The main operational bases for German U-Boat sorties were strung along the French Biscay coast, with massive concrete fortifications rendering their harbouring sites immune to Allied bombing. The strategic aim of U-Boat attacks on Allied convoys was to destroy shipping tonnage at a rate beyond replacement capacity; in their Happy Times this was achieved, endangering effective Allied prosecution of the war. But in summer 1942, powerful Leigh carbon-arc searchlights aided bomber detection of German submarines, evening up the battle. Then, in mid-1943, the introduction of VLR (very-long-range) bombers equipped with centimetric radar, which was undetectable by U-Boats, dramatically swung the battle. With North Africa now secured by the Allies, air patrols from there were able to supplement those from Southwest England, and inflict carnage on U-Boats in Black May. In the second half of the year the hunters increasingly became hunted, as U-Boats were picked off along the Biscay perimeter as they surfaced to recharge batteries.
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