Convoy ONS 5 ONS 5 was the 5th of the numbered ONS series of Slow trade convoys Outbound from the British Isles to North America . The North Atlantic battle surrounding 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 period of a week , and involved more than 50 Allied ships and their escorts , and over 30 U - boats . It saw heavy losses on both sides . However it was almost the last Allied convoy to do so , while losses inflicted on attacking U - boats and U - boat groups became a besetting feature of the campaign ; As such it is seen as the point when the tactical and strategic advantage passed to the Allies , and ushered in the period known to the German Navy as Black May . Background . ONS 5 consisted of 43 ships bound from Liverpool to Halifax . The ships were either in ballast or carrying trade and export goods . The convoy departed Liverpool on 21 April 1943 , and would arrive in Halifax three weeks later on 12 May . It was under the command of JK Brook RNR as Convoy Commodore , traveling in the Norwegian freighter " Rena " . The escort was provided by Mid - Ocean Escort Force group B7 , 7 warships under Captain Peter Gretton , in the destroyer " Duncan " . Also in the group were the destroyer " Vidette " , frigate " Tay " , and corvettes " Sunflower " , " Snowflake " , " Loosestrife " , and " Pink " . The group also contained 2 trawlers , " Northern Gem " and " Northern Spray " as rescue ships , and the fleet oiler " British Lady " for mid - ocean re - fuelling . The convoy was joined by other escort vessels as the battle progressed . ONS 5 was just one of the allied convoys at sea at the end of April ; also in the Western Approaches were ON 180 , just leaving , and HX 234 , just arriving . Approaching the Americas were ONS 4 and ON 179 ; departing was SC 128 , while in mid - Atlantic , due to pass ONS 5 east of Greenland , was SC 127 . Two other east - bound convoys , HX 235 and HX 236 , were also in mid - Atlantic , following a southerly route . This accounted for over 350 ships on the move in the north Atlantic at that time . Ranged against them were 58 U - boats in 3 patrol lines ; " Specht " ( Woodpecker ) with 17 boats south of Greenland on the western side of the Air Gap ; " Meise " ( Bluetit ) with 30 boats east of Greenland covering the northern route , and " Amsel " ( Blackbird ) with 11 boats , south of " Meise " covering the southern route . " Meise " had been deployed to catch SC 127 , which had been identified by B - Deinst , but on 26 April SC 127 had slipped through a gap in the line and escaped undetected . Realizing what had happened on 27 April , and aware that a slow west - bound convoy was imminent , " Meise " was re - configured ; the easternmost boats ( 16 in all ) formed the patrol line " Star " ( Starling ) to intercept it . At 8am on 28 April " U - 650 " sighted ONS 5 , and group " Star " quickly gathered for the attack . Ships involved . ONS 5 comprised 42 ships and 16 escorts , ( though not all were present at the same time ) ; 13 ships were lost in the course of the 7 day engagement . Wolfpacks " Star " and " Finke " comprised 43 U - boats ( though again , not all had been in contact throughout ) ; 6 of these were lost . Action . Merchant ships departing Liverpool on 21 April 1943 were met by Escort Group B7 at 1400 on the 22nd ; and the convoy formed up in high winds and a heavy sea . At 2200 , the Polish freighter " Modlin " had to turn back with engine trouble . Station - keeping for the remaining ships became increasingly difficult as weather worsened on the 23rd . 24 April . At 1655 on the 24th , in a moderate gale , Fortress C of 206 Squadron RAF from Benbecula , dropped six depth charges on the swirl of a diving U - boat . Fortress D found " U - 710 " 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 10 miles ahead of the convoy , though probably unaware of its proximity . 25 – 27 April . Freighters " Bornholm " and " Berkel " collided on the evening of the 25th as a moderate west - northwest gale reduced convoy speed to two or three knots . " Bornholm " began taking on water and left the convoy on the 26th attempting to reach Iceland . " Vidette " joined the convoy with three merchant ships from Iceland on the 26th 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 the 26th , 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 . The weather moderated enough for " Duncan " , " Vidette " and " Loosestrife " to refuel from the escort oiler " British Lady " on the 27th and a salvage tug from Iceland rescued " Bornholm " that evening . Battle of 28 – 29 April . On the 28th ONS 5 arrived at the " Star " patrol area , and was sighted at 0900 by " U - 650 " . " U - 650 " held contact despite being forced to dive three times to avoid approaching aircraft at 1014 , 1150 and 1518 . By nightfall " U - 650 " had been joined by " U - 375 " , " U - 386 " , " U - 528 " and " U - 537 " . The U - boats ' contact reports alerted Commander Gretton to the presence of U - boats on the convoy ' s port bow , beam , and quarter and astern . " Duncan " and " Tay " made an unsuccessful depth charge attack after sighting a U - boat on the port bow at 1830 . Gretton mounted a vigorous defence as the U - boats attacked after dark . At 2000 " Sunflower " detected a RADAR contact at ; and upon closing lost the RADAR contact , gained a doubtful ASDIC contact , and dropped two depth charges . At 2245 " 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 " detected 