Black May (20 page)

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 6
1
°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:

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 yards 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:

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: