'Fink' was a U-boat wolfpack operation, in tandem with 'Amsel I', 'Amsel II', 'Specht' and 'Star', in the Atlantic against the SC.128 and ONS.5 convoys (4/6 May 1943).

The wolfpack comprised U-125, U-168, U-192, U-209, U-226, U-231, U-260, U-264, U-270, U-358, U-378, U-381, U-413, U-438, U-514, U-531, U-533, U-552, U-584, U-614, U-628, U-630, U-648, U-650, U-662, U-707, U-732 and U-954, and for the loss of Kapitänleutnant Ulrich Folkers’s U-125, Oberleutnant Werner Happe’s U-192, Korvettenkapitän Heinrich Heinsohn’s U-438, Kapitänleutnant Herbert Neckel’s U-531 and Oberleutnant Werner Winkler’s U-630 sank eight ships (38,241 tons) in attacks on the ONS.5 convoy.

In this decisive phase of the 'Battle of the Atlantic', the ONS.5 slow convoy found itself pitted against elements of five U-boat wolfpacks during its passage westward across the Atlantic between 29 April and 6 May from the UK to Canada, and the battle for the convoy in April and May 1943 is seen as the turning point of the Battle of the Atlantic in which the initiative passed from the Germans to the Allies. The fighting continued for eight days, and was characterised by heavy losses on each side.

The ONS.5 convoy, under the command of Commodore J. K. Brook, was almost the last Allied convoy to suffer losses on this scale, however, while the high casualties which the escorting warships inflicted on the wolfpacks which attacked the convoy became increasingly standard from this time onward. Thus the battle for the ONS.5 convoy was the point at which the tactical and strategic advantage in the 'Battle of the Atlantic' passed from the Germans to the Allies, and ushered in the period known to the U-boat arm of the German navy as schwarz Mai ('Black May').

The ONS.5 convoy was a slow westbound convoy, and eventually comprised 48 ships (including two escort oilers in the form of the 6,952-ton US Argon and 6,098-ton British British Lady and a few which joined from Iceland) bound from Liverpool to Halifax, the ships either in ballast or carrying trade and export goods. The convoy departed Liverpool on 21 April 1943, and was scheduled to reach Halifax three weeks later on 12 May. Three of the merchant vessels turned back, one of them with collision damage and two with mechanical defects.

Support was provided by Commander P. W. Gretton’s British Escort Group B7 (the destroyers Duncan and Vidette, the frigate Tay and the corvettes Sunflower, Snowflake, Loosestrife and Pink). The group also contained two trawlers, Northern Gem and Northern Spray, as rescue ships. The convoy was joined by other escort vessels as the battle progressed.

ONS.5 was just one of several Allied convoys at sea at the end of April: 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, and in mid-Atlantic, and due to pass ONS.5 at a point to the east of Greenland, was SC.127. Two other eastbound convoys, HX.235 and HX.236, were also in mid-Atlantic, following a more southerly route. These movements thus comprised more than 350 ships in the North Atlantic at one time.

Ranged against them were an eventual 58 U-boats in a four major wolfpacks: the 'Specht' pack with 17 boats was to the south of Greenland on the western side of the 'air gap' beyond the range of Allied warplanes, the 'Meise' pack with 30 boats was to the east of Greenland covering the northern route, and the 'Amsel' pack with 11 boats was to the south of the 'Meise' pack covering the southern route. The 'Meise' pack had been deployed against the SC.127 convoy, which had been identified by the Beobachtungsdienst (B-Dienst, or naval radio interception and codebreaking organisation), but on 26 April the SC.127 convoy had slipped through a gap in the line and escaped undetected.

Realising on the following day what had happened, and also aware that a slow westbound convoy was imminent, the commander-in-chief of the German navy, Grossadmiral Karl Dönitz, still in day-to-day command of the U-boat arm via Konteradmiral Eberhard Godt, the chief of U-boat operations, reconfigured the 'Meise' pack: the easternmost boats (16 in all) became the 'Star' pack to intercept it.

At 08.00 on 28 April Kapitänleutnant Ernst von Witzendorff’s U-650 located the ONS.5 convoy, and his sighting report allowed the 'Star' pack to concentrate for the attack. The ONS.5 convoy had departed Liverpool on 21 April, and was quickly involved in incident. On the first night, the 3,569-ton Polish freighter Modlin had to turn back with engine trouble. On the following day, in a moderate gale, the air escort from Scotland spotted and sank Oberleutnant Dietrich von Carlowitz’s U-710, which was just 10 miles (16 km) ahead of the convoy but probably unaware of its proximity. On 26 April the freighter Penhale was damaged in a storm and detached to return to the Hvalfjörður in south-western Iceland under escort of Northern Gem, which rejoined the convoy on the following day. Also on the same day Vidette joined with three merchant ships from Iceland.

On 28 April the ONS.5 convoy arrived in the patrol area of the 'Star' pack, and was sighted by U-650. By the fall of night this boat had been joined by Kapitänleutnant Jürgen Könenkamp’s U-375, Oberleutnant Hans-Albrecht Kandler’s U-386, Kapitänleutnant Georg von Rabenau’s U-528 and Kapitänleutnant Peter Schrewe’s U-537, and at 24.00 these boats began their attack. Warned by the radio chatter of their presence, Gretton mounted a vigorous defence, and during the course of this first night none of the Allied ships was hit, while two of the U-boats, U-386 and U-532, were damaged and forced to return to base: U-386 reached St Nazaire on 11 May, but U-532 was attacked in the Bay of Biscay and sunk by aircraft on the same day.

The German assault continued into the daylight of 29 April. At 12.00 on this day the 6,198-ton US McKeesport was sunk by Rabenau’s U-528, which was itself damaged and had to return toward base, being sunk en route during 11 May by British air and sea attack.

