Black May

by Michael Gannon

  The RN engineers realized the difficulty of reducing an installation to weight and size that would fit on a small escort vessel. So did the engineers of the German Navy, who particularly thought that the antenna required for HF/DF reception was far too large to be fitted to an escort vessel, such as a destroyer or frigate. Lacking apparently the imagination to conceive the impossible, Donitz’s technical advisors persisted in that view long after B-Dienst intelligence, visual observation, and U-boat experience had amply demonstrated otherwise, and, in fact, they remained obdurate on the point up to the end of the war, always explaining Allied detection successes as the work of radar.33

  The British resolved the weight and size problem in 1940, when they mounted a prototype FH 1 set and antenna on the destroyer H.M.S. Hesperus in March of that year. Though the FH I’S performance was disappointing, the experiment proved that seaborne installations were possible. An improved FH 3 set, which gave an aural presentation of target data (requiring earphones) was fitted to two fleet destroyers, H.M.S. Gurkha and Lance, in July 1941. And in October of the same year, an FH 4 set with visual presentation on a cathode ray tube, having met all test expectations, went into escort service on board the ex-American four-stack destroyer H.M.S. Leamington, which accompanied the troop convoy WS.107 to Madagascar in March 1942.

  While a single HF/DF set could detect the azimuth bearing of a U-boat transmitting to base or to another boat on a high-frequency wave band, and even determine from the ground wave whether it was near or far (25–30 nautical miles being its maximum range), the crosscut bearing provided by a second HF/DF-equipped escort gave a fairly exact fix, and an escort could be detached to pursue the fix, attack the surfaced U-boat, drive it off, or force it to submerge, after which asdic would be employed. This action would have a particular value if the transmitting boat was a shadower, since underwater its observations and communications were greatly limited. Thus, seaborne detection made possible aggressive tactical operations that were not possible with shore-based detection. Too, HF/DF at sea provided data that even shipborne radar could not deliver, since HF/DF’s range was about 25 miles while Type 271 radar’s on a surfaced, trimmed-down U-boat was only 3,000–5,000 meters, depending on sea conditions. Production of HF/DF equipment was slow, however, owing to radar’s greater popularity as an “active” detection system. The number of sets at sea on RN and RCN escorts in 1942 was very small, but by spring 1943 at least two escorts with each convoy had either FH 3 or FH 4 equipment.

  In the United States, production of a shipborne system called DAQ, based on the Busignies design, trailed the British work by many months. Though a USN decision was taken in March 1942 to build sets for ships, manufacturing delays prevented deployment until 1943. Even midway into that year, the first successful U.S. anti-U-boat attack mounted by an American HF/DF-equipped ship, the escort carrier U.S.S. Bogue (CVE-9), on 22 May 1943, was based on bearings taken by a British set just recently installed at Liverpool. The attack, made by Bogue’s TBF-1 Avenger aircraft, resulted in the surrender and scuttling of U-569 (Kptlt. Hans-Peter Hinsch). In his report on the action, Carrier Captain and Commander, Sixth Escort Group, Giles E. Short, U.S.N., stated:

  From the time the BOGUE left Belfast a continuous watch had been maintained on the newly installed HF/DF. Three radiomen from the BOGUE manned this equipment under the supervision of Sub-Lieutenant J. B. Elton, R.N.V.R., who had been assigned … to assist on this trip. The HF/DF equipment proved invaluable…. An HF/DF bearing was directly responsible for the attack on the sub-marine which surrendered…. Without doubt the … transmission at 1727Z was made by the U-Boat and wrote its death warrant.34

  The second successful HF/DF-directed U.S. attack, by aircraft from the escort carrier U.S.S. Card, would not come until August 1943, well after the U-boat war had been decided. Meanwhile, from June 1942 through May 1943, British HF/DF-equipped escorts employed their new equipment with great effect, chasing down bearings unknowingly supplied by the loquacious German boats and attacking their surprised crews. Admiral Donitz was fully aware that the British were attempting to monitor his and the U-boats’ communications from shore-based stations, and though he was advised by Naval Staff engineers that no DF bearings of any accuracy could be acquired from high-frequency signals, he warily restricted radio use by his boats.35 For necessary traffic such as position updates, convoy sightings, and damage reports he had the engineers devise Kurzsignale (short signals), letter codes (by which, for example, a damage and position report could be made using only four letters of the alphabet), rapid frequency changes, and electronically compressed messages that went out in bursts, or “squirts.” These attempts to elude the shipborne HF/DF receivers generally failed, and the message content in its various forms was successfully unscrambled by the interception stations and GC&CS.

  Prior to a convoy engagement, the attacking “wolfpack” patrol line normally observed radio silence, except that on some occasions (see the Fink line boats in chapters 4 and 5) the boats gave noon or evening position reports in great proliferation. Once the shadower boat reported a sighting, however, that boat’s signals to base became frequent; and with the battle joined, other boats soon joined in the chatter: When on 4–9 February 1943 U-boat Groups Landsknecht and Haudegen attacked Convoy SC.118, the U-boats made 108 radio transmissions in a period of seventy-two hours, and one boat alone, U–402, sent forty-one signals during the four-day battle, all of which were detected by shipborne HF/DF.36

  Although Dönitz was prepared to take some risks of DF detection with his high-volume command and control communications net, he never realized during the war how thoroughly his boats at sea were being exposed by that system; just as, of course, he never knew that his own “rudder commands from the beach” were being read by GC&.CS and Rodger Winn. Because it brilliantly exploited the German reliance on radio control, Britain’s mostly unheralded shipborne HF/DF deserves to be recognized as one of the principal tools employed by the Allies against the U-boats up to May 1943—hence its extended treatment here—and it would play a particularly significant role in the two May battles for transatlantic Convoys ONS.5 and SC.130 (described in chapters 4, 5, 6, 7, and 10).

  During the crossing of ONS.5 the destroyer H.M.S. Duncan (Senior Officer Escort Group B7, Commander Peter Gretton), which was equipped with FH 4, counted 107 DF contacts on U-boats before having to leave the convoy because of fuel depletion. The frigate H.M.S. Tay, equipped with FH 3, counted 135 transmission intercepts, many of them shared as cross-cuts with Duncan before 3 May, when the latter withdrew.37 The same two escorts, again under Gretton, accompanied SC.130 later in the month, when the surface escorts had excellent air cover, and collected 104 DF bearings between them. Later, Gretton described how Duncan and Tay were “able to get fixes on U-boats transmitting near us with great accuracy and to send aircraft quickly after them.”38 In his analysis of the battle from the other side of the hill, Admiral Dönitz, of course, attributed the British success to radar:

  These attacks could only be attributed to a very good radar location device which enables the aircraft to detect the boat above the clouds even, and then to make a surprise attack from the clouds. The amazing thing is that apparently at the time only 1 to 2 machines [aircraft] in all were escorting the convoy, according to intercept messages of aircraft operating. Each machine, however, detected during the whole day one boat more frequently than every quarter-hour, from which it must be concluded that the enemy’s radar hardly missed a boat.39

  German historian Jürgen Rohwer has concluded: “If we analyze the great convoy battles between June 1942 and May 1943 … the remarkable fact is that the outcome of the operation always depended decisively on the efficient use of HF/DF.”40 Although effective use of radar detection by escort ships and aircraft was also progressing from strength to strength during that same period, and while granting that it was the new shipborne centimetric radar operated by the escorts of ONS.5 on the fogbound night of 5/6 May 1943 that made possible that most p
ivotal Allied victory in the U-boat war (chapters 6 and 7), still, in normal visibility conditions, more U-boats in the convoy battles of 1942–1943 were first detected by HF/DF than were by radar.41

  What the British at this time called RDF (for Radio Direction Finding, which was a deliberate cover) and what the Americans called Radar (for Radio Detection And. Ranging), an acronym coined by USN Lieutenant Commanders Samuel M. Tucker and F. R. Furth, is an electronic tool understandable to modern readers familiar with its use in air traffic control, weather forecasting, and police speed guns. Instead of passively receiving radio signals, as in HF/DF, a radar set actively generates a stream of short radio energy pulses that, once transmitted through an antenna, return echoes from any physical objects the stream encounters—objects as solid as an airplane and as gossamer as a cloud. The presence of these objects is displayed on a cathode ray tube (CRT) in such a way that the radar operator can determine mass, bearing, and range.

