'HX' was the designation of Allied slow convoys (together with a numerical and sometimes a literal suffix) plying from Halifax, Nova Scotia (New York from September 1942) to Liverpool in the UK, and as such reciprocals of the 'ON' series (September 1939/June 1945).

There were 377 of these convoys. which absorbed the identically suffixed BHX convoys from Bermuda en route, for ships able to maintain a cruising speed of at least 9 kt. The convoys eventually involved about 20,000 ships. Some 38 of the 377 convoys came under attack, resulting in the loss of 110 ships within the convoy, another 60 were lost after straggling, and 36 were lost while detached or after dispersal. With loses from accidents and other causes, the total loss was 206 ships, or about 1% of the total.

The HX designation perpetuated a similar series which was operated in World War I during 1917 and 1918. The HX convoys were organised right at the start of the Battle of the Atlantic and ran without major changes until the end of the war as the longest continuous convoy series of World War II. The convoys of the HX series were initially considered to be fast convoys, and comprised merchant able to cruise at between 9 and 13 kt. A parallel series of slow convoys, the SC series, was run for ships unable to cruise at 8 kt, while ships able to cruise at faster than 13 kt sailed independently until the 14-knot CU convoys were organised late in 1943. The largest convoy of World War II was HX.300, which sailed for the UK via New York on 17 July 1944, with 167 merchant ships and arrived in the UK without incident on 3 August 1944.

The first of the series was HX.1 of 16/30 September 1939 with 18 merchant vessels and four escorts, and the last was HX.358 of 23 May/6 June 1945 with 59 merchant vessels and 23 escorts.

Of the 40 Allied convoys which lost more than six ships during the Battle of the Atlantic, five were convoys of the HX series. Arguably the most significant of these convoys were HX.79, HX.84, HX.106, HX.112, HX.156, HX.212, HX.228 and HX.229.

The HX.79 convoy of 8/23 October 1940 with 49 merchant vessels and 16 escorts came under severe U-boat attack on 19/20 October in the Western Approaches and suffered major losses. Together with the attack on the SC.7 convoy during the the previous day, this represented the worst two-day shipping losses of the whole Battle of the Atlantic.

Bound for Liverpool, the 49 ships of the HX.79 convoy were laden with war materials, and on 19 October, four days from landfall, the convoy entered the Western Approaches and caught up with the slower SC.7, which was already under attack. The escort for the crossing had been small even by the limited standards of the day, comprising the armed merchant cruisers Alaunia and Montclare against the possibility of attack by a surface raider, but these had departed as the convoy approached British waters. Alarmed by the weight of the attack on the SC.7 convoy, the Admiralty had ordered the despatch of a notably strong local escort, which was scheduled to join the convoy during the morning of 19 October. Under the command of Lieutenant Commander A. B. Russell, this comprised the destroyers Sturdy and Whitehall, corvettes Arabis, Coreopsis, Heliotrope and Hibiscus, minesweeper Jason, anti-submarine trawlers Angle, Blackfly and Lady Elsa, and Free Dutch submarine O-14.

Before the local escort arrived, the HX.79 convoy was sighted by Kapitänleutnant Günther Prien’s U-47. Already an 'ace', Prien immediately sent a sighting report and began to shadow the convoy, while Vizeadmiral Karl Dönitz ordered the assembly of a pack to attack the convoy. The U-boats which had attacked SC.7 and were still able to fight (three had departed to re-arm after expending all their torpedoes) were directed to the scene. The four boats which joined U-47 during the day were Kapitänleutnant Joachim Schepke’s U-100, Oberleutnant Engelbert Endrass’s U-46, Kapitänleutnant Heinrich Bleichrodt’s U-48 and Kapitänleutnant Heinrich Liebe’s U-38.

Undeterred by the size of the escort, the U-boats moved in to the attack as night fell, using the darkness to cover an approach on the surface. Prien penetrated the escort screen from the south to attack from within the convoy, while Endrass, who had learned his trade as Prien’s first officer), did the same from the north.

