Black May

by Michael Gannon

  At 1200 on 26 April, owing to changes unexpectedly introduced by Berlin in naval Enigma settings, GC&CS and the OIC Submarine Tracking Room abruptly ceased reading U-boat traffic, and would not read it again until the afternoon of 5 May, a critical period in ONS.5’s westward voyage, since it was during those nine days of cryptographic intelligence blackout that Commander Gretton’s convoy would enter the longitudes where U-boat packs were known to be maneuvering in strength.16 With what Rodger Winn called “no precise information,” the most that he and Patrick Beesly in the Submarine Tracking Room could tell CinCWA Admiral Horton after 26 April was that three U-boat groups were still thought to be “in the general area off Newfoundland.” As Winn wrote later, “Thus [where ONS.5 was concerned] it was not possible to attempt any evasive routing although the convoy had in the first place been routed as far north as possible to avoid U-boats.'17 Three westbound convoys earlier in April, ON.178, ONS.3, and ONS.4, also had been routed north by the tip of Greenland, with small losses to the first two and none to the third.18 Perhaps ONS.5 would be as lucky.

  The OIC Tracking Room was not without alternative sources of information, as noted earlier. Wireless transmissions from the U-boats could be DFed by a shore-based HF/DF network that supplied cross-bearings in the Atlantic theater; the U.K. had twenty stations, the U.S. sixteen, and Canada eleven.19 Furthermore, short signals (Kurzsignale) in the old, still readable Hydra cipher (as Heimische Gewässer had been renamed on 1 January 1943) transmitted by Räumboote (motor minesweepers) escorting U-boats in and out of their Baltic and Biscay bases enabled Winn and Beesly to calculate daily the number of boats at sea; and they took into account the fact that by 26 April BdU had three Type XIV tanker and supply boats—Milchkühe—U-487, U-458, and U—461, on station in mid-Atlantic, which could extend the time some of the attack boats, after refueling, could remain on operations. No alternative source, however, could give Winn and Beesly the same confidence that their Main North Atlantic Plotting Table represented actual conditions at sea as that provided, when readable, by crisply accurate “Z,” particularly at those times when U-boat groups were regularly forming, re-forming, or shifting their operational areas.

  The consequences for ONS.5 were immediate, and dangerous. Unknown to Winn and Beesly, hence unknown also to Horton and Gretton, on 27 April BdU established Gruppe Star (Starling), consisting of sixteen newly assembled boats, along a north-south patrol line, or “rake,” at 3o°W between latitudes 61°5o’ and 57°oo’N about 420 nautical miles east of Greenland. On the Kriegsmarine grid chart the new patrol line ran from AD 8731 via AK 3523 to AK 0329. The boats were to be in place by 0900 GST on the 28th.20 The line’s northernmost wing just brushed the course of ONS.5. At 0800 on the 28th the convoy was at position 61°45’N, 29°ii’W, turning southwest. By 0800 the next day it would pass through 30°W to 34°51'W.21 Group Star had been created expressly to catch the next westbound ONS convoy departing on the eight-day cycle, which was ONS.5.

  Winn and Beesly were unable to recommend evasive routing because the order creating Star, unknown to them, went out the day after GC&CS’s reading of naval Enigma ended. Commander Gretton could not have had that piece of information. Nor could he have known that while GC&CS’s eyes were shut, the eyes of B-Service were still open. The German radio monitoring and cryptographic service was reading, though not always in real time, the Anglo-American Naval Cipher No. 3 used for convoy routing. The encrypted transmissions included the daily Admiralty or USN U-Boat Situation Reports (which should have been another clue to BdU that its own cipher had been compromised).

  When data from B-Service on the composition, sailing date, course, and speed of a convoy reached the green baize table in BdU’s situation room in Berlin/Charlottenburg, it was assigned a number. Convoy ONS.5 was assigned No. 33. We do not know the exact form in which information about ONS.5 was first communicated to BdU because that daily or hourly message traffic no longer exists; but the B-Service’s extant summary for 26 April—2 May discusses the convoy by name in connection with both the Third Escort Group (EG3), which will appear as a Support Group for ONS.5 later in this narrative, and a USN U-Boat Situation Report:

  The Third Convoy Escort Group was positioned at 47°2o’N, 50°03'W on 29 April at 2100, course approximately 100°-200°, speed 15 knots, heading toward ONS.5; it should have changed course at 47°oo’W by 11–13 degrees. The American U-Boat Situation Report of 30 April identified up to 20 boats in the general area 59°61'N, 30°43'W as a result of various wireless direction finding methods, and that a few of those boats continue to be positioned near ONS.5.

