Black May (11 page)

All the while, the best way to defeat asdic was to employ the night surface attack, whether delivered singly or in a pack, since asdic was effectively blind to surface targets. By this means Admiral Donitz neutralized the early advantage that asdic had given the British, and he might thereby have earned an advantage for himself had not the British and Americans presented him with two other
shipborne
electronic marvels: high-frequency (radio) direction-finding (HF/DF, or “Huff-Duff“) and, more widely fitted, radar.

By use of a transformer circuit called a radiogoniometer, HF/DF receivers could determine the direction from which a radio signal was transmitted. Both Britain and the United States had ground-based HF/DF stations in operation during the early years of the war, and these took bearings around the clock on U-boat radio transmissions. By 1942 British DF receivers and antennas covered the North Atlantic sea lanes at listening posts ranging from the Shetlands around to Land’s End; south to Gibraltar, Ascension, Freetown, and Cape Town; and west to Iceland, Newfoundland, and Bermuda.

All of the bearings taken by these stations were communicated to
the main station at Scarborough, England, and thence to the OIC Tracking Room at the Admiralty, where retired Lt.-Cmdr. Peter Kemp, R.N., headed a plotting team of never more than seven men and women who graphically presented the bearings taken on a particular boat’s transmitter by black strings drawn across a chart of the North Atlantic. Where the strings from three or more stations converged, that, it was estimated, marked the position of the U-boat. With as many as six bearings establishing the intersection, or “cut,” it was thought that a boat could be “fixed” within 25 nautical miles. Until naval Enigma gave more precise data in June 1941, this was the most reliable information on U-boats at sea that the OIC possessed, and convoys were vectored around estimated U-boat positions on the strength of DF intelligence.
²⁹
The network would play a critical role again during the Triton blackout of February-December 1942.

In the United States the USN had two similar HF/DF networks at work in 1942, one on the Pacific coast to monitor Japanese traffic, and another of seven stations that dotted the eastern seaboard from Winter Harbor, Maine, to San Juan, Puerto Rico. Analysis of U-boat transmission bearings received by the eastern net was done in the Atlantic Section, Intelligence Center (Op-20-G, later F-21), a clone of the OIC Tracking Room in the Navy Department (Main Navy) at Washington, D.C. Exchanges of data took place freely between the two rooms, and they with a similar Canadian Navy room at Ottawa. Early USN DF technology lagged behind that of the British, and the bearing computation experience of operators was so small that USN fixes were often qualified as being within 200 miles of the U-boat targeted.
³⁰
An intense training program improved the performance of the operators. And the equipment deficit was corrected by the gift of two superior systems, one British and one French. In the spring of 1941, an Army Navy technical mission to England returned with a complete Marconi Adcock HF/DF installation, which the naval members of the mission judged to be “far ahead of us in these developments.”
³¹
And an even more advanced system, antedating that of the British, instantaneous, automatic-indicating HF/DF, invented by French engineer Henri Busignies and smuggled out of German-occupied France, made its way, along with the inventor, to the United States in December 1940,
where it was reconstructed by the International Telephone and Telegraph Corporation (ITT) and first supplied in prototype models to the USN net in the fall of 1941. Busignies’s system became the basis for future U.S. development in the field.
³²

There were problems with the usefulness of shore-based HF/DF that were shared with Ultra, namely, that (a) they gave the Allies information at too great a distance for tactical attack purposes, and with a wide margin for inaccuracy since, many more times than not, they gave no indication of the direction in which a targeted U-boat was proceeding; and (b) they suffered time lags of collating and plotting or of decryption that allowed a U-boat Commander to nip out of the area where he had been tagged. What was needed, the RN decided early on, was shipborne HF/DF equipment that permitted
immediate
pursuit at sea of a
close-by
target.

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:

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:

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 pivotal 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.