Hunt and Kill

by Theodore P. Savas

  King believed that unified control over antisubmarine operations in the part of the Atlantic for which the United States was responsible was essential to victory. To meet that need, he established Tenth Fleet under COMINCH control on May 20, 1943.38 King took command of the fleet believing supreme naval authority was required for the ASW mission. Still, he usually left day-to-day command authority to “Frog” Low, Tenth Fleet chief of staff, whom he promoted to rear admiral. Orders from Tenth Fleet to the operating forces, however, were issued in King’s name, a procedure that left no room for argument from subordinates.39

  King and Low set out to create a unit composed of highly intelligent people, both regular navy and reservists, officers and enlisted, in a small, flexible, responsive organization.40 Its elements included a Convoy and Routing Division, made up of a staff Tenth Fleet inherited from the former F-35; an ASW Organization (ASWORG), consisting of approximately 100 civilian experts, and a core staff of approximately 50 officers and enlisted.41

  There was a reason Tenth Fleet did not possess a single ship: it did not have a need for them. By King’s explicit order, Tenth Fleet had the power to commandeer any U.S. naval forces as needed for antisubmarine operations.42 These Allied hijacking directives were normally dispatched as “suggestions” to Commander in Chief, Atlantic Fleet (or CINCLANT), though King fully expected CINCLANT to comply immediately.43 The end result was that CINCLANT’s commander, Admiral Royal E. Ingersoll, virtually relinquished command of his escort carrier task groups, allowing Frog Low to coordinate their operations.44

  For his part Low fully appreciated that Tenth Fleet required intelligence support to perform its tasks. While the organization had unrestricted access to the British Admiralty’s ASW resources, most of Tenth Fleet’s intelligence was obtained from U.S. naval sources.45 Both Low and King agreed Tenth Fleet did not need its own intelligence division. Therefore, Knowles’s F-21 was tasked with fulfilling the fleet’s intelligence requirements since it was already devoted almost exclusively to issues relating to U-boats.46 The division of labor worked well. F-21 maintained the operations plot for Tenth Fleet, which in turn passed its “suggestions” to CINCLANT, which complied.47 F-21’s output was also forwarded to Tenth Fleet’s Convoy and Routing Division (or F-0), which used the intelligence to direct shipping away from U-boat infested waters.48

  In addition to its daily contact with the operating forces through CINCLANT, Tenth Fleet issued a regular publication. Commonly known as the “Yellow Peril” after the color of its cover, U.S. Fleet Antisubmarine Warfare Bulletin was issued every month from June 1943 to June 1945, serving as Tenth Fleet’s primary method of communication with commanding officers at sea. It provided a wealth of reliable and useful information about new enemy weapons, U-boat tactics and capabilities, and ASW developments and doctrine.49

  Small yet multifaceted, Tenth Fleet labored largely in obscurity, as did the various elements of COMINCH and ONI that supported the antisubmarine effort. Any glory in the Battle of the Atlantic went to the operating forces. Still, those chasing down U-boats in the wide expanse of the Atlantic or escorting convoys safely across the sea fully understood the value of the intelligence they received and unstintingly praised the naval intelligence organizations that made their exploits possible.50

  Allied anti-submarine warfare in the Atlantic benefited from a strong spirit of cooperation among the United States, Great Britain, and Canada. This relationship, however, took some time to develop because codebreakers at Bletchley Park were reluctant to share Enigma intelligence with their American counterparts.51 It took an exchange of visits between Bletchley officials and the Office of Naval Intelligence and a written agreement to establish a solid foundation for the sharing of intelligence gleaned from Enigma. The British influence in antisubmarine warfare was also put to use when the U.S. Navy built its ASW organization on the Royal Navy’s model, a result that largely came about with Knowles traveling to England to meet with Winn to learn the science and art of submarine tracking and how the Royal Navy set up its ASW organization.52

  Intelligence Sources and Methods:

  The Role of All-Source Intelligence in the Capture of U-505

  The publication of F. W. Winterbotham’s The Ultra Secret in 1974, detailing for the first time the story of how the Allies had broken the Enigma machine cipher, captured the popular imagination of people around the globe. The Allies’ success in keeping the secret for nearly three decades after the end of World War II only added to the mystique of the accomplishment. Less romantic intelligence disciplines, although vital to the Allied antisubmarine effort, passed largely ignored.