  From this entry it is not possible to pinpoint the day when ONS.5 was first identified to BdU. The dates given, 29 and 30 April, relate to events reflected on after a week’s decryption effort. Was word of ONS.5 passed to BdU as early as 26 April, when the week began? We may never know the answer. What is known is that in establishing the position of the Star line, the BdU war diary of 27 April stated: “The object of this is the interception of the next ONS convoy at present proceeding in the North…. A slow southwest-bound convoy is expected there on 28 April.”22 Advantage: Germany.

  At 0900 on the 28th, a time when some late-arriving boats were still maneuvering to their Star stations, Oblt.z.S. Ernst von Witzendorff had U—650 on the surface in naval square [qu] AD 8761. Conditions were a moderate sea, wind from the southeast Force 3, and visibility 12 nautical miles. One of the lookouts with binoculars on the conning tower bridge sighted, “Mastspitzenn!” Von Witzendorff wrote in his KTB: “Mastheads sighted. I am closing them to see what we have. It’s a convoy proceeding southwest.” At 0942 he transmitted an Ausgang F. T. (outgoing wireless message) to BdU: CONVOY AD 8758. Seventeen minutes later he sent again: CONVOY STEAMING AT 8–10 NAUTICAL MILES, COURSE 270°. At 1040 Witzendorff was heartened to see another U-boat of Group Star surface a short distance away on the port side. Three minutes later, he received a message from BdU directed to all Star boats: GROUP STAR SHOULD ATTACK ON BASIS OF WITZEN-DORFF’S REPORT. WITZENDORFF IS FREE TO ATTACK AS SOON AS ANOTHER BOAT HAS CONTACT. At 1110 U-650 updated her first report: WESTBOUND CONVOY NOW IS AD 8728, SPEED 8 KNOTS.23

  The IIIO transmission was picked up and DFed by Duncan and Tay, as well as by escorts supporting eastbound Convoy SC.127, about 60 nautical miles due south of ONS.5.24 Gretton now knew that ONS.5 was being shadowed—but on behalf of how many boats? He sent the corvette Snowflake to search down the bearing for the transmitting U-boat, which Gretton calculated, based on its strong signal, was “close ahead.” Meanwhile, he altered course of the convoy 35 degrees to starboard and maintained 296° until 1600, when he returned to the original course. Snowflakes search was fruitless, as was a ten-mile high-speed sweep ahead of the convoy by Duncan. Visibility declined to three miles.

  At 1650 a U-boat signal detected from astern at 085 degrees indicated that the course alteration, which placed ONS.5 north of the Star rake, had been successful, for the time being. Tay hunted down that bearing and Vidette tracked another, but made no contact. Gretton worried that the U-boat shadower was part of a large pack—one wholly unanticipated—and that it would not divide its forces between his convoy and SC.127 but concentrate them solely on him, and at night, when the U-boats had 17–18-knot surface speed. As dusk came he worried, furthermore, that he could expect no help from aircraft. The air escort given ONS.5 from 24 April had been discontinued at midnight on the 27th/28th, since there were no OIC reports of U-boats in the vicinity. The convoy had been sighted in the late afternoon by a distant USN Catalina, but the possibility of air cover this night, if requested, was remote, since Iceland was socked in by weather.

  At 1838, as a heavy head sea formed, Duncan DFed a U-boat close on the port bow, bearing 210°. He chased it at maximum speed and ordered Tay to make a parallel search to port. At 1920 Duncans bridge sighted a cloud of spray thrown up around a U-boat’s conning tower, about two miles bearing 146°. Gretton altered course to pursue the boat, but at a range of about 3,000 ya
rds it dived. In the rough sea Duncan’s asdic failed to make contact, but Gretton fired a ten-charge pattern of D/C by plot. Duncan and Tay then carried out operation “Observant” for an hour. Observant was an asdic square search of two-mile sides with the “Datum Point” (contact point) at the center; one of the escorts could either reinforce the square (sometimes called box) or operate within it. Leaving Tay to sit on the submerged boat while the convoy passed, Duncan returned to establish night stations with the convoy at 2130.25

  The merchantmen, now beginning a southwest leg, were on course 240°, speed 7.5 knots. The wind was freshening from the southeast at 16–20 miles per hour and the sea was rough, with moderate long swell.