Over the six-hour period which followed in the night of 19/20 October, the five U-boats torpedoed 14 ships, of which which 12 sank, for a total of 75,089 tons. Liebe’s U-38 sank the 7,653-ton British Matheran and 6,856-ton Dutch Bilderdijk; Endrass’s U-46 sank the 4,548-ton British Ruperra and 9,965-ton Swedish Janus; Prien’s U-47 sank the 4,966-ton British Uganda, 4,947-ton British Wandby, 5,185-ton British La Estancia and 5,026-ton British Whitford Point, and damaged the 6,023-ton British Shirak and 8,995-ton British Athelmonarch; Bleichrodt’s U-48 sank the already damaged Shirak; and Schepke’s U-100 sank the 8,230-ton British Caprella, 6,218-ton British Sitala and 5,452-ton British Loch Lomond, which was the convoy’s rescue ship.

None of the attacking U-boats was damaged.

Despite its strength, the escort was ineffective largely as a result of the fact that, at this comparatively early stage of the war, the ships were unco-ordinated at the tactical level, being unused to working together, and having no common battle plan or tactics; the escorts had arrived singly, being dispatched as and when available, this being the common practice at the time; escort command fell to the senior officer present, and could therefore change as each new ship arrived; ny tactical arrangements had to be made on the spot, and communicated by signal lamp to each ship in turn; and, finally, the presence of an Allied submarine was less than helpful: O-14 had no targets, and was twice attacked by mistake by other escorts.

The failure of such a substantial escort led to a number of changes in escort policy. The first of these to take effect was the formation of escort groups, which were groupings of escort ships which operated together under specified leadership. This paved the way to the development of consistent tactics and teamwork, and thus an increasing level of capability.

The 38 laden ships of the HX.84 convoy departed Halifax for Liverpool on 28 October 1940 under escort of Captain E. S. F. Fegen’s armed merchant cruiser Jervis Bay. On 5 November 1940, Kapitän Theodor Krancke’s German pocket battleship Admiral Scheer located the convoy and immediately attacked. Fegen took his Jervis Bay straight into an attack on the raider, which he hoped to delay and thereby give the convoy time to escape. The armed merchant cruiser was completely outclassed in terms of armament and protection, and was quickly sunk with the loss of 190 of her crew. Nevertheless, the sacrifice of Fegen and his crew did indeed give the convoy an opportunity to scatter, allowing all but six of the merchant ships to escape.

The 7,908-ton British Maiden, 5,201-ton British Trewellard, 5,225-ton British Kenbame Head, 10,042-ton British Beaverford and 4,955-ton British Fresno Bay were sunk and the 8,073-ton British tanker San Demetrio was damaged, but failing light now allowed the rest of the convoy to escape. San Demetrio was abandoned by her crew, but two days later some of the crew, now in lifeboats, sighted the tanker, still afloat and still ablaze, reboarded her, got the engines running and brought her in to port.

The battle of the HX.112 convoy resulted in the loss of two of Germany’s most successful U-boat 'aces', Korvettenkapitän Otto Kretschmer and Kapitänleutnant Joachim Schepke, together with their boats, U-99 and U-100 respectively, on 15/17 March 1941. The convoy departed Halifax, Nova Scotia, on 1 March 1941 for Liverpool in the UK with 41 ships laden with war matériel; many of the ships were tankers carrying fuel oil. The convoy was escorted by Commander D. G. F. W. MacIntyre’s 5^th Escort Group, which consisted of two destroyers, MacIntyre’s own Walker and Vanoc, and the corvettes Bluebell and Hydrangea. Because of the cargo’s importance, the 5^th Escort Group was reinforced on this occasion by three more destroyers, Sardonyx, Scimitar and Volunteer, and rendezvoused with the convoy as it entered the Western Approaches.

On 15 March the convoy was sighted by Kapitänleutnant Fritz-Julius Lemp’s U-110 which radioed a sighting report and started to shadow the convoy. U-110 was joined during the day by four other boats, namely Kretschmer’s U-99, Schepke’s U-100, Korvettenkapitän Asmus Nicolai Clausen’s U-37 and Korvettenkapitän Eitel-Friedrich Kentrat’s U-74. The German boats began their attack on the night of 15/16 March. U-100 was able to torpedo the 9,314-ton British tanker Franche Comté, which burst into flames but survived to reach port, and the other attacks that night were frustrated by the escorts. Keeping up with the convoy on the surface during the day, the German wolfpack tried again as night fell on 16 March. U-99 managed to penetrate the convoy from the north, on its port side, and sank four vessels (8,136-ton Norwegian tanker Beduin, 7,375-ton Canadian J. B. White, 5,728-ton British Venetia and 6,593-ton Norwegian tanker Ferm) in less than one hour. Remaining with the central column of the convoy the boat sank another freighter, the 6,673-ton Swedish Korshamn, 15 minutes later before making its getaway.