  In the case of U-505, as it was with the greater anti-submarine war effort, the reality was more complex. Radio direction finding technology matured quickly and was used for tactical intelligence both ashore and aboard ship. Less sophisticated codes and ciphers were exploited both for the information they held and as aids to decrypting Enigma traffic. Human intelligence stripped from the interrogation of prisoners of war yielded information that was used in defeating the U-boats. Material captured from enemy surface ships and submarines was exploited profitably in signals intelligence (SIGINT) operations and in the development of countermeasures.

  High Frequency Direction Finding:

  Unglamorous, but Effective

  The British initiated efforts to obtain high frequency direction finding (HF/DF, or Huff-Duff) bearings on U-boats as early as January 1941. By that time seven radio intercept stations were operational in the Atlantic area.53 Within four months the U.S. Navy and the British Admiralty began exchanging DF information, and the Americans quickly demonstrated skills comparable to those of the British.54 As the war continued improvements in equipment, training, and doctrine made direction finding at least as important to the defeat of the U-boats as any other single intelligence asset, with the possible exception of Ultra.55

  At its peak the Atlantic HF/DF network consisted of 51 shore stations along the coasts staffed by American, British, and Canadian operators. Their first task was to copy the text of any radio traffic to or from U-boats at sea. Their second task was to obtain a bearing on the source of the transmission. With six or seven bearings (known as “cuts”), a U-boat’s location could sometimes be plotted to within 25 miles of its actual location.56 Because of the peculiarities of radio-wave propagation, most fixes were less precise. Still, knowledge of the approximate location of a U-boat often allowed the Allies to guide antisubmarine forces to an area where the submarine could be attacked, or route target ships away from the stalking submarines.

  Unlike other technical intelligence assets, HF/DF had the advantage of rapid exploitation. Because of the time required for decryption, Ultra remained too slow for tactical use. In contrast, improvements in communications eventually made it possible to transmit a fix to COMINCH within 30 minutes of a U-boat’s transmission.57 When shipborne equipment became widely available, HF/DF became a true tactical asset. Fixes were often forwarded immediately to every ship and aircraft in a task group, improving the chances of a successful antisubmarine operation.58

  U-boats Highly Vulnerable to HF/DF

  Admiral Karl Dönitz, the commander of German U-boat operations, insisted on a centralized system of control for his U-boat fleet and frequent reports from his captains. This system allowed for the organizing of “wolf pack” task groups directly from his headquarters. Unfortunately for the German war effort, Dönitz’s system generated a tremendous amount of radio traffic between submarines at sea and BdU, Dönitz’s headquarters. Even during the Enigma blackout, HF/DF profitably exploited the radio waves themselves.59

  Dönitz was a sailor, not a technical man. Born outside Berlin in 1891, he entered the Navy in 1910 and cut his military teeth aboard the cruiser SMS Breslau during the war’s early years before transferring to U-boats in 1916. The younger Dönitz made five successful patrols in the Mediterranean aboard U-39 as I.W.O. (First Watch Officer) before getting his own boat, UC-25, a minelayer and atta
ck craft of 417 tons. He made two patrols between late February and September 1918 with UC-25, sinking several ships and earning the highly coveted Cross of Hohenzollern with Swords. His calm demeanor, “competence and energy” brought him respect and the command of a larger boat, UC-68. These early boats, Dönitz would later note, had a tendency to dive sharply; they also tended to be manned by largely inexperienced crews. These two factors probably came together on October 4, 1918, during a convoy encounter. While submerged the boat experienced a stabilization problem and plunged deeply by the bow. When the tanks were blown the boat went out of control and broke the surface near enemy destroyers. Out of compressed air and thus unable to dive, he ordered the boat abandoned while under fire and spent the next nine months as a prisoner of the British. Dönitz, however, had time to think about and analyze the entire U-boat experience. The result was the formulation of the early doctrine of what would evolve into his 1939-1945 U-boat tonnage war.