  The technology was co-invented in 1934–35 by the British (Scottishborn) engineer Robert Watson Watt, superintendent of the Radio Department of the National Physical Laboratory near the Berkshire town of Slough, and by American engineers, notably Robert Morris Page, Albert Taylor, and Leo Young, at the Naval Research Laboratory at Anacostia in Washington, D.C. Thanks to the original research of Watson Watt, who is customarily called the “father” of radar, British engineers were able to erect the famous Chain Home radar network that, by providing range, course, and altitude of incoming Luftwaffe bombers and fighters, helped the RAF to win the Battle of Britain in 1940.

  Quickly miniaturized, RDF sets were installed in RN escort vessels beginning in fall 1940. This first equipment, Type 286 (1.5 meter wavelength), could obtain echoes on a trimmed-down U-boat at no more than 1,000 meters; hence, except in moonless nights or in fog, it was frequently outperformed by a human lookout. By March 1941 about ninety escorts were so equipped. In the same month and year Type 271 (10-centimeter, or S-band) radar was fitted to an escort, the corvette H.M.S. Orchis. When 271 entered general service with the escort fleet in 1941–42, U-boat detection range jumped to 3,000–5,000 meters. By May 1942, 271 was mounted on 236 RN ships of all categories.

  The 271 was made possible by a remarkable device called the resonant multi-cavity magnetron valve. The invention of two British physicists at the University of Birmingham, John Randall and Henry Boot, the cavity magnetron’s central feature was a cathode and anode structure built into a block of copper through which either six or eight symmetrical holes were bored. The high-frequency radio oscillations produced by the device enabled a radar apparatus to operate on a wavelength of 9.7 centimeters—rounded out in popular usage to “10-centimeter,” or “centimetric” radar.42 This represented an extraordinary advance in power, range, and accuracy over the previous “metric” radar, and constituted, in the words of Britain’s most accomplished air and naval operations research scientist, physicist Patrick M. S. Blackett, “one of the most decisive technical developments made during the war.”43

  Its narrow horizontal beam width enabled a single escort to find, fix, and hold a nearby U-boat on the surface at night and in fog. And from its first operational use until the fall of 1943, its beam was not detectable by any German search receiver then at sea. A U-boat Commander proceeding on the surface at night near a convoy escort had no way of knowing that he was being “painted.” But in the plot, or operations room, of the nearby escort, that U-boat was exposed starkly on the 271's plan position indicator (PPI), where a sweeping radial line from the center of the circular CRT screen rotated in synchronization with an outside antenna and, each time it passed the U-boat’s position, displayed it as a bright phosphorus-lit spot.

  In September 1940, a seven-member British Technical and Scientific Mission to the United States, headed by physicist Henry Tizard, arrived in Washington, D.C., with a black solicitor’s box containing, among other scientific objects and blueprints, a hand-sized eight-cavity magnetron. The gift was not entirely magnanimous: the British knew that to win their war they would need American technological assistance and industrial capacity. Members of Tizard’s mission, which was conducted at the height of the Battle of Britain, were mindful, too, that their homeland might soon be invaded; if the war was lost in the Old World, this means for continuing the fight would be in the hands of the New.

  Grateful U.S. radar specialists acknowledged that the gift put them two years ahead of the curve, and James Phinney Baxter III, official historian of the U.S. Office of Scientific Research and Development, was moved to write in 1947, “When the members of the Tizard Mission brought [a cavity magnetron] to America in 1940, they carried the most valuable cargo ever brought to our shores.”44 The encomium should not be accepted uncritically to mean that centimetric radar alone, or even principally, won the war at sea. By itself it was not a war-winner, though one could certainly say it was a battle winner, as in the final stage of the surface battle for Convoy ONS.5 (chapter 7), where centimetric radar was the triumphant technology. In the regular structure of surface engagements between escort vessels and U-boats, centimetric radar fell into place as one of five technological innovations that, taken together, swept the field. The first four were, in the order in which they were employed: (1) HF/DF; (2) radar; (3) hydrophone effect (using asdic to listen for underwater noise such as cavitation from a U-boat’s propellers); and (4) asdic echo contact. (Close in, one must not discount the Mark I eyeball.) The fifth innovation was TBS (Talk Between Ships), an American-developed very high frequency (VHF) voice radio-telephone (R/T) system, introduced in early 1941 and universally fitted on escorts a year later. At low power and short range it was immune to DFing. TBS enabled an escort commander to give instantaneous direction to the movement of his ships and to converse with overhead air cover. It also enabled the individual surface escorts to coordinate attack maneuvers between and among themselves. It is clear that by the date of the May 1943 battles, an Atlantic escort was an electronics platform of daunting authority.

