Black May

by Michael Gannon

  “There was great promise in this situation,” Raushenbush wrote privately in 1948. “The danger in it was that the new weapon might (like tanks in 1916) be used in too small numbers, with too small effect, and that the Germans would consequently be given ample notice of the new weapon before it could be used against them with telling effect, and would be ready for it.” He anguished, he wrote, over the possibility that a centimetric radar installation would first be used in an area such as the Mediterranean or the European mainland, where it might be captured and compromised.7 As it happened, a few Io-centimeter sets were flown by Coastal aircraft out of Gibraltar in February before their use in the Bay. And Raushenbush’s worst-case scenario—though it is not known that he was aware of it at the time—unfolded on 2 February when an RAF Bomber Command Stirling bomber equipped with centimetric radar went down at night near Rotterdam. The radar set was Type H2S, in which the radar pulses were used in a “look-down” mode for picking out coastlines, lakes, waterways, and (less successfully) cities.

  Coastal had forcefully opposed that use of 10-centimeter radar prior to its use in the Bay precisely because capture of the equipment, which seemed likely, would ruin Coastal’s chances of obtaining surprise in the Biscay transit area. But Bomber Command spoke louder, claiming that for the success of the night-bombing campaign over Germany—always the overriding imperative in the Prime Minister’s mind—the bombers desperately needed H2S as a navigational aid. Churchill gave approval for the new radar’s use over enemy territory beginning in January, with, as Coastal feared, predictable results. Though the Stirling’s radar equipment was badly damaged, German technicians were able to reassemble the Rotterdam Gerät, as they called it, at the Telefunken laboratories in Berlin. By chance, the device was badly damaged a second time in an RAF bombing raid. Again, it was reconstructed, this time in a bombproof bunker. After flight-testing the magnetron valve equipment, the technicians realized that the Allies had achieved a major technological breakthrough, and, where the maritime war was concerned, had leapfrogged the Fu.MB (Metox). News of the disclosure was passed at once to BdU, where on 5 March the Dönitz/Godt war diary reported a confirming incident at sea and ruminated on the Rotterdam Gerät.

  U-333 [Oblt.z.S. Werner Schwaff] was attacked by enemy aircraft at night without previous radar [detection by Fu.MB] in BF 5897. Slight damage, aircraft was shot down in flames.… [The aircraft was L/L Wellington “B” of No. 172 Squadron, which had just begun Bay patrols with ASV Mark III.] The enemy is working on carrier waves outside the frequency range of the present Fu.MB receivers. The shooting down over Holland of an enemy aircraft apparently carrying an apparatus with a frequency of 9.7 centimeters is the only indication at present of this possibility.8

  The secret was out, and it appeared likely that the Germans would now neutralize the centimetric wavelength in the same way that Metox had neutralized the metric. But the Telefunken Company experienced problems in replicating parts of the Allied equipment, and administrative muddles further checked what was to have been a crash program to develop a new G.S.R., with the result, astonishingly, that an effective detector called Naxos-U was not shipped to the U-boats until October, far later than the two or three months predicted by Watson Watt, and long after the issue in the Atlantic had been decided.

  Raushenbush began his calculations with a review of U-boat performance figures. The optimum (as against maximum) speed surfaced for charging batteries was 12 knots. The optimum speed for running submerged was 1.75 knots. The average battery capacity on entering the 200-mile-deep transit channel was 51 miles submerged, after which a U-boat had to surface for maximum recharge for a period of 6.77 to 7.77 hours, during which it would travel 81 to 93 miles. After another 51 miles submerged, it would have to surface for charging at least once again, briefly, until completing the 200 miles (assuming a direct course) in a total traverse time of 76.37 hours.

  Thus, a U-boat in transit would be on the surface and vulnerable to air attack for at least one lengthy period. Any attempt to remain underwater beyond 41 hours would exhaust the air supply, although a boat could surface for 5 to 10 minutes to ventilate. A surfaced U-boat forced to dive by aircraft would later have to charge for approximately seven minutes to compensate for the 100 ampere hours used in one cycle of crash-diving and resurfacing. Since the average density of boats in the transit area at any given time was 15.8 boats, that number together would be exposed from 1,280 to 1,470 miles during their passage. Raushenbush calculated that there would be a density of one surfaced boat per 3,800 square miles.