  During the height of the Second World War the staff of the Supreme Naval Command repeatedly told Dönitz—and he believed it—that the use of high frequency radio would make direction finding impossible. Kriegsmarine scientists were even more reluctant to consider the possibility that the Allies were installing HF/DF gear aboard ships.60 Not until June 15, 1944, when U-505 was already en route to Bermuda under tow, did BdU send a message to the fleet warning of the threat from shipborne HF/DF. By that late date, of course, much of the damage had already been done.61

  The large volume of radio traffic, much of it inconsequential, facilitated Allied direction finding and helped bring about the destruction of 22 U-boats between May 1943 and April 1945.62

  HF/DF Technology

  Throughout the war, American HF/DF benefited from technological advancements. For example, the electronics firm International Telephone and Telegraph (IT&T, now known as ITT) developed a shore-based system far superior to previous units, and the Naval Research Laboratory (NRL) created a practical shipboard system; both yielded faster and more accurate bearings than had been possible at the beginning of the war.

  How, exactly, were bearings obtained? In 1941, obtaining a bearing meant rotating a directional antenna until the received signal was at its weakest (known as a “null”) and plotting a bearing at right angles to the null. Bearings were taken from several stations located at some distance from each other, which were then plotted on a chart to determine the location of the transmitting station. The margin of error averaged 25 miles under good conditions, but a transmitting boat 500 miles offshore could be as much as 60 miles from where the DF indicated it was operating.63 A wide number of variables affected these bearings, including cold fronts at sea, electrical storms, improper electrical grounding of DF stations, weather conditions between the transmitting U-boat and the DF station, radio propagation conditions (influenced by the height of the ionosphere and the aurora borealis, among other phenomena), and the skill of the operator. As the Allies soon discovered, a better system was available.64

  In 1936, IT&T engineer and French émigré Henri Busignies had developed Model DAJ, an instantaneous-reading HF/DF.65 His DAJ used a fixed Adcock antenna array for distant stations and a quadrature loop antenna for signals transmitted within 200 miles. In either case it was unnecessary to rotate an antenna because the Adcock and quadrature arrays sensed the difference in phase as the signal traveled between antennas, and converted the phase shift into a line of position.66 A cathode-ray tube displayed the bearing as a glowing line on the screen. It was a stunning advancement in HF/DF technology.67 The Army and Navy did not become aware of the DAJ system until a conference with IT&T in January 1941. The only catch was that it was necessary to modify the system to use standard American vacuum tubes and 110-volt, 60-Hz alternating current, which was not the European standard. As a result, the system did not become operational until 1943.68

  The new technology did not eliminate errors, however. When compared to the locations logged in U-505’s war diary, for example, the positions shown in the U.S. DF log were off by distances ranging from 81 to 299 nautical miles. Much like anti-submarine warfare in general, in many ways direction finding was an art as much as a science, and intelligence analysts had to bring their abilities to bear to produce more accurate position estimates than those available through DF alone.

  HF/DF at Sea

  Because of the delay involved in relaying a fix to COMINCH and then disseminating that fix to forces in position to make use of the intelligence, land-based direction finding was too slow to be a true tactical asset.69 Therefore, the development of shipborne HF/DF gave escorts and hunter-killer groups an important advantage because the information was immediately available to the officer in tactical command, and the effective range of ship-borne HF/DF was approximately 30 nautical miles—far exceeding either radar or sonar.70

  The first shipboard HF/DF to employ a cathode-ray tube to display bearing information was called FH4, an experimental system installed on HMS Culver (Y-87), the former U.S. Coast Guard cutter Mendota, in October 1941. Unfortunately, the ship was torpedoed and sunk in January 1942.71 The British also began installing less sophisticated HF/DF aboard escort vessels. These detection units were initially used primarily for evasion, but were occasionally employed to set up attacks on U-boats. The first HF/DF-directed sinking of a U-boat occurred on March 27, 1942, when escorts from the convoy WS-17 sank Kptlt. Ulrich Borcherdt’s U-587 in the North Atlantic.72

  Despite the loss of Culver, the Navy decided to proceed with further development of shipboard HF/DF.73 On June 26, 1942, Admiral King ordered half of all new U.S. destroyers (DD) and destroyer escorts (DE) to be fitted with the IT&T DAQ (shipborne) system or any improved models developed thereafter.74 Larger ships were also fitted with HF/DF, including the escort carrier USS Bogue (CVE-9), which carried an operational DAQ by May 1943.75 IT&T went on to produce about 4,000 HF/DF sets before the end of the war.76 High frequency direction finding aided in the sinking of 22 German submarines between May 22, 1943, and April 8, 1945. This credible figure is in addition to the capture of U-505 and damage inflicted to many other U-boats that were not sent to the bottom.77 By comparison, Ultra information was used in 54 sinkings.78 Clearly, HF/DF was no panacea, but it did play an important role in the Battle of the Atlantic and the capture of U-505.