Meanwhile the escorts, searching for U-boats outside the convoy perimeter, found U-100 around 01.30 as the boat moved in on the surface. U-100 dived, but Walker attacked with a depth charge pattern at close range. U-100 evaded damage and surfaced, only to be sighted and rammed by Vanoc just after 03.00: Schepke was killed and U-100 went down with most of her crew.

As this was happening, U-99 was making its escape. The boat nearly collided with a destroyer in the dark and dived. Picked up on sonar by Walker, the U-boat was depth-charged and severely damaged. Kretschmer brought his boat to the surface, where it came under fire from the British warships. U-99 was sunk, but Kretschmer and most of his crew survived and were taken prisoner.

There were no further attacks on HX.112, and the convoy reached Liverpool on 20 March. HX.112 had lost six ships totalling 50,000 tons, but the German loss of two U-boat 'aces', one of them the highest scoring submarine commander of World War II, was a severe blow to the U-boat offensive. The defence of HX.112, coupled with the successful defence of OB.293 and the loss of another U-boat 'ace', Korvettenkapitän Günther Prien, during the preceding week, marked a minor turning point in the Atlantic campaign.

The HX.156 convoy of 43 laden ships departed Halifax on 22 October 1941 for Liverpool, and was met two days later at sea by the US Navy’s Task Unit 4.1.3, comprising the destroyers Niblack, Reuben James, Tarbell, and Benson class destroyers Benson and Hilary P. Jones. Kapitänleutnant Erich Topp’s U-552 sighted the convoy at dawn on 31 October 1941, and torpedoed Reuben James as the destroyer approached to investigate the HF/DF bearing of the sighting report. The torpedo struck the port side and detonated the forward magazine; the hull aft of the third stack remained afloat for five minutes, and 44 of the 159-man crew were rescued. Reuben James was the first US warship to be sunk in World War II, some five weeks before the Japanese 'Ai' attack on Pearl Harbor brought the USA formally into the war.

U-552 was driven off by the escort, but Kapitänleutnant Engelbert Endrass’s U5-67 relocated the convoy during the afternoon of the same day. TU4.1.3 handed responsibility for the convoy to the British 6^th Escort Group on 1 November. On this day U-552 and U-567 made two unsuccessful torpedo attacks on 1 November, and maintained contact with the convoy to 3 November, and the convoy reached Liverpool on 5 November without suffering any losses.

The HX.212 convoy of 45 laden ships departed New York City on 18 October 1942 and was met on 23 October by the Mid-Ocean Escort Force’s US Escort Group A3, which comprised the US coast guard cutter Campbell, destroyer Badger and corvettes Dianthus, Rosthern, Trillium, Dauphin, Alberni, Summerside and Ville de Quebec. The first six escorts had worked together previously, but the last three were attached to the escort only for passage to the eastern Atlantic in preparation for assignment too the naval forces supporting 'Torch'. Summerside was the only escort equipped with modern Type 271 centimetric-wavelength radar.

As the western Atlantic coastal convoys brought an end to the 'second happy time' of 'Paukenschlag', Admiral Karl Dönitz, the Befehlshaber der Unterseeboote, shifted the focus of the U-boat into the mid-Atlantic region in order to avoid the aircraft patrols which had started to make life very difficult for his boats. Although the Allied convoy routing was less predictable in the middle of the Atlantic, Dönitz anticipated that the increased numbers of U-boats being produced would allow the wolfpacks to search more effectively search for convoys with the advantage of intelligence gained through the B-Dienst’s decryption of the key British Naval Cypher No. 3. However, only 20% of the 180 trans-Atlantic convoys sailing from the end of July 1942 until the end of April 1943 lost ships to U-boat attack.

