Atlantic · 1 Mar 1943 – 31 May 1943
Black May: the U-boat war turns
From March’s costly convoy battles through the May attacks on ONS 5 and SC 130, Allied escorts and aircraft inflicted heavy U-boat losses. Dönitz ordered most boats out of operations on 24 May.
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Black May formed the final part of the Atlantic crisis that had built through March and April 1943. In March, U-boats attacked heavily defended convoys and sank large amounts of Allied shipping. One account recorded 82 Atlantic ships lost that month and 12 U-boats sunk; the U.S. Naval History and Heritage Command described 567,000 tons of Allied shipping lost to U-boats. The different totals reflected differences in how sources counted losses, but both accounts described March as the high point of the convoy crisis.
April brought some relief as many U-boats returned to port for repairs and replenishment, although attacks continued. At the end of the month, U-515 sank seven ships from Convoy TS 37 off Freetown. That episode showed that the danger had not passed. Meanwhile, longer-range Allied air cover, escort numbers, and anti-submarine methods had improved. Escort groups had become more effective at hunting submarines, and new weapons had entered service. These changes did not act alone; they combined with the growing ability to cover Atlantic convoy routes.
The opening battle of Black May centred on Convoy ONS 5, which had sailed from Liverpool toward Halifax. Between 29 April and 6 May, U-boats attacked the convoy in the North Atlantic. The sources differed on the merchant losses: one reported 13 ships sunk, while another reported 12. They agreed that the escorts sank six U-boats and damaged seven. ONS 5 therefore showed the increasing cost of attacking a convoy, even though its merchant ships suffered heavily.
The balance worsened for the U-boat arm over the rest of May. The next three attacked convoys lost seven Allied ships, while seven U-boats were sunk. Convoy SC 130 then passed without losing a ship, while five attacking U-boats were lost. The peak in German operational strength—240 boats, of which 118 were at sea—did not produce a recovery in shipping losses. Aircraft and surface escorts destroyed U-boats both in convoy battles and on patrol, including in the Bay of Biscay.
On 24 May, Karl Dönitz ordered a temporary halt to the campaign and withdrew most boats from operational service. Accounts differed over the total May U-boat losses: one gave 43, while others gave 40 or 42. The sources also reported 37 U-boats damaged. Allied shipping losses were reported as 58 ships worldwide during May, of which 34 were lost in the Atlantic. Those figures reflected different geographical scopes, and the sources warned that totals varied with counting methods. The immediate result was clear: the U-boat offensive had suffered an exceptional monthly loss while sinking fewer Allied ships, and the boats did not regain their earlier advantage.