stylistic:tone, linguistic:voice_change, stylistic:style_transfer, quality:correction, information:filtering
Climax of the campaign (March–May 1943, "Black May")

On March 10, 1943, the Germans added a refinement to the U-boat Enigma key, which blinded the Allied codebreakers at Bletchley Park for 9 days. That month saw the battles of convoys UGS 6, HX 228, SC 121, SC 122 and HX 229. One hundred and twenty ships were sunk worldwide, 82 ships of 476,000 tons in the Atlantic, while 12 U-boats were destroyed.

The supply situation in Britain was such that there was talk of being unable to continue the war, with supplies of fuel being particularly low. The situation was so bad that the British considered abandoning convoys entirely. The next two months saw a complete reversal of fortunes.

In April, losses of U-boats increased while their kills fell significantly. Only 39 ships of 235,000 tons were sunk in the Atlantic, and 15 U-boats were destroyed. By May, wolf packs no longer had the advantage and that month became known as Black May in the U-boat Arm (U-Bootwaffe). The turning point was the battle centred on slow convoy ONS 5 (April–May 1943). Made up of 43 merchantmen escorted by 16 warships, it was attacked by a pack of 30 U-boats. Although 13 merchant ships were lost, six U-boats were sunk by the escorts or Allied aircraft. Despite a storm which scattered the convoy, the merchantmen reached the protection of land-based air cover, causing Dönitz to call off the attack. Two weeks later, SC 130 saw at least three U-boats destroyed and at least one U-boat damaged for no losses. Faced with disaster, Dönitz called off operations in the North Atlantic, saying, "We had lost the Battle of the Atlantic".

On 13 April RAF Coastal Command started its second Bay Offensive with operation Derange. Seventy-five long range aircraft equipped with the new centrimetric ASV Mark III radar with PPI display patrolled regions in the Bay of Biscay with known concentrations (through enigma decrypts) of U-boats in transit. The German metox radar detector operated only in the metric band and did not detect the new centrimetric radar emissions. As a result, many U-boats were surprised and attacked. In response Dönitz ordered his U-boats to stay on the surface and fight it out with the aircraft. Some U-boats were converted to " flak boats" with extra and new anti-aircraft guns, but to no avail: In May five U-boats were sunk and another seven were forced to abort.

In all, 43 U-boats were destroyed in May, 34 in the Atlantic. This was 25% of German U-boat Arm's total operational strength. The Allies lost 58 ships in the same period, 34 of these (totalling 134,000 tons) in the Atlantic.