After a fruitless pursuit of the ON.139, the 'Puma' (ii) wolfpack was reinforced and continued its search farther into the Atlantic, and on 26 October Kapitänleutnant Günther Seibicke’s U-436 sighted the HX.212 convoy, made its sighting report and and shadowed the convoy without being detected by the convoy escort. On the next day U-436 launched a salvo of five torpedoes at 21.10, hitting the 10,107-ton British Sourabaya (carrying the 291-ton LCT-2281 as deck cargo), 8,225-ton US Gurney E. Newlin and 7,350-ton Norwegian Frontenac. Alberni and Summerside dropped back to rescue survivors from the torpedoed ships, of which the first sank with the embarked LCT and the other two were damaged.

At 03.54 on 28 October Oberleutnant Hans-Heinrich Döhler’s U-606 torpedoed the 16,966-ton Norwegian Kosmos II on the starboard side. Barrwhin dropped back to rescue survivors, and both Kosmos II and Gurney E. Newlin were sunk as the convoy proceeded. A Consolidated Liberator patrol bomber of the RAF’s No. 120 Squadron, operating from a base in Iceland, prevented five other U-boats from reaching attack positions during daylight hours but the 4,000-ton British Bic Island and 7,701-ton US Pan-New York were torpedoed after the fall of night, the former by Oberleutnant Hans-Karl Kosbadt’s U-224 and the latter by Oberleutnant Ulrich Graf von Soden-Fraunhofen’s U-624.

Its northern routing made it possible for the convoy to pass through the narrowest portion of the 'air gap', thereby reducing its period of greatest vulnerability to U-boat attack, and continuous daylight air patrols forced the U-boats to remain submerged and thereby lose contact with the convoy. The anti-submarine trawlers Bodo and Molde escorted the convoy through the Western Approaches on 1 November, and the convoy reached Liverpool on 2 November.

The HX.228 convoy was one of a series of four convoy battles which took place during the crisis month of March 1943. The HX.228 convoy of 60 laden ships, plus local contingents, departed New York on 28 February for Liverpool, and was joined at sea by the Mid-Ocean Escort Force’s British Escort Group B3, which had sailed from St John’s under the leadership of Commander A. A. Tait in the destroyer Harvester; the other ships of this group were the destroyers Escapade and Burza and Garland, the corvettes Narcissus and Free French Aconit, Renoncule and Roselys. The group was backed by the US escort carrier Bogue and US destroyers Belknap and Osmond, which sortied from Argentia on 5 March.

Arrayed against the convoy and its escorts were the 'Wildfang' (i), 'Burggraf' and 'Neuland' (ii) wolfpacks, although in the event only a reconfigured 'Neuland' (ii) wolfpack, comprising 13 U-boats, engaged HX.228.

Early in March some of the U-boats came in contact with SC.121, a slow eastbound convoy several days ahead of HX.228, and engaged it. Information about this battle led the Admiralty to divert the HX.228 to the north-east in order to avoid the SC.121 battle, but this took the HX.228 convoy straight into the 'Neuland' (ii) wolfpack’s patrol area.

On 10 March the first contact was made by Kapitänleutnant Hans Hunger’s U-336 in heavy weather. The boat made the standard sighting report, and during the rest of the day eight other boats were directed to join U-336. Ironically, at this point the weather forced Bogue and her two destroyers to detach, as it was impossible to fly off aircraft in the storm.
The escort carriers departed for Argentia, and took no part in the action.

It was during the evening of 10/11 March, during a snow squall, that the U-boats made their first attacks. Oberleutnant Hans-Hartwig Trojer’s U-221 attacked three ships, sinking two ammunition ships, the 5,412-ton British Tucurinca and 6,565-ton US Andrea F Luckenbach, and damaging a third vessel, the 7,197-ton US Lawton B. Edwards. After this U-221 was vigorously counterattacked, and had to withdrew from the battle to effect repairs.

Oberleutnant Albert Langfeld’s U-444 and Oberleutnant Friedrich Deetz’s U-757 each fired on the 7,197-ton US William C. Gorgas, carrying explosives and also the 291-ton LCT-2398 tank landing craft, which was severely damaged and dropped back to sink later.