  On the air side, Raushenbush called for an additional 160 long-range aircraft, all equipped with ASV Mark III and many with Leigh Lights, to make up a total force of 260 aircraft. Such a large, coordinated force, trained to capitalize on the Allied advantage of centimetric radar, could be expected to make 7.5 sorties per aircraft per month, to make 1.8 attacks on each of 150 U-boats entering the transit channel each month, to make a minimum of twenty-five kills per month, and to cause damage to a further thirty-four boats. Over the projected 120 operational days of this effort, 100 boats would be destroyed and 136 damaged, thus “paralyzing” the U-boat fleet and throwing it on the defensive. The damaged boats would play their role in the paralysis effect by jamming and overloading the Biscay repair bases.

  There were two critical factors in the Raushenbush Plan: (1) the attack program must be put into effect promptly, before the enemy devised a centimetric search receiver; and (2) the attacking force must be sufficiently large from the outset; “no small driblets” of additional aircraft would make the plan work. On the second point he elaborated that a law of increasing returns could be developed to show that up to a certain point, a large but still less than adequate force would produce only minor results; but that once enlarged to and beyond a certain critical mass, the effectiveness of that force was in high progression. He concluded:

  The morale of the remaining U-boat fleet may be broken by such an effort. If in four months (May-August 1943 inclusive) 100 U-boats are killed, and 136 damaged, and every one is attacked 1.8 times in transit, the U-boat fleet based on Biscay would have lost about 36 per cent of its numbers and the crews of an additional 136 would have been shaken up. The unkilled 175 U-boats may thereby be so broken in morale as to impair their effectiveness greatly.9

  Raushenbush went on to suggest crew “mutiny” as a possibility, which was going somewhat over the top; the suggestion probably showed the degree to which his views were shaped by British associates, among whom the morale war seems to have been a preoccupation. One suspects, knowing how U-boat crews put out to sea unflinchingly in 1945, when certain to near-certain destruction faced every boat, that infidelity to duty in the U-Bootwaffe was never a consideration.

  The Raushenbush Plan was endorsed by Captain Solberg, and, upon his recommendation, by Admiral Harold R. Stark, U.S.N., Commander, United States Naval Forces in Europe, who had it printed up for presentation to the Prime Minister’s A.U. Committee on 24 March. In the meantime, it received strong support from the operations research team at the Admiralty, though those politically savvy people knew that the Plan would not fly unless it passed the inspection of Churchill’s personal science advisor, Professor Lindemann, now Lord Cherwell. Accordingly, Professors Blackett and Williams (the latter now also with the Admiralty) joined Raushenbush to form a special committee under the chairmanship of Sir Stafford Cripps, Minister of Aircraft Production and vice-chair of the A.U. Committee, for the purpose of bringing Cherwell into camp. In that endeavor they were not entirely unsuccessful.

  Cherwell was at first dismissive of the Raushenbush Plan as “based upon somewhat speculative foundations,” calling it “unduly optimistic.” Without directly challenging any of the American’s numbers or calculations, he rejected the “largely theoretical” proposals in the Plan as diverging from prior practical experience in the Bay, where the dividends had been very few. Furthermore, he argued, the presumed advantage of 10-centimeter radar would be overcome �
�very easily” by a new German search receiver; and the probability that the enemy would sprinkle the Bay with radio decoys seemed to have been treated “rather lightly” by Raushenbush. It would be better, Cherwell said, to devote aircraft resources to the more fruitful duty of protecting menaced convoys. In fact, better still would be the allocation of Coastal Command aircraft to the bombing of German cities, which “must have more immediate effect on the course of the war in 1943.” All that said, however, Cherwell did allow that it could be an “interesting experiment” to give the Raushenbush advocates a free run to see how they fared.10

  Two other events transpired before the plan devised by the U.S. Naval Attache’s one-man Bay research branch was formally presented. First, the Admiralty produced its own similar plan for the Bay. Second, a trial of the two plans was flown by Coastal Command from 6 to 15 February under the code name Operation Gondola. Although authorship of the Admiralty’s plan was credited to Blackett, he suggested in a eulogy of Williams (who died in 1945) that the calculations had been done by Williams during the winter of 1942–1943, when “he worked out in great detail the best methods of conducting such an offensive by a balanced force of day and night aircraft equipped with the latest forms of 10 cm. radar.”11