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 single 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 0045 . At 0132 " 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 . " 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 safely at St Nazaire on 11 May , but " U - 528 " was attacked in the Bay of Biscay and sunk by aircraft on the same day . " U - 258 " and " U - 650 " maintained contact through the night , and " U - 258 " was submerged ahead on the convoy at dawn on the 29th . As the convoy passed overhead at 0530 , " 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 single depth charge on a doubtful SONAR contact at 0605 and two more depth charges at 0615 after contact was regained at a range of . " McKeesport " was abandoned and sunk by the escort to prevent discovery of classified documents by German boarders . " Northern Gem " rescued all but one of the crew . " U - 258 " had also been damaged , and was forced to return to base . The Admiralty arranged reinforcements for ONS 5 in response to this battle . HMS " Oribi " was detached from SC127 , and destroyers " Penn " , " Panther " , " Impulsive " , and " Offa " , of the 3rd Support Group under Capt . J . M . McCoy , RN , sailed from Newfoundland . Weather rapidly deteriorated , and the convoy was sailing into a full gale by late afternoon of the 29th . About 1700 " Sunflower " was struck by a wave which filled the crow ' s nest with water . " Oribi " was slowed to 11 knots by the storm , but joined the convoy at 2300 as " Tay " was attacking a U - boat astern . At 2312 " Duncan " obtained an ASDIC contact at ; and both " Duncan " and " Snowflake " dropped depth charges to discourage the U - boats . 30 April . ONS 5 found itself making less than 3 knots headway into a Force 10 gale . The convoy started to be scattered , some ships ending up 30 miles 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 the 30th before the weather again made re - fuelling impossible , and a number of the destroyers became so low on fuel as to throw doubt on whether they could continue . At 2305 " Snowflake " made a RADAR contact at and dropped a single 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 . 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 , and tasked with intercepting westbound convoy SC128 . The 3rd Support Group destroyers joined the convoy at 0100 2 May , but the fuel situation aboard destroyers became increasingly desperate as weather and frequent course adjustments to avoid icebergs prevented refueling . At 1400 on the 3rd Gretton was forced to take " Duncan " to St John ' s at economical speed ( 8 knots ) ; and he arrived there with only 4 percent fuel remaining . In Gretton ' s absence , command was assumed by Lt - Cdr RE Sherwood , of HMS " Tay " . The SONAR set aboard " Tay " failed just as Sherwood assumed command of the escort group . " Impulsive " also detached to Iceland at 1900 3 May , with " Northern Gem " carrying the survivors from " McKeesport " , while " Penn " and " Panther " detached for Newfoundland at 0600 4 May . 4 May . By 4 May the weather had abated to Force 6 , and ONS 5 was now making up to 6 knots , though reduced to 30 ships and 7 escorts . The rest were scattered and proceeding independently , including a group of four with " Pink " , trailing some 80 miles behind the main body . The 1st Support Group sailed from Newfoundland at midday with frigates " Wear " , " Jed " , " Spey " and sloops " Pelican " and " Sennen " to replace " Oribi " and " Offa " whose fuel state would become critical on the 5th . " U - 628 " of group " Finke " , assembled to catch convoy SC 128 , sighted convoy ONS 5 at 2018 . Two of the gathering group " Finke " U - boats were attacked by RCAF Cansos in separate incidents . One 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 . At 2220 " Vidette " detected " U - 514 " on RADAR at and approached until " U - 514 " dived when the range dropped to . " Vidette " punished " U - 514 " with a pattern of 14 depth charges causing damage putting " 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 2237 . " Vidette " detected " U - 662 " on RADAR at and , upon closing , sighted " U - 732 " at . The conning tower was still visible at a range of ; and a pattern of 14 depth charges dropped by eye caused damage requiring " U - 732 " to return to base . 5 May . " U - 264 " and " U - 628 " each launched five torpedoes shortly after midnight . " Harbury " was hit at 0046 ; " Harperly " was hit by two torpedoes at 0104 ; and " West Maximus " was hit by one torpedo at 0103 , another at 0110 , and a third at 0135 . Both U - boats claimed three ships ; but modern historians credit the first freighter to " U - 628 " and the other two to " U - 264 " . One of the torpedoes passed within of " Snowflake " . At 0122 " Snowflake " started closing a RADAR contact illuminated by star shells fired by " Oribi " ; and both ships dropped depth charges . The gunfire encouraged " U - 264 " to dive , and the depth charges forced " U - 270 " to return to base . ) . " U - 358 " torpedoed " Bristol City " at 0225 , and " Wentworth " at 0230 . At dawn , " Lorient " was missing from the convoy . No witnesses to her destruction survived the battle . Before " U - 125 " was sunk , she sent a radio report about sinking a steamship sailing independently ; and modern historians assume " Lorient " straggled from the convoy and was torpedoed by " U - 125 " . " Northern Spray " picked up 143 survivors from " North Britain " , " Harbury " , " Harperly " , and " West Maximus " by 0700 and was detached to take the rescued men to Newfoundland . " Loosestrife " assumed the role of rescue ship and picked up the survivors from " Bristol City " and " Wentworth " . At 1057 " Oribi " sighted a surfaced U - boat at . " U - 223 " , " U - 231 " , " U - 621 " , and " U - 634 " dived as " Oribi " approached . " Oribi " dropped 14 depth charges after the U - boats dived . " U - 638 " torpedoed " Dolius " at 1240 . " Sunflower " gained an ASDIC contact at within minutes and destroyed " U - 638 " with a pattern of ten depth charges before rescuing survivors from " Dolius " . " Tay " , " Oribi " , and " Offa " refueled from convoy oilers that afternoon . " Selvistan " , " Gharinda " , and " Bonde " were hit by a salvo of four torpedoes from " U - 266 " within the space of a few minutes about 1950 . " Selvistan " and " Bonde " sank within two minutes . " Tay " rescued survivors from the three ships while " Offa " made depth charge attacks damaging " U - 266 " , which was sunk by aircraft on 15 May while attempting to reach base for repairs . At midday , " Pink " made a firm ASDIC contact ahead of her small convoy proceeding separately . " Pink " spent 90 minutes making five depth charge and hedgehog attacks . " Pink " received post - war credit for destruction of " U - 192 " ; but later analysis concluded the victim , " U - 358 " , returned to base after being damaged . " U - 584 " torpedoed " West Makadet " while " Pink " was attacking " U - 358 " . " Pink " rescued the survivors . Night of 5 – 6 May . As May 5 faded into darkness , " Tay " counted seven U - boats surfaced in the convoy ' s path ; but ONS 5 was entering the fog formed where the warm Gulf Stream meets the cold Labrador Current off the Grand Banks of Newfoundland . Visibility dropped to by 2202 and to by 0100 . British centimetric RADAR enabled the escorts to see while the U - boats could not . Many of the U - boats involved never returned to base to file their reports ; so historians still struggle to correlate individual reports of the dozens of ships interacting briefly in no fewer than 24 attempted attacks on the night of 5 / 6 May . At 2309 " Vidette " made a RADAR contact at , and a second appeared while closing the first . " Vidette " dropped a pattern of ten depth charges on a submarine seen submerging ahead , and then moved on to drop a pattern of five depth charges on the second contact which became visible at . Historians suggest the first attack destroyed " U - 531 " . At 0030 " Loosestrife " made a RADAR contact at . The U - boat turned away when the range reached and fired two torpedoes at " Loosestrife " from its stern tubes while diving . " Loosestrife " dropped a pattern of ten depth charges as it overran the diving U - boat . A reported slick of oil and debris is believed to have been produced by destruction of " U - 192 " . At 0252 " Oribi " collided with " U - 125 " first seen at a range of while investigating an ASDIC contact , but lost contact after the collision . While pursuing an ASDIC contact , " Snowflake " detected " U - 125 " on RADAR at 0354 , observed heavy conning tower damage by searchlight at a range of , and watched the crew detonate scuttling charges and abandon ship . The escorts chose to continue protective patrolling around the convoy rather than attempting rescue of the U - boat crew assumed to have sunk the " Lorient " . At 0406 " Vidette " made an ASDIC contact at , and made a hedgehog attack causing two explosions . Historians suggest this attack destroyed " U - 630 " . At 0443 " Sunflower " made an ASDIC contact at and subsequently sighted a surfacing U - boat . " Sunflower " rammed " U - 533 " and dropped two depth charges as " U - 533 " attempted to dive . Both " Loosestrife " and " U - 533 " were able to repair damage and remain at sea . At 0552 " Pelican " was leading the 1st Support Group to reinforce the convoy escort when it detected a RADAR contact at . " Pelican " made visual contact at , dropped a pattern of ten depth charges where the U - boat dived , and dropped a second pattern of nine depth charges after regaining contact . Historians suggest these attacks destroyed " U - 438 " . " Finke " had already outlasted its usefulness , and faced mounting losses if the attack continued . Realizing his mistake , Dönitz called off the assault on 6 May and ordered " Finke " to retire . Conclusion . In the course of a week , ONS 5 had been the subject of attacks by a force of over 40 U - boats . With the loss of 13 ships totalling 63 , 000 tons , the escorts had inflicted the loss of 6 U - boats , and serious damage on 7 more . This battle demonstrated that the convoy escorts had mastered the art of convoy protection ; the weapons and expertise at their disposal meant that henceforth they would be able not only to protect their charges and repel attack , but also to inflict significant losses on the attacker . ONS 5 marked the turning point in the battle of the Atlantic . Following this action , the Allies inflicted a series of defeats and heavy losses on the U - boat Arm , a period known as Black May . This culminated in Dönitz withdrawing his forces from the North Atlantic arena . The official historian , Stephen Roskill commented : “ This seven day battle , fought against thirty U - boats , is marked only by latitude and longitude , and has no name by which it will be remembered ; but it was , in its own way , as decisive as Quiberon Bay or the Nile ” . ,