The battle of Convoy ONS5 of 28 April -6 May 1943 was a major defeat for the U-boats, and was part of a dramatic shift in fortune in the battle of the Atlantic. The German attack on Convoy HX229 in mid-March was the most successful U-boat attack of the entire war, and had seen twenty-one ships sunk for the loss of a single U-boat. Admiral Dönitz, by then both Commander-in-Chief of the German Navy and of the U-boat force, had been able to concentrate 38 U-boats against HX229 and the nearby Convoy SC122, and had overwhelmed the escort forces.
By the end of April Dönitz had even more U-boats in place in the North Atlantic – two groups (one of seventeen and one of fifteen) at the western edge of the Atlantic air gap, one group of thirteen at the eastern corner of the gap and another, also of thirteen, south east of Cape Farewell at the southern tip of Greenland.
ONS5 was a westward bound convoy, protected by eight escorts (three destroyers, one frigate, two corvettes and two rescue trawlers), led by Commander P.W. Gretton on the destroyer HMS Duncan. None of the new escort carriers were available to protect the convoy, but the 3rd Escort Group (five destroyers under Captain J.A. McCoy of HMS Offa) was close by, and would join the convoy before the U-boat attack began. The 1st Escort Group (one sloop, three frigates and an ex US coast guard cutter) would also come into the picture towards the end of the battle.
U-258 was the first to find the convoy, sighting it on 28 April in the middle of a full Atlantic gale. If the attack had come at the end of April, then the battle may have ended very differently, for the weather prevented the long range Liberators from flying, while the 3rd Escort Group simply couldn’t find the convoy, but on 1 May the U-boats also lost contact with OSN5.
The bad weather had weakened the convoy escort – on 3 May Gretton had to leave the convoy to take on fuel, and the same problem forces two of the 3rd Escort Group ships away on the following day. When the wolf pack attack began on 4 May the convoy was protected by ten escorts. Allied airpower struck the first blows – a Flying Fortress of Coastal Command sank one U-boat, while a flying boat of the RCAF sank U-630 on 4 May.
The most successful phase of the German attack came on the night of 4-5 May, when the U-boats sank five merchantmen. The next day wasn’t much better, with four ships lost, but the weather did improve enough for the escorts to refuel at sea, while the Flower class corvette Pink sank U-192.
The biggest German attack came on the night of 5-6 May, with attacks reported from every direction but ahead. This was the moment at which the battle turned against the Germans. That night four U-boats were sunk: U-125 by the hedgehog mortar of the destroyer Vidette, U-439 by the sloop Pelican, U-531 after being rammed by the Oribi and U-638 by the corvette Loosestrife. The next day the 1st Escort Group arrived, and Dönitz called off the attack.
The attack on Convoy ONS5 had cost the Germans seven U-boats destroyed and five badly destroyed. Twelve merchant ships had been sunk, but a rate of exchange of under two merchant ships for each U-boat lost was more than Dönitz could afford. Attacks on Convoys HX237 and SC129 in mid-May and on SC130 and HX239 towards the end of the month produced equally poor rates of returns, and by the end of May Dönitz had been forced to withdraw his wolf packs from the North Atlantic. Although the Germans would attempt to return later in the war, the battle of the Atlantic had been won.