  Knowing that U-boats preferred to attack down sea, so that spray did not betray their approaches, and knowing, too, from HF/DF that the U-boats were on the port bow beam, and quarter, and astern, Gretton “placed his field” with strength to that side, leaving the starboard bow uncovered.26 Attack abaft the port beam was most probable. Where the Germans were concerned, by nightfall only four of Star’s other U-boats had rallied to U–650's reports: U-386, U-378, U-532, and U-528. That more had not assembled on the convoy’s course, despite BdU’s urging to do so, was owed in part to the “ha2y weather” and “strong wind” against which the boats “had to struggle during their pursuit of the enemy.” This, at any rate, was the assessment of BdU on 1 May, after all the excuses were in.27 If only five boats were on the scene by darkness on the 28th, the rest were not unheard from, however, as every Commander made his evening position report to Berlin. The W/T traffic—“like a chattering of magpies”—was DFed in England as well as by B7's two HF/DF sets.

  For Gretton there were two immediate good results from this chatter. In view of the concentration around ONS.5, CinCWA detached destroyer H.M.S. Oribi from the escort of SC.127 and sent her, at 20 knots, to his assistance. And since this convoy was likely to be targeted by the western U-boat packs, when it reached those longitudes a Support Group (EG3) of four Home Fleet destroyers, H.M.S. Offa in command, was ordered to steam out of Newfoundland at 15 knots to meet ONS.28 With those pledges of reinforcement to brace their spirits, Gretton and his Captains prepared for the night battle sure to come.

  In an interview conducted fifty years later, Lieutenant [now Sir] Robert Atkinson, R.N.R., Captain of the corvette Pink, remembered the night of 28 April:

  Well, I remember once getting a fantastic signal. I will give you an example—in H.M.S. Pink. We were about a hundred and fifty miles west-southwest of Iceland, approaching Greenland, and there was a moonlight night—going to be a moonlight night, a very nasty night, windy. And we received a signal from the Commander in Chief [Horton]: “You may expect attack from down moon at approximately 0200.” Now they knew and were able to interpret in Whitehall [the Admiralty] the various radio activity and signals by the German U-boats—great activity. They knew where the moon would be and when it would rise and where the U-boats might attack from—he liked a profile. And by the feverish increased activity of the radio signalling, they knew attacks were imminent. Now we didn’t know that, of course, but the fact that we had a signal telling us to be ready for attack about 0200 made all the difference. And Admiral Gretton, who was Commander Gretton then, we were so highly trained he sent a signal round to our escort group, and do you know what that signal said?—one word, “Anticipate,” that’s all he said. Didn’t get excited, and didn’t tell the men.to do this, or not do the other. It wouldn’t have been any good. We were trained; we knew what to do. And do you know what I did? It was about five o’clock, pitch black, windy as hell, and I said, “Hands to tea, six o’clock.” Cleared lower deck and said, “There’s going to be a hell of a battle tonight. I’m not sure how many of us will see daylight. I intend to see it if I can.” So it was up to us.29

  The night battle began earlier than Horton, or Atkinson, expected, at exactly 2358 when one of the four Star boats in the sea to port made the first of six attempted attacks that took place between that hour and daybreak on the 29th. Gretton wrote up his report of the action in unadorned telegraphic style:

  The first attempt was made at 2358, when SUNFLOWER on the port bow got an RDF [radar] contact bearing 170°, range 3000 yards. She ran out towards, but the U-Boat dived, and as no A/S [asdic] contact was obtained she dropped two charges and resumed station. TAY was in station by 0300 [29 April] and at 0045 (the second attempt) DUNCAN got an RDF contact bearing 100°—3500 yards and turned to attack. The U-Boat dived at 2500 yards and A/S contact was picked up at 1500 but almost at once lost. I dropped one charge, ran out and back over the firing position and was resuming station when at 0114 (the third attempt) I obtained an RDF contact bearing 296°—2500 yards. I chased at best speed until at 0119 at range 1100 yards, the U-Boat dived and I reduced to operating speed. RDF plot gave U-Boat’s course as 320° and at 0122 A/S contact was obtained on last RDF bearing, her wake was sighted, and an accurate ten-charge Minol [explosive charge] pattern was dropped. At 0130, while running out after this attack another RDF contact was obtained bearing 146°—4800 yards (the fourth attempt). I turned towards, chased and at 0140, the U-Boat dived at range 3000 yards. No A/S contact was obtained so one charge was dropped by plot. The chasing course was into the wind and seaspray was flying mast high and the U-Boat saw us coming earlier than when we had chased down sea.