  Williams (or Blackett) shared the plan with Raushenbush, who drew up a one-page summary of comparisons and differences between the two sets of numbers. Both plans called for a total force of 260 heavy aircraft. Where Raushenbush estimated that the force required 160 additional aircraft, Williams estimated 190. Where Raushenbush envisioned a four-month offensive, Williams called for a full year’s endurance of effort. Both plans anticipated 150 U-boat transits a month in the Bay during spring 1943 (which proved to be too high). The average number of sorties per aircraft per month were approximately the same, as were the ratios of sightings to attacks, attacks to kills, and attacks to damaged U-boats. Where Raushenbush predicted twenty-five kills per month and thirty-four boats damaged, Williams anticipated twenty-two kills and twenty-two damaged.’12

  The nine-day Gondola trial did not exactly replicate either plan, since the aircraft of only three of the sixteen squadrons participating in whole or in part were equipped with 10-centimeter radar: these were United States Army Air Forces (USAAF) Liberator Squadrons Nos. 1, 2, and 224. Altogether, 136 individual aircraft, including L/L Wellingtons and L/L Catalinas, took part in standard patrols that “fanned” southward over the Inner Bay (East), where during the operational period forty U-boats traversed the area, and the Outer Bay (West), where thirty-eight boats transited. Eighteen sightings resulted (only two initiated by centimetric radar), leading to seven attacks. One U-boat was believed sunk by Liberator “T” of No. 2 Squadron, but a recent NHB/MOD reassessment finds that the U-boat attacked, U-752 (Kptlt. Karl-Ernst Schröter), escaped serious injury. Still, the numbers, particularly those of sightings, and of the reduced flying hours required to make them, seemed provisionally to validate the Raushenbush/Admiralty Plans, taking into account the fact that most aircraft, as noted, were not equipped with centimetric radar. After the end of the operation there was a marked drop in the ratio of sightings to flying hours, back to the former low level.

  In early March, to Coastal’s great regret, U.S. Admiral King requested the transfer of two USAAF Liberator squadrons from St Eval in Cornwall to Morocco. Air Marshal Slessor stated that their crews had shown “intense energy and enthusiasm” in the anti-U-boat war, and “were just getting into their stride.”13 The loss of these centimetric-equipped aircraft as well as No. 405 Halifax Squadron, which had to be returned to Bomber Command, was a blow to both the Raushenbush and Admiralty Plans. Nonetheless, with the aircraft remaining, including this time the newly operational No. 172 Squadron of centimetric-equipped L/L Wellingtons, another combat trial in the Bay called Operation Enclose was laid on by Coastal for dusk 20 to dawn 28 March.

  Curiously, as will be shown below, this was at just the time that Coastal was officially denigrating the Bay Offensive as an uneconomical use of Coastal assets; and indeed, it was on the 22nd that Air Marshal Slessor sent his Note to the A.U. Committee (seen in chapter 3) recommending that the Bay be consigned to the condition of a “residuary legatee.”14 Yet Peyton Ward tells us that his naval liaison staff at Northwood made the suggestion for a new trial and that Slessor supported it.15 (This was not the last example of Slessor’s paradoxical behavior.) In P. W.'s conception, the Gondola patrol fan (so-called because it spread out slightly to the east and west below the south England and Welsh bases) should be replaced by a single patrol “ribbon” 140 miles wide running north and south across the Bay between longitudes 7° and 10½° W. The width of the ribbon represented the probable maximum distance traveled by a U-boat in 24 hours regardless of the ratio of the time spent surfaced or submerged. The scheme called for aircraft to form a constant stream passing south into the ribbon as far as 44½° N and returning on nearly reciprocal courses. P. W. and his staff added a fillip to the nighttime flights that was calculated to sow uncertainty and carelessness among the U-boat crews: in addition to the 10-centimeter pulses, aircraft still fitted with metric equipment should send the old familiar metric pulses.

  No. 19 Group stood down for a week beforehand in order to conserve energy for a seven-and-a-half day intensive effort. Then, at dusk on the 20th, 115 individual aircraft—10 cm.-equipped Liberators of USAAF No. 224 Squadron, 10cm. L/L Wellingtons, other Wellingtons, Halifaxes, Fortresses, Sunderlands, Whitleys, and one Catalina—began patrolling the ribbon. A week and twelve hours later, their expenditure of 1,300 flying hours had produced twenty-six sightings and fifteen attacks leading to the sinking of U-663 (Oblt.z.S. Hans-Jürgen Haupt) by Whitley “Q” of No. 10 Squadron Operational Training Unit (O.T.U.), and damage to U-332 (Oblt.z.S. Eberhard Hüttemann) by Wellington XII “T” of No. 172 Squadron. Since forty-one U-boats crossed the ribbon—the estimate having been forty-two—the significant numbers were one-half the Gondola hours per sighting and twice the ratio of sightings to U-boats on passage. Though those results were still not up to the Raushenbush/Admiralty projections, they were sufficiently promising that Coastal planners began scheduling Enclose II for April—at just the moment, it bears repeating, that AOC-in-C Slessor was proposing to concentrate his air resources on close cover of threatened convoys “at,” he said, “the expense of the Bay patrols.”16

  When sitting for its twelfth meeting at 6:00 p.m. on Wednesday 24 March in the Cabinet Room at 10 Downing Street, S.W.I, the A.U. Committee, with Churchill in the chair, found three Papers on their agenda. The first was a Note proposing the Raushenbush Plan, to which Admiral Stark, who since the previous meeting had been made a member of the Committee, was prepared to speak. The second was the Note by Marshal Slessor proposing emphasis of air cover for threatened convoys in preference to Bay patrols. And the third was a Memorandum by the First Lord of the Admiralty, Mr. A. V. Alexander, M.P., and First Sea Lord and Chief of Naval Staff, Admiral of the Fleet Sir Dudley Pound, urging that Bomber Command launch new heavy raids on the Biscay bases.17 Because both the U.S. proposal, which the Committee called the Stark Plan, and the Admiralty’s called for the diversion of bombers to the Bay or its bases, and the sense of the Committee was that for the moment those aircraft could only come from Bomber Command’s operations over Germany, it was decided to defer discussion of the three Papers until the next meeting and to invite the Air Officer Commanding-in-Chief, Bomber Command, Air Chief Marshal Sir Arthur T. Harris, to present a Paper, if he wished, and to attend the meeting.18 Two days before that meeting, the Secretary of the War Cabinet, Sir Edward Bridges, circulated a Note specifying that only thirteen members directly concerned with the agenda Papers should attend. By the meeting date there were three additional Papers on the agenda: the invited response from Air Chief Marshal Harris; Cherwell’s comments on the Raushenbush document; and a new position paper from the Admiralty proposing the Blackett/Williams Plan while supporting the Stark Plan “for its striking and independent support of the Admira
lty view.… ”19

  Not surprisingly, in the meeting of 31 March as in his Memorandum to the Committee (dated 29 March), Marshal Harris took aim at that section of the Admiralty’s latest document that called for the transfer of 190 long-range bombers from the bombing campaign over Germany to the Bay Offensive. The loss of so many aircraft, Harris contended, would mean calling off bomber operations against Germany for the next four months and throwing the whole brunt of fighting Germany upon the Soviet Union—points his Naval opposites no doubt thought exaggerations. The Minutes read: “He did not think it was fully realized what great damage was done by the attacks on U-boat construction yards and accessory factories. There was continuous confirmation that the U-boat construction programme was being considerably interfered with by these attacks and if they were stopped he was certain that the output of U-boats per month would increase.”

  As for new attacks on the Biscay bases, which the Admiralty’s earlier Memorandum advocated, the U-boats and their essential services were sheltered under impenetrable concrete, Harris reminded the Committee, and the 10,000 tons dropped recently on the bases at Lorient and St.-Nazaire had, as the Admiralty themselves conceded, no appreciable effect on U-boat operations.20 (Slessor, too, was critical of the bombing, at this stage, of the Biscay bases, “which was actually quite useless and resulted merely in spoiling several nice old French towns.”21) Chief of the Air Staff Portal spoke up in support of “Bomber” Harris, as he was known in the Force, saying of the U.S. Navy and Admiralty proposals that he deprecated the transfer of any of Harris’s bombers to Bay patrols on the strength of “a theoretical calculation.”