U-757 next fired on the 5,001-tpn Norwegian Brant County, which was also carrying ammunition. The merchant ship caught fire and exploded, and U-757 was damaged, being forced to return toward base. This and another two boats were later attacked in the Bay of Biscay by British aircraft, but all survived.

Kapitänleutnant Walter Schug’s U-86 and Kapitänleutnant Horst Dieterichs’s U-406 both claimed hits using the new FAT pattern-running torpedoes. The 5,464-ton British Jamaica Provider was damaged during this stage of the battle.

On the morning of 11 March, Harvester sighted Langfeld’s U-444 on the surface and ran in to attack. The destroyer opened fire and then rammed the U-boat, suffering damage to her propellers in the process. It was originally thought that Harvester had sunk U-444, but the boat was found later on the surface and finished by Aconit. While in the area, Harvester found and picked up 50 survivors from William C. Gorgas and one from U-444. While attempting to rejoin the convoy the destroyer’s engines failed, and she summoned Aconit for aid. However, while it lay helpless, Harvester was sighted and torpedoed by Eckhardt’s U-432 and sank with 149 of her crew including Tait.

Arriving on the scene, Aconit gained sonar contact with -U432, which was lying motionless at periscope depth, her commander and crew celebrating their victory. Aconit attacked the boat with depth charges, blowing her to the surface and sinking her later with gunfire. Aconit picked up 20 survivors from U-432, 48 from Harvester, 12 from William C. Gorgas and the one from U-444, to join the three men previously picked up.

Despite further action that day and during the night of 11/12 March there were no further losses on either side, and on 12 March Dönitz called off the attack, and HX.228 reached Liverpool on 15 March.

The battle for the HX.228 convoy cannot be seen as a victory for either side. The convoy had lost four merchant ships and one warship, and also Tait, who had been an effective and well-respected escort group commander. The 'Neuland' (ii) wolfpack had lost two boats, a potentially ruinous rate of exchange.

During this series of clashes there also took place the largest convoy battle of all time, on 16/19 March 1943, involving the HX.229 and SC.122 convoys: for the loss of only one U-boat, the Germans sank 13 ships (93,502 tons) of the HX.229 convoy’s 60 ships and five escorts, and nine ships (53,694 tons) of the SC.122 convoy’s 40 ships and eight (later more) escorts.

At this time the German tactics against convoys were based on multi-boat wolfpacks making nearly simultaneous surface attacks under cover of darkness, for the increased level of Allied air patrols was by now severely restricting the ability of U-boats to converge on convoys during daylight. The North Atlantic winters offered the longest periods of darkness to conceal surfaced U-boat operations, and the winter of 1942/43 saw the largest number of U-boats deployed to the mid-Atlantic before the number and range of the Allies increasingly comprehensive anti-submarine aircraft patrols could be extended into that area.

Thus, during March, there was a series of fierce convoy battles which became, for the Allies, the crisis point of the whole of the Battle of the Atlantic. The 100 merchant ships of the HX.229 and SC.122 trade convoys encountered the 'Raubgraf', 'Stürmer' and 'Dränger' wolfpacks, totalling 38 U-boats, in a single sprawling action. German radio reported this as the 'the greatest convoy battle of all time;, and a British naval report later concluded that 'The Germans never came so near to disrupting communications between the New World and the Old as in the first twenty days of March 1943.'

SC.122 was a slow eastbound convoy of 60 ships, routed from New York to Liverpool. (This was during the period when SC convoys were switched from Sydney, Cape Breton, to New York, though this later was reversed once more as a result of congestion problems in New York.) The SC.122 convoy sailed on 5 March 1943, protected at first by one destroyer and five corvettes of the Western Local Escort Force. On 6 March, off Cape Cod, two ships put back to New York as a result of heavy weather, and on 8 March another six abandoned the crossing and put in to Halifax.

The remainder of the convoy pressed on, changing escorts on 13 March off Cape Race. The Western Local Escort Force left, after the Mid-Ocean Escort Force’s British Escort Group B5 joined from St John’s. This escort group comprised eight warships, led by Commander R. C. Boyle in the destroyer Havelock, and otherwise comprised the US destroyer Upshur, the British frigate Swale, the British corvettes Buttercup, Godetia, Lavender, Saxifrage and Pimpernel; there was also a trawler as rescue vessel.