On the same day the Admiralty arranged reinforcements for the escort of the ONS.5 convoy: the British destroyer Oribi, detached from the SC.127 convoy’s escort, would arrive on 30 April, while Captain J. A. McCoy’s 1^st Support Group (destroyers Penn, Panther, Impulsive and Offa) would arrive on 1 May.

During the following few days, the course of the fighting was dictated by the weather. In worsening conditions the ONS.5 convoy found itself making less than 3-kt headway into a Force 10 storm. The convoy started to become scattered, some ships ending as far as 30 miles (48 km) from the convoy, and the escorts were kept busy locating the stragglers and bringing them back to the convoy.

On 30 April and 1 May the promised British reinforcements arrived, thereby easing the task of the escort commander. The weather made refuelling impossible, and a number of the destroyers ran so low on fuel that their continued presence became problematical. On 3 May Gretton was forced to take Duncan to St John’s, Newfoundland, at his destroyer’s economical speed of 8 kt: he arrived with sufficient fuel for only a few hours' more steaming. In Gretton’s absence, command was assumed by Lieutenant Commander R. E. Sherwood of Tay. Later in the same night Impulsive also detached to Iceland, with Northern Gem carrying the survivors of McKeesport, and Penn and Panther also detached for Newfoundland. Meanwhile the U-boats tried to maintain contact, though any attack was impossible.

Only six of the 'Star' wolfpack’s boats had been able to make contact, and of these three had been forced to return to base. Dönitz decided that nothing could further be gained and on 1 May ordered boats from the 'Star' and 'Specht' packs, together with some newly arrived boats, to establish a new patrol line to the west as the 'Fink' pack, which was in place on 3 May and totalled 27 boats tasked with the interception and destruction of the westbound SC.128 convoy. This reorganisation was also not without incident. On 1 May two of the newcomers were attacked by aircraft in separate incidents while joining the 'Fink' pack: one was thought to have been Winkler’s U-630 and was believed to have been sunk, but is now believed to have been Kapitänleutnant Heinrich Brodda’s U-209, which was damaged in the attack and succumbed to an air attack while attempting to return to base. The other boat, Korvettenkapitän Heinrich Heinsohn’s U-438, was only slightly damaged in the attack.

By 4 May the weather had abated to Force 6, and the ONS.5 convoy was now making up to 6 kt, though reduced in numbers to 30 ships and seven escorts. The rest were scattered and proceeding independently, including a group of four with the corvette Pink, trailing some 80 miles (130 km) behind the main body.

At 12.00 on 4 May ships were sighted by Kapitänleutnant Heinrich Hasenschar’s U-628 of the 'Fink' wolfpack: these were not part of the SC.128 convoy, as expected, but of the ONS.5 convoy. The 'Fink' wolfpack’s boats started to concentrate, and by the evening of the same day were ready to make their attack. The assault started at the fall of night and continued throughout the hours of darkness. The 4,737-ton British Lorient, 4,635-ton British North Britain, 5,081-ton British Harbury, 5,561-ton US West Maximus and 4,586-ton British Harperley were torpedoed by U-125, U-707, U-628, U-264 and U-264 again, and a number of U-boats were damaged.

The German attack continued into the day, the U-boats switching from surfaced to submerged attacks, and the 2,864-ton British Bristol City, 5,212-ton British Wentworth, 5,507-ton British Dolius and 1,570-ton Norwegian Bonde were lost to U-358, U-358 again, U-638 and U-266 respectively. This last boat also sank the 5,306-ton British Gharinda and 5,136-ton British Selvistan.

Several boats also found Pink's group and attacked it: one ship, the 5,565-ton US West Makadet, was sunk by U-528, while Pink mounted a determined attack on a contact, and was credited with the destruction of Happe’s U-192, though the victim was more likely to have been Kapitänleutnant Rolf Manke’s U-358, which was damaged.

Dönitz ordered the assault to be continued into the night, but the advent of thick fog saw the tactical advantage pass to the escorts. The U-boats lost sight of their targets and the escorts, while the radar-equipped escorts were able to locate the surfaced U-boats and surprise them. The German attack was therefore repulsed with no merchant ships lost, but Kapitänleutnant Oskar Staudinger’s U-638, Kapitänleutnant Ulrich Folkers’s U-125 and Kapitänleutnant Herbert Neckel’s U-531 were sunk.

On the morning of 6 May further British reinforcement arrived in the form of the 1^st Support Group, led by Commander G. N. Brewer in the sloop Pelican. The group announced its appearance on the scene by attacking and sinking U-438; while the ex-US cutter Sennen, sent to help Pink's party, made an attack which was also credited as a kill but later found to have been unsuccessful. With the other members of the 1^st Support Group (the frigates Jed, Wear and Spey), the escort now comprised 13 warships.

The 'Fink' pack had clearly outlasted its usefulness, and faced mounting losses if the attack continued. Realising his mistake, Dönitz called off the assault on 6 May and ordered the 'Fink' pack to pull back. Although it did not reach port for another week, the ONS.5 convoy was not attacked again, which was just as well for the Allies as Offa and Oribi had to be detached on 6 May as they were acutely short of fuel, while the Escort Group B7’s ships, short of ammunition as well as fuel, were in little shape for more action. Only the ships of 1^st Support Group were in any state to fight.

In the course of a week the ONS.5 convoy had been attacked a force of more than 40 U-boats, but while 13 ships totalling 63,000 tons had been lost, the escorts had sunk six U-boats and seriously damaged another seven.

This battle demonstrated that the convoy escorts had now mastered the art of tactical convoy protection: the weapons and skills which were now available meant that henceforth the escorts would be able not only to protect their charges and repel attack, but also to inflict significant losses on the attacking U-boats.