  RAF Coastal Command began the war with a fleet of Avro Ansons (301), Lockheed Hudsons (53), Vickers Vildebeests (30), Short Sunderlands (27), Saro Londons (17), and Supermarine Stranraers (9). The most numerous aircraft, the Anson “Annies,” which entered RAF service in 1936, were obsolescent, and by the close of 1941 had been replaced by Wellingtons, Halifaxes, Whitneys, and other advanced designs; so, too, the Vildebeests, Londons, and Stranraers were struck off the inventory. The Hudsons, an American passenger plane refitted as a bomber, continued to be purchased and used in large numbers.

  But the principal survivors among the original list, which in fact soldiered on to the end of the war, were the Sunderland flying boats. Admirably equipped for long-range anti-submarine patrols, the “Queens,” as air crews called them (U-boat crews, noting their languorous flight maneuvers, called them mtide Bienen, “tired bees”), would score the second-highest U-boat tally of the war. Third in ranking would be the Vickers Wellington, a two-engine bomber never designed for maritime operations, but greatly effective in the Bay of Biscay (see chapter 8). Two American designs that came on the scene in 1941 and performed well were the Boeing B-17 Flying Fortress, a four-engine heavy bomber, and the Consolidated PBY-5 and PBY-5A (amphibian) Catalina twin-engine flying boat.

  The overall favorite aircraft in Coastal Command and the most successful sighter and destroyer of U-boats was the four-engine Consolidated B—24 Liberator heavy bomber. Though somewhat harder to handle, more demanding in maintenance time, and certainly more drafty than the Fortress, the Liberator was esteemed for its range. In a V.L.R. (for Very Long Range) modification, where weight reduction was achieved by removing self-sealing liners (if present) to the fuel tanks, most of the protective armor plating, the turbo-superchargers, and the belly gun turret, the Liberator had a low-altitude operational range of 2,300 nautical miles at an economical 150 knots, while carrying, on takeoff, 2,000 gallons of high-octane fuel and eight 250-pound depth charges (gravity bombs with hydrostatic fuses).
This was the aircraft that would give overhead coverage to threatened convoys in distant midocean lanes, force shadowing U-boats to submerge (which until May 1943 they uniformly did on sighting a reconnaissance bomber), and thus retard their speed, maneuverability and visibility, hence, their potential for organizing packs. This was the aircraft that, operating from both shores of the Atlantic, would eventually plug the Air Gap between Iceland and Newfoundland in May 1943—assisted by newly introduced carrier-borne aircraft.

  No. 120 Squadron received the first Mark I Liberators in June 1941 and in September began flying nine of them southwestward from bases in Northern Ireland and Iceland to their Prudent Limit of Endurance (PLE), governed by fuel remaining. But the numbers of these aircraft in Coastal remained depressingly few, as American Admiral Ernest J. King, Commander in Chief, United States Navy (after 30 December 1941), hoarded most Navy-assigned Liberators (designated PB4Y-1) for the Pacific theater, while almost all of the RAF-assigned Liberators that made it to the U.K. were claimed by Bomber Command. By September 1942 Coastal still had only one squadron (No. 120) containing V.L.R. Mark Is, six in number. The V.L.R.s were “being allowed to die out,” Coastal complained to the Under-Secretary for Air. The squadron list also included two Mark IIs (range 1,800 miles) and three Mark Ills (1,680 miles). Other squadrons had PBY-5 Catalinas (1,840 miles) and PBY-5AS (1,600 miles), but the V.L.R. was the most desperately needed long-distance performer. (There were none in Newfoundland, Canada, Gibraltar, or West Africa.) And Coastal’s Atlantic operations were being denuded of other essential resources, including, between October 1941 and January 1942, 166 air crews and whole squadrons of Catalinas shipped to overseas bases.