Tom Spencer explains how US Navy PBY Catalinas employed a novel device to successfully track submerged U-boats transiting the Strait of Gibraltar, during spring 1944...

Cat: Consolidated PBY-5A Catalina ‘P-18’ of VP- 63 over the Strait of Gibraltar in the spring of 1944. Note it carries a full load of Retrobombs under the wings

Patrol Squadron 63’s unit badge
The Allied battle against Germany’s U-boat menace during World War Two was a relentless game of cat-and-mouse, as both fought to gain any tactical or technological advantage possible while working to counter the other’s advances.