ONS 5 was the 5th of the numbered ON/ONS convoys of Slow trade convoys Outbound from the British Isles to North America. The North Atlantic battle around it in May 1943 is regarded as the turning point of the Battle of the Atlantic in World War II. The battle ebbed and flowed over a week and involved more than 50 Allied ships and their escorts, against more than 30 U-boats. It saw severe losses on both sides and was almost the last Allied convoy to do so. Losses inflicted on the U-boats became a besetting feature of the campaign. It is seen as the point when the tactical and strategic advantage passed to the Allies, ushering in the period known to the Kriegsmarine as Black May.
Background Convoy Convoy ONS 5 consisted of 43 ships bound from Liverpool to Halifax. The ships were either in ballast or carrying export goods. The convoy departed Liverpool on 21 April 1943, and would arrive in Halifax three weeks later on 12 May. The Convoy Commodore was J. K. Brook RNR, travelling in the Norwegian freighter Rena.Seth (1961) p. 69 The escort was provided by Escort Group B7 (Captain Peter Gretton) of the Mid-Ocean Escort Force, comprising the destroyers , , the frigate and the corvettes , , and . The group also contained the trawlers, and as rescue ships, and the fleet oiler for re-fuelling. The convoy was joined by other escort vessels as the battle progressed. Convoy ONS 5 was one of several Allied convoys at sea at the end of April; also in the Western Approaches were the outboud Convoy ON 180, and the inbound Convoy HX 234. Approaching the Americas were Convoy ONS 4 and Convoy ON 179; departing was Convoy SC 128, while in mid-Atlantic, due to pass Convoy ONS 5 east of Greenland, was Convoy SC 127. Two other eastbound convoys, Convoy HX 235 and Convoy HX 236, were also in mid-Atlantic, following a southerly route; over 350 ships were in the north Atlantic.
U-Boats
Ranged against the convoy were 58 U-boats in 3 patrol lines, (Woodpecker) with 17 boats south of Greenland on the western side of the Air Gap, (Blue Tit) with 30 boats east of Greenland covering the northern route, and (Blackbird) with 11 boats, south of Meise covering the southern route. Meise had been assembled to catch Convoy SC 127, which had been identified by B-Dienst, but on 26 April the convoy had slipped through a gap in the line and escaped undetected. On 27 April, BdU re-organised and the boats furthest east formed a new (Starling), of sixteen U-boats, in a north–south line (a rake) at 30°W between latitudes 61°50′ and 57°00′N, roughly east of Greenland. The boats were to be ready by the morning of 28 April to intercept the next ON convoy on the eight-day cycle and the northern end of the line was adjacent to the track of Convoy ONS 5. In the morning of 28 April the convoy was at 61°45′N, 29°11′W, turning onto a south-westerly heading. By the morning of 29 April it would pass through 30°W to 34°51′W.
Ships involved
Convoy ONS 5 comprised 42 ships and 16 escorts (including reinforcements and detachments) 13 ships were sunk during the seven days that the convoy was under attack. Gruppe Star and comprised 43 U-boats, not all in contact throughout; six were sunk.
Action Merchant ships departing Liverpool on 21 April 1943 were met by Escort Group B7 at 2:00 p.m. on 22 April and the convoy formed up in high winds and a heavy sea. At 10:00 p.m., the Polish freighter Modlin had to turn back with engine trouble. Station-keeping for the remaining ships became increasingly difficult as weather worsened on 23 April.Seth (1961) pp.77–79
24 April At 4:55 p.m. in a moderate gale, B-17 C of 206 Squadron, from Benbecula, dropped six depth charges on the swirl of a diving U-boat. B-17 D found on the surface less than an hour later and sank it with depth charges as the U-boat attempted to fight back rather than submerge. U-710 was just ahead of the convoy, though probably unaware of its proximity.Seth (1961) pp. 83–85
25–27 April Freighters Bornholm and Berkel collided on the evening of 25 April as a moderate west-northwest gale reduced convoy speed to two or three knots. Bornholm began taking on water and left the convoy the next day for Iceland. Vidette joined the convoy with three merchant ships from Iceland on 26 April while rescue trawler Northern Spray was standing by the straggling freighter Penhale until the straggler was ordered into Reykjavík so the trawler could rejoin the convoy. At noon on 26 April, BdU changed the Enigma machine cypher being used to transmit instructions to the U-boats. Allied intelligence services were unable to decrypt message traffic until the afternoon of 5 May.Gannon (1998) p. 127 The weather moderated enough for Duncan, Vidette and Loosestrife to refuel from the escort oiler British Lady on 27 April and a salvage tug from Iceland rescued Bornholm that evening.Seth (1961) pp. 85–88
28–29 April On 28 April, Convoy ONS 5 arrived at the Star patrol area and was sighted at 9:00 a.m. by U-650, that held contact despite being forced to dive to avoid approaching aircraft at 10:14 a.m., 11:50 a.m. and 3:18 p.m. By nightfall U-650 had been joined by U-375, U-386, U-528 and U-537. The U-boats' contact reports alerted Gretton to the presence of U-boats on the convoy's port bow, beam, quarter and astern. Duncan and Tay made an unsuccessful depth charge attack after sighting a U-boat on the port bow at 6:30 p.m.Seth (1961) pp. 90−92
Gretton mounted a vigorous defence as the U-boats attacked after dark. At 8:00 p.m. Sunflower gained a radar contact at , on closing lost the radar contact, gained a doubtful ASDIC contact, and dropped two depth charges. At 10:45 p.m. Duncan detected a radar contact at and upon closing lost the radar contact, gained an ASDIC contact at , lost contact at and dropped one depth charge. Upon returning to station Duncan got a radar contact at and upon closing sighted a U-boat which dived at and appeared on ASDIC at . Duncan dropped a pattern of ten depth charges and while turning for another attack, gained another radar contact. The radar contact disappeared at a range of . Duncan made no ASDIC contact, but dropped a depth charge at the estimated diving position before making another radar contact at . As Duncan closed, the U-boat dived at a range of . Duncan gained a good ASDIC contact and dropped a pattern of ten depth charges over a visible wake. Two more depth charges were dropped when a weak ASDIC contact was regained at 12:45 a.m.
