On 26 April, the Allies suffered a rare blackout in their ability to read the German cipher, just as 53 U-boats regrouped for an assault on the convoy routes. Miraculously, two eastbound convoys, SC.128 and HX.236, escaped destruction, but ONS.5, a weather-beaten, westbound slow convoy of 30 merchant ships escorted by 7 warships stumbled into the middle of the wolf packs on 4 May. During the next 48 hours, the U-boats sank 12 ships but at an unacceptable cost: escort vessels sank 6 U-boats, and long-range air patrols claimed 3 others. Radar in aircraft and escort vessels had played a decisive role in giving the numerically overmatched escorts a tactical edge in the battle.
The commander of the German U-boat arm, Admiral Karl Dönitz, was aware of the tilting balance, but he urged his U-boat commanders not to relent. Yet many of the vessels did not even reach their areas of operations. The determined antisubmarine offensive in the Bay of Biscay by aircraft of the Royal Air Force Coastal Command destroyed 6 U-boats during May and forced 7 others to return to base.
In the second week of May, the ragged survivors of the North Atlantic wolf packs, which had operated against Convoys ONS.5 and SL.128, regrouped and deployed against HX.237 and SC.129. Only 3 merchantmen were sunk, at the expense of the same number of U-boats. In addition to radar, the small escort carrier Biter, which had provided air cover for HX.237 as well as for SC.129, was vital in denying the German submarines tactical freedom on the surface near the convoys. When the U-boats renewed their attacks against Convoy SC.130 between 15 and 20 May, escort vessels sank 2 U-boats, and shore-based aircraft claimed 3 others. SC.130 suffered no casualties. The U-boat offensive failed entirely against HX.239, a convoy with a rather generous organic air cover (aircraft attached to the convoy) provided by the escort carriers USS Bogue and HMS Archer. Not a single U-boat managed to close with the convoy, and on 23 May, a U-boat fell victim to the rockets of one of the Archer's aircraft. The following day, Dönitz recognized the futility of the enterprise and canceled all further operations in the North Atlantic. During the month to that point, more than 33 U-boats had been sunk and almost the same number had been damaged, nearly all of them in convoy battles in the North Atlantic or during transit through the Bay of Biscay. The month went down in German naval annals as "Black May," with the loss of 40 U-boats. At the end of May 1943, the British Naval Staff noted with satisfaction the cessation of U-boat activity. SC.130 was the last North Atlantic convoy to be seriously menaced during the war.
Blair, Clay. Hitler's U-Boat War. Vol. 2, The Hunted, 1942–1945. New York: Random House 1998.; Gannon, Michael. Black May. New York: Harper Collins, 1998.; Morison, Samuel Eliot. History of United States Naval Operations in World War II. Vol. 5, The Struggle for Guadalcanal, August 1942–February 1943. Boston: Little, Brown, 1949.