  As I was turning to resume station after this attack, yet another RDF contact was picked up bearing 210°—4000 yards at 0156 (the fifth attempt) and again I turned and chased at best speed. The U-Boat was heading for the convoy at about twelve knots, but at 0204 at range 1500 yards, he dived and I reduced to fifteen knots. At 0203, the ship passed through a patch of oil about fifty yards diameter, so this U-Boat may have been previously damaged. Good A/S contact was obtained and an accurate ten-charge Minol pattern fired. At the moment of the firing, his wake was clearly seen under the port bow. Contact was regained astern, but lost at 800 yds each time I attempted to attack, so that the idea of a hedgehog [impact-fused bombs fired forward] attack had to be abandoned. I dropped two deep charges on him by plot and resumed station at high speed. At 0054, I had ordered TAY to take position R [port quarter] in my absence. I was back in station by 0310, and TAY ordered to resume position S [astern].

  SNOWFLAKE in position P [port beam] drove off the sixth attempt, at 0339, sighting a U-Boat on her port bow, range 1100 yards, steering towards the convoy. SNOWFLAKE attacked and fired two ten-charge patterns, a torpedo narrowly missing her in return. By 0348, SNOWFLAKE had dropped astern into the port quarter position so I moved up to the port beam in her place. At 0354, TAY, in position S, gained and attacked a good A/S contact—a possible but unlikely seventh attempt. By then, dawn was starting to break, and at 0416 I ordered day stations, so that ships would have plenty of time to gain bearing into the ahead stations for the expected dawn attack submerged. The night had been a busy one, the convoy unscathed, and I felt that the U-Boats must be discouraged by our night tactics and might try day attack.30

  The course of Convoy ONS.5 and the positions of U-boat patrol lines. Star, Specht, and Amsel are shown on the German grid chart. Nautical positions courtesy of Jürgen Rohwer. Cartographer: Paul Pugliese

  It was a good night’s work, and Gretton was elated. Against what he estimated were “five or six” U-boats the B7 team had performed splendidly. Not a single ship in the convoy had been sunk or damaged. But he was sure that the U-boats had suffered, and he was right: of the four boats that actually participated in the attacks, two, U-386 (Oblt.z.S. Hans-Albrecht Kandler) and U-528 (Oblt.z.S. Georg von Rabenau) were heavily damaged and forced to withdraw to base, though it is not possible to determine whether it was Duncan or one of the corvettes, Sunflower or Snowflake, that was responsible for either or both. The Type VIIC U-386 limped into home port at St.-Nazaire on II May. On the same day, the Type IXC/40 U—528, also struggling to return from what was her first combat patrol, was approaching the Bay of Biscay when she was bombed by Halifax II �
�D” of 58 Squadron and finished off by H.M.S. Fleetwood and Mignonette, escorting convoy OS.47, with the loss of eleven killed, forty-five captured.31 Gretton certainly thought he had a kill on the third U-boat attempt, which he called “an accurate attack,” and this may have caused damage to one of the two boats cited. In any event, it was, as he said, “a most successful night.”

  On her maiden escort with the Group, Duncan had detected and attacked four separate U-boat advances in the space of one hour and fifty minutes, and in weather conditions where the ship was pitching and rolling wildly, and seas washing down the quarter deck made the work of loading and reloading the heavy D/Cs both difficult and dangerous.32 Gretton used the loud-hailer to praise the behavior of his ship’s company during their first baptism of fire. The two corvettes had also given a good account of themselves, although each committed an error: In dropping two D/Cs at 2208 to scare off any U-boats in her vicinity, Sunflower inadvertently dropped a calcium flare that lit up the entire seascape—“rather an unnecessary advertisement,” as her Canadian Captain, Lt.-Cmdr. J. Plomer, R.C.N.V.R., dryly put it. Sunflower turned back to extinguish the light and, on a second attempt, by going hard to starboard, drew it through the propeller stream.33.