The HX.229 convoy departed New York on 8 March with 40 ships and the local escort. A further 34 ships which should have been included in this convoy were delayed as a result of the congestion at New York, and in fact departed on the following day as the HX.229A convoy. The first few days of the convoy were uneventful. HX.229 met its Mid-Ocean Escort Force on 14 March, whereupon the warships of the local escort departed. The ocean escort was the British Escort Group B4, which had sailed from St John’s with four destroyers and one corvette. The group was led on this occasion by Commander G. J. Luther of the destroyer Volunteer, as its regular leader was in dock for repairs. Luther had only recently joined the group, and this was only his second crossing. The other ships of the Escort Group B4 were the destroyers Beverley, Mansfield and Witherington and the corvette Anemone, although Witherington had to detach on 15 March, to be replaced by the corvette Pennywort for the crossing.

Deployed against the two convoys were the patrol lines of three wolfpacks. The 'Raubgraf' pack comprised eight boats which had already worked together in a battle with the preceding HX.228 convoy: it was sent to patrol to the east of Newfoundland, at the western edge of the 'air gap'. The 'Stürmer' pack was a new grouping of 18 boats, which were to form in the middle of the 'air gap' and comprised boats from the 'Westmark' pack, which had previously engaged the SC.121 convoy. The 'Dränger' pack comprised 11 boats formed to the east of the 'Stürmer' pack: some of these boats were from the 'Neuland' (ii) pack, which had also been in the battle with the HX.228 convoy, and the others were newly arrived boats.

The Germany navy’s B-Dienst signals interception and decryption organisation had already given warning of an eastbound convoy, and by 20.00 on 13 March had established the location of the SC.122 convoy. Dönitz ordered the 'Raubgraf' wolfpack to intercept, forming a new patrol line farther to the west. A westerly gale gave a push to the speed of SC.122, which therefore passed through the 'Raubgraf' wolfpack’s patrol area on the morning of 15 March just 24 hours before the patrol line was formed.

Allied 'Ultra' intelligence, which decrypted German messages enciphered using the 'Enigma' machine and had already helped the Admiralty to divert previous convoys away from wolfpacks, had been 'blinded' on 10 March when the Germans introduced a new short weather report. This starved the British codebreakers of the 'cribs' necessary to break 'Shark', the cipher currently used by the U-Boats. The U-boat tracking room at the Admiralty’s Operational Intelligence Centre was therefore not in the position to divert convoys around the U-boat wolfpacks. Fortunately for the Allies, a message from a U-boat gave away its position, which was fixed by HF/DF, and the SC.122 convoy was diverted around the estimated danger area.

The Allied Cipher No. 3 used by the convoy escorts had been broken by the Germans, however, and this made it possible for them to position wolfpacks in the way of HX.229, which was following a course similar to that of SC.122. The convoy passed through the 'Raubgraf' wolfpack’s patrol line during the night of 15/16 March in adverse weather, and this latter meant that the Germans did not spot the Allied convoy. On the morning of 16 March Korvettenkapitän Gerhard Feiler’s U-653, which had been detached from the 'Raubgraf' wolfpack to return to base with mechanical problems, sighted HX.229 heading to the east and sent a sighting report. Dönitz immediately ordered 'Raubgraf' to pursue and intercept, while the 'Stürmer' and 'Dränger' wolfpacks were ordered to the west to form a line ahead of the convoy. Dönitz saw in this an opportunity to inflict major damage on an eastbound convoy of merchant ships laden with war matériel bound for Europe and with the full width of the 'air gap' still to cross.

The 'Raubgraf' wolfpack caught up with HX.229 during the evening of 16 March, and attacked in the course of that night. Three ships were sunk and another five on the morning of 17 March, a total of eight ships in just eight hours. The attacking boats reported that the escort was weak, as two of its ships had dropped out to pick up survivors. The other escorts chased three contacts during the night, but with no result. During the rest of the day, boats from the 'Stürmer' wolfpack began to arrive. One of these was attacked by a destroyer, but again without success.