At 1:32 a.m. Snowflake approached a hydrophone contact, and U-532 was detected visually and on radar at a range of . U-532 launched six torpedoes. The closest one missed Snowflake by about . After U-532 dived, Snowflake dropped three depth charges on the initial ASDIC contact and ten depth charges when contact was regained at . Snowflake regained ASDIC contact at and dropped another pattern of ten depth charges. A short time later Tay dropped depth charges on a good ASDIC contact astern of the convoy.Seth (1961) pp. 94–98 U-532 returned to base to repair depth charge damage. U-386 and U-528 were also damaged by these attacks and forced to return to base. U-386 arrived at St Nazaire on 11 May but U-528 was attacked in the Bay of Biscay and sunk by aircraft on the same day.Gannon (1998) pp. 136–138
U-258 and U-650 maintained contact through the night and U-258 was submerged ahead of the convoy at dawn on 29 April. As the convoy passed overhead at 5:30 a.m., U-258 rose to periscope depth and launched two torpedoes at McKeesport. After one torpedo hit McKeesport on the starboard bow, Northern Gem detected U-258 and dropped three depth charges. Snowflake dropped a depth charge on a doubtful Asdic contact at 6:05 a.m. and two more depth charges at 6:15 a.m. after contact was regained at a range of . McKeesport was abandoned, Northern Gem rescued all but one of the crew and it was scuttled by the escort.Seth (1961) pp. 99–105 U-258 had also been damaged and was forced to return to base.
The Admiralty arranged reinforcements for Convoy ONS 5. was detached from Convoy SC127 and the destroyers , , and , of the 3rd Support Group (Capt. J. M. McCoy) sailed from Newfoundland. The weather rapidly deteriorated and the convoy was sailing into a full gale by late afternoon of 29 April. About 5:00 p.m. Sunflower was struck by a wave which filled the crow's nest with water. Oribi was slowed to by the storm, but joined the convoy at 11:00 p.m. as Tay was attacking a U-boat astern. At 11:12 p.m. Duncan obtained an ASDIC contact at Duncan and Snowflake dropped depth charges to discourage the U-boats.
30 April The convoy found itself making less than headway into a Force 10 gale. The convoy started to be scattered, some ships ending up from the convoy, and the escorts were kept busy rounding up stragglers. Oribi was able to refuel from the convoy oiler when the storm abated briefly on 30 April before the weather again made re-fuelling impossible, and a number of the destroyers became low on fuel, throwing doubt on whether they could continue. At 11:05 p.m. Snowflake made a radar contact at and dropped a depth charge after the U-boat dived when illuminated by star shell. The escorts dropped some random depth charges until dawn and Admiral Dönitz cancelled the chase on the evening of 1 May.Seth (1961) pp. 105–108
Regrouping 1–3 May On 1 May Dönitz ordered boats from Star and Specht, with some newcomers to form a new patrol line to the west. This was group Finke (Finch) which was in place on 3 May numbering 27 boats to intercept the westbound Convoy SC 128. The 3rd Support Group destroyers joined the convoy at 1:00 a.m. 2 May, but the fuel situation aboard destroyers became increasingly desperate as weather and frequent course adjustments to avoid icebergs prevented refuelling. At 2:00 p.m. on 3 May Gretton was forced to take Duncan to St John's at economical speed [] and he arrived with only 4 per cent of the fuel remaining. Command was assumed by Lt-Cdr R.E. Sherwood, of HMS Tay. The Asdic set aboard Tay failed just as Sherwood took over. Impulsive also detached to Iceland at 7:00 p.m. on 3 May, with Northern Gem carrying the survivors from McKeesport, while Penn and Panther detached for Newfoundland at 6:00 a.m. on 4 May.Seth (1961) pp. 110–114
4 May By 4 May the weather had abated to Force 6, and the convoy was making up to , though reduced to 30 ships and 7 escorts. The rest were scattered and proceeding independently, including a group of four with Pink, trailing behind the main body. The 1st Support Group sailed from Newfoundland at noon with the frigates Wear, Jed, Spey and the sloops Pelican and Sennen to replace Oribi and Offa whose fuel state would become dangerously low on 5 May. U-628 of group Finke, assembled to catch Convoy SC 128, sighted Convoy ONS 5 at 8:18 p.m. Two of the gathering group Finke U-boats were attacked by RCAF Cansos. One U-boat, thought to have been U-630 was sunk but is now believed to have been U-209, which was damaged in an attack by Canso W and foundered later while attempting to return to base. The other, U-438, was only slightly damaged in attacks by Canso E.Seth (1961) pp. 114–121
At 10:20 p.m. Vidette detected U-514 on radar at and approached until U-514 dived when the range dropped to . Vidette fired a pattern of 14 depth charges causing damage that put U-514 out of the battle until 7 May. North Britain was straggling astern of the convoy and sank within two minutes of being torpedoed by U-707 at 10:37 p.m. Vidette detected U-662 on radar at and upon closing, sighted U-732 at . The conning tower was still visible at and a pattern of 14 depth charges dropped by eye caused damage requiring U-732 to return to base.
5 May