was again its source of strength, namely invisibility. It could disappear and hide. Going down was a matter of seconds, but once there it was sluggish--and its time submerged limited. For it was not truly an underwater boat.

    The German Navy knew of the weaknesses of the U-boat. The handbook of 1942 listed and discussed them.

    Low speed (principal weakness) which, results from the necessity, for sake of "invisibility," of two kinds of propulsion, thus doubling the machine installation and resulting in a corresponding reduction in the performance of either one singly. This comparatively low speed " of fundamental significance for the U-boats tactical use."

    Limited Underwater crusing range.
    Limited look-out.
    Great vulnerability.

3. Communications Intelligence and Perspective on U-boat war from beginning of 1943 to end of war.

On the whole; U-boat traffic read in OP-20-G covers the period of U-boat weakness. It would be surprising if this were not so. The U-boat was at a distinct disadvantage throughout this period because it had lost its "invisibility." Its movements, were known, whether by decryption intelligence or D/F, or radar, or sonic devices--or by a combination of all these Allied location devices. It was the U-boat that was more and more "surprised." In the fall of 1942, the U-boat had already begun the long and involved history of remedies, but not until the decisive defeat of the winter and spring offensive of 1943 did U-boat Command's search for counter-devices and restoratives become constant and urgent, finally resulting in a major change in submarine warfare.

Throughout the search for causes of the U-boats loss of "invisibility" and "its one advantage in, combat: surprise," there was one Allied location measure which remained unknown to


the Germans. Behind D/F and radar there was a further locating device, decryption intelligence, whose existence the Germans may have feared on occasions but in which they apparently never really believed. At least those responsible for final decisions never acted as though they believed in it; the changes which might have countered the Allied cryptanalytic attack were never introduced. Knowing that the U-boat had lost its secret strength and had become the hunted instead of the hunter, U-boat Command persisted in reducing the mystery of Allied location by radar, i.e. to something which could be seen or could be heard, given adequate CRT and receiver equipment. It was of course considered that the secret might not lie solely in Allied transmissions, but also in U-boat radiations... Every kind of U-boat radiation was studied: heat and electronic which might be picked up in special allied receivers, U-boat transmitter radiations in tuning, U-boat receiver radiations, and their, own radio transmissions D/F'd by Allied intercept net. In truth, the critical radiations were the radio transmissions and the receiver was the Allied intercept net. But the Allies were not simply D/F'ing the unit transmissions, they were reading the messages--U-boat Command's as well as those from units. U-boat Command's messages were more revealing than those from individual U-boats.

4. The contribution of decryption intelligence to the U-boats loss of invisibility.

  1. Record of decryption.

    Well over 115,000 individual German naval messages were decrypted in OP-20-G and read in the Atlantic section. (This figure does not include short-signals and WW's). During the first 9-1/2 months of reading, i.e. from December 1942 to mid-September 1943, U-boat traffic was not decrypted solidly nor always within the time limit for immediate operational effectiveness. 233 out of the 289 days in this period were read, 127 within 5 days, of which 61 within 2 days. Thereafter, from mid-September 1943 to the end of the war, but especially after 1 November, it is no exaggeration to say that U-boat traffic was consumed on the spot, continuously, solidly, and currently for 19-1/2 months.


Record of U-boat Traffic Decryption (Atlantic)
December 1942--May 1945
Month No. Days
Not Read
No Days Read Total No. Days
Read in 5 Days
or Less
No. Days
Read in 2 Days
or Less
2 to 5 Days
Dec 42 12 19 8 5 3
Jan 43 3 28 17 5 12
Feb 1 27 23 18 5
Mar 5 26 19 8 11
Apr 11 19 19 12 7
May 3 28 14 9 5
Jun 9 21 7 1 6
Jul 2 29 3 2 1
Aug 6 25 10 1 9
Sep 1-15 4 11 7 0 7
Total to 15 Sep 43 56 233 127 61 66
Sep 15-30 All days read 12 3 9
Oct All days read 25 18 7
  Average daily delay in hours (GCT)
Nov All days read   26.1  
Dec All days read   26.0  
Jan 44 All days read   23.7  
Feb All days read   32.6  
Mar All days read   22.0  
Apr All days read   18.0  
May All days read   21.1  
Jun All days read   21.5  
Jul All days read   18.6  
Aug All days read   16.2  
Sep All days read   36.4  
Oct All days read   45.2  
Nov All days read   37.0  
Dec All days read   40.3  
Jan 45 All days read   25.2  
Feb All days read   28.1  
Mar All days read   56.7  
Apr All days read   33.4  


    After months of effort the German machine cipher had been mastered and the recovery of the daily keys reduced to a minimum delay.
  1. Weakness of U-boat operating system which increased value of decryption intelligence: operational bookkeeping by radio.

    The above statement on the quantity of material made available by decryption is not complete without a statement concerning certain peculiarities of the German operating system which greatly increased the value of U-boat special intelligence. The constant and close personal direction of the U-boat from the shore necessitated complete reliance on radio communications. U-boat radio traffic constituted in effect a system of operational bookkeeping which required daily posting if the U-boats were to function as Command intended. The following are some of the major points in the operating system, all dependent upon radio communications, which made available to the Allies Dönitz's (COMSUBs) own entries in his system of accounting for every U-boat at sea.

    1. Heading points and operational plans were ordered by radio after the U-boat had put to sea. There were few exceptions to this procedure. Occasionally boats sailed with written orders for special tasks, but even they were told by radio to "Proceed according to Special Task."

    2. Passage reports: U-boats were required to send passage reports after clearing outer Biscay or after crossing 60N when outbound from the Baltic or Norway for Atlantic operations.

    3. Complete control of operations from shore: U-boats were not only told where to go after they had put to sea but also when they were expected to arrive in their heading area. Patrol lines were formed on the hour. Every U-boat Commander who was to be in the line was addressed by name. There were to "be in line at 0800/17" at stated intervals between two points defined down to minutes by means of grid squares. There they were to expect an "eastbound convoy" beginning at dusk. From there on they continued to be told exactly what to do. It is a mistake to think of the "wolfpack" as


      a kind of independent task force. There was no senior officer present who directed the operation from a flag-boat. COMSUBs decided everything, and thus demanded the most detailed descriptions of the situation from each U-boat, transmitted on frequencies that could be heard equally well on this side of the Atlantic. There was a provision for a "group circuit" to be used only when Control could not hear what was going on. If communications were entirely broken off (very rare) and the "group circuit" ordered, then COMSUBs would name other U-boats as radio relays so that he could still keep himself informed. The shadowing U-boat had to send homing signals on medium frequencies, but at the same time it had to report to Control on high frequencies.
    1. Position reports: If a U-boat failed to append his position to a transmission or failed to transmit for several days, he usually received a special request to report position. (U-boat sinkings were judged on the basis of these reports.)

    2. Modifications: No U-boat could deviate from orders without permission requested and granted by radio.

    3. Returning: U-boats could start their return home only after a specific order to do so or after a request with reasons had been submitted and granted--by radio. The decision to return was left up to the Commander in some cases, such as those involving a damaged U-boat, but the damage had to be reported and the return cruise announced. On their way in, U-boats had to transmit their expected time of arrival off escort points and their requests for radio beacons.

    4. Fuel on hand had to be appended to every transmission. Refueling at sea was handled in the same detailed manner. The refueling station was announced well ahead of time. What U-boats were to be serviced, how much fuel they should have left on arrival, and how much fuel they were to be given were all sent on the air, down to the last cubic meter. And after the job was done, the supply submarine had to make his report by radio; telling what U-boats he refueled, how many cubic meters each received and when, and how much of everything the supply submarine had on board.


    1. While mechanical details of new equipment were not always available in U-boat traffic, the essentials could be put together in short order, because here again either Command could not wait to see how the things were working out or something would come apart and the U-boat would need advice.
    2. General orders. estimates, etc., situation reports. The German habit of bookkeeping did not stop with the details of U-boat movements, but went on to make far more information available to the Allies then was actually necessary for the U-boats day to day operations. The tempo of the war at sea, and considerations of security, may have necessitated the custom of, giving the U-boat his heading orders after he put to sea. The fact that some U-boats were always at sea may have made it difficult to effect a timely delivery of general orders and estimates of the current situation without reliance on radio. But whatever the German difficulties may have been, their solution by radio kept the Atlantic Section's books posted along with those of the U-boats at sea. When the U-505 was captured, her volume of Current War Orders, for example, was already contained in the Atlantic section's files and long since in the hands of COMINCR. In fact, U-505's copy was out of date, having been superseded by additions and corrections which she had failed to intercept herself or which had been transmitted after her capture.