At the north-eastern end of the 'Stürmer' wolfpack’s patrol line, Kapitänleutnant Manfred Kinzel’s U-338 had sighted the SC.122 convoy as it headed to the east, about 120 miles (195 km) from the HX.229 convoy’s position. After sending a sighting report the attacked, sinking four ships (7,886-ton Dutch Alderamin, 5,072-ton British King Gruffydd, 4,898-ton British Kingsbury and 4,071-ton Panamanian Granville); a fifth, the 7,134-ton British Fort Cedar Lake, was damaged and sunk later in the day by Oberleutnant Hans-Jürgen Haupt’s U-665.

Two more ships from the HX.229 convoy were lost during the day. Two boats from the 'Stürmer' pack were able to penetrate the defences at about 12.00 on 17 March, but the escorts were able to fend off any further attacks, assisted by brief visits from Allied very long-range aircraft flying at extreme range. The SC.122 convoy was also able to resist further attacks until evening.

During the night of 17/18 March the German assault on the two convoys, now just 70 miles (115 km) apart, continued. It was during this period that Kinzel’s U-338 sank the freighter Granville, surviving a fierce counterattack by the escorts, and after 24.00 Kapitänleutnant Rudolf Bahr’s U-305 sank two more ships, in the form of the 8,789-ton British Port Auckland and 4.256-ton British Zouave.

The HX.229 convoy’s escort suffered a blow as Mansfield was forced to detach during the night of 17/18 March, but assistance was on its way in the form of the destroyer Highlander, under Commander E. C. L. Day. Arriving on 18 March Day, as a senior and more experienced officer, took command of the Escort Group B4 for the rest of the engagement. Also en route from the Hvalfjördur in south-western Iceland were the destroyers Vimy and US Babbitt for HX.229, and the US Coast Guard cutter Ingham for SC.122. These had been dispatched during the morning of 18 March, and arrived with their convoys on the following day.

During the afternoon of 18 March, Oberleutnant Robert Schetelig’s U-221 succeeded in sinking two of HX.229’s ships, which avoided further losses. Highlander joined the convoy during that afternoon, and was a most welcome addition as Escort Group B4 had been reduced by this time to just five warships.

During the night of 18/19 March the two convoys were running in tandem, though sailing independently. All attacks on both convoys were repelled this night, and six firm contacts were attacked but little damage was inflicted. One ship of the HX.229 convoy was lost after it had broken away to proceed independently: this ship, the 5,848-ton US Matthew Luckenbach, ran into the melée around SC.122 and was torpedoed and damaged, to be sunk later on 19 March by Kapitänleutnant Herbert Uhlig’s U-527. A straggler from SC.122, Clarissa Radcliffe, was also lost, disappearing without trace.

On 19 March the escorts were reinforced by the arrival of Vimy and Babitt for HX.229, which was also joined by the corvette Abelia detached from another convoy, and Ingham for SC.122. Also on 19 March, Oberleutnant Hans-Achim von Rosenberg-Gruszcynski’s U-384 was attacked and sunk by an air patrol to the north of SC.122. There were no further losses to the convoys on this day: faced with stiffening resistance and sensing that nothing further would be achieved without disproportionate losses, Dönitz called off the assault.

The convoys thus continued to the east. More changes to the escort occurred on 20 March as reinforcement arrived in the form of the destroyer British Sherbrooke, while Upshur and Ingham were detached. The local escort groups met the convoys on 23 March, and HX.229’s surviving 27 ships reached Liverpool on the same day, with SC.122’s surviving 42 ships arriving at the same port later in the same day.

The double battle had involved 100 merchant ships and 16 escort ships, though not all of the latter were present at the same time. The Allies lost 22 merchant ships (13 from HX.229 and nine from SC.122), a overall loss in the order of of 146,000 tons, and more than 300 merchant seaman had died. In total, 38 U-boats had taken part, though throughout the battle not all had been in contact, and one U-boat had been lost with all hands.

This was the largest convoy battle of the Battle of the Atlantic. The desperate nature of the battle was made clear in a later British naval report, which concluded that 'It appeared possible that we should not be able to regard convoy as an effective system of defence.'