  Top Secret ULTRA: Enigma and the Bombe

  “An extraordinarily important component”

  Earlier in the war, before the Allies broke the Enigma cipher, Royal Navy hunter-killer groups scoured vast expanses of the Atlantic in search of U-boats. Shore-based direction finding was a useful tool, but a U-boat whose presence was first revealed by a HF/DF fix in the middle of the North Atlantic was likely to be well out of range of the nearest available ASW assets. The British recognized the importance of the Enigma cipher and devoted enormous resources to its solution. Through statistical analysis, foreign material exploitation, and their extension of Polish work in decryption by machine, the codebreakers at Bletchley Park eventually kept pace with the three-rotor Enigma machine. When the four-rotor Enigma was introduced to the U-boat fleet in February 1942, outstripping Bletchley’s capabilities, the U.S. Navy cooperated with American industry to develop a high-speed machine capable of solving the new problem.

  The eventual availability of Ultra intelligence tipped the scales for the Allies, largely because of BdU’s (U-boat headquarters) meticulous accountability and control. BdU routinely sent operational orders by radio after U-boats had sailed, which in turn made the orders available to the Allies as soon as they were decrypted. BdU also required periodic reports from U-boats at sea, which often included position information.79 The U.S. and Royal navies used this information to send carrier groups into promising areas, from which airborne patrols and shipboard HF/DF were used to pinpoint a U-boat’s location, after which it was often hunted to destruction.80 When combined with intelligence from other disciplines, Ultra truly became “an extraordinarily important component that orchestrated a vast symphony of techn
ological achievements that would defeat the U-boat.”81

  German Overconfidence and Mistakes

  In theory, the Enigma had 3 x 10 to the 114th power (or 3 x 10114) ciphering possibilities.82 German experts estimated a team of 1,000 cryptanalysts working continuously would require 900 million years to test every possible key—even if each analyst was working with a captured or copied Enigma machine.83 The overconfidence implied by these numbers induced the Germans to make serious mistakes in operating procedure and communications security. In addition to facilitating HF/DF, BdU’s system of centralized control and frequent reporting provided Allied codebreakers with enormous amounts of raw material for analysis.84 Even when the four-rotor Enigma was introduced in 1942, the keys were identical to those of the three-rotor machine; only the fourth rotor setting was different, which simplified the task of Allied cryptanalysts.85

  Worse yet for the Kriegsmarine, senior officers refused to consider the possibility Enigma had been compromised. After the introduction of the four-rotor Enigma, neither Dönitz nor any other high commander ever ordered technical or procedural changes that might have blunted the Allied cryptanalytic attack.86 German experts studied radio and heat radiation from U-boats, transmitter radiation while tuning, the intermediate frequency oscillators of radio receivers, direction finding of U-boat transmissions, and the possibility of an undetectable Allied radar, but apparently never gave serious thought to the possibility of decryption intelligence. It was a serious oversight that had far-reaching consequences for the German war effort.87

  Early Success against Enigma

  The internal workings of the Enigma machine were “cracked” by Marian Rejewski, a young Polish mathematician employed by his country’s Biuro Szyfrow (BS), or Cipher Bureau. He began his work in September 1932 using higher mathematics than had ever been employed in cryptanalysis, as well as information purchased from German spy Hans-Thilo Schmidt. By the end of the year he had broken the code for the first rotor.88 Rejewski worked out a mathematical method for determining Enigma keys when the indicators in three messages met certain conditions. AVA Radio Manufacturing Company built the first bomby machines designed to exploit Rejewski’s work and delivered them to BS in November 1938.89