    3. Communications: To carry this heavy burden of traffic the German navy developed elaborate, complex, flexible, and highly efficient radio service which is probably unique in the history of military communications. Its workings, however, were likewise revealed in U-boat traffic. The Atlantic Section translated more than a hundred corrections to "Communications War Orders" in addition to approximately 150 series of messages announcing frequency and circuit changes.


5. Uses of decryption intelligence in the Battle of the Atlantic.

The effect on the enemy of this wholesale misdirection of his own radio communications is difficult to calculate in detail. In mentioning some of the more obvious uses of intelligence emphasis should be given to the most general use, background information.

  1. Accumulation of information on U-boat navy.

    As will be evident from the items already referred to, the files of processed U-boat traffic steadily gathered a vast amount of information, both general and detailed, on every aspect of the U-boat, including the edicts of the Grand Admiral and the welfare of the U-boat crews, their ailments and proxy marriages. Orders of the day and reports from those at sea could be read against a broad background. Something of the enemy's temper could be determined, his habits and character appreciated. One had considerable evidence with which to appraise his intentions and limitations, with which to judge what he could or would do--or not do.

    On the basis of traffic U-boats could be identified by number and by commander. The Atlantic Section periodically issued a master list of operational U-boats. Their types could be identified, if not already known, in various ways, as, for example, by reports of fuel on hand.

    From a knowledge of the enemy's communications and operating systems one could frequently fill in information that had been missed. To mention only one example: if a U-boats passage report had been missed, the order to change from "Coastal" to "Ireland" radio circuit would give the same information in another way. There were also cases in which errors made by the Germans were caught here before they were cleared up by Control.

    If this background was invaluable in the interpretation of present and future U-boat movements, it was equally important in breaking into current keys, for one knew the sort of message which would accompany or come out of a given situation at sea. One waited for a message of the right length or with a D/F fix in the right area.


  1. The protection of Allied convoys and shipping.

    During the first 5 months of 1943, the primary use of radio intelligence was in convoy diversion, for at this stage sufficient antisubmarine and escort forces were not available for an immediate and direct offensive.

  2. Offensive use.

    The defensive use of ULTRA intelligence continued throughout the war, but with the increase of anti-submarine forces the direct offensive use became more and more characteristic. One of the first instances of the US Navy offensive use of ULTRA was the attack delivered by the USS Bogue on U-boat group Trutz early in June 1943. From that time on U.S. Navy CVE forces carried out frequent attacks, giving special attention to U-boat refueling rendezvous in the Middle Atlantic. As a result of these attacks U.S. Navy contributed in a major way to the destruction of the U-boat supply fleet. In addition, U.S. Navy forces practically cut off Axis supply line to and from the Far East by intercepting the submarine cargo runners, after having played a full part in the sinkings of surface blockade runners. Of the 489 U-boats sunk by Allied action at sea, beginning in January 1943 U.S. Navy forces sank approximately 63 with direct aid of ULTRA information plus some 30 more with indirect aid of ULTRA.

  3. Allied decryption intelligence as countermeasure.

    An important contribution made by the Atlantic Section on the basis of its own correlation of U-boat traffic was the detection of German cryptanalytic success with Allied codes and ciphers--principally the Anglo-U.S. combined naval cipher.

  4. Technical intelligence.

    Communications intelligence correlated, evaluated and disseminated advance intelligence and currently significant details about devices and techniques under development or in use by the enemy. Such information was extensive both in variety and in volume. A related factor in the constant


    struggle for technical supremacy, namely enemy knowledge of Allied devices and procedures, was also the subject of continual attention. A review of the nature and history of the more important devices concerning which information was passed leaves no room for doubt that this aspect of the Atlantic Section's activity was of vital significance. The advantage of superior intelligence about the enemy's endeavors in the field of radar, anti-radar search receivers and deceptive devices (such as Aphrodite), weapons, underwater diesel propulsion (schnorchel) and other technical matters helped the Allies to gain and hold the initiative in the battle for superiority in technical devices.


Merchant Shipping Sunk in Escorted Convoys by U-boats


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