'Enclose II' was the British successor to 'Enclose I', undertaken in the Bay of Biscay in conjunction with 'Derange' to locate and sink U-boats transiting the Bay of Biscay on the surface (5/13 April 1943).
Kapitänleutnant Friedrich-Karl Marks’s U-376 was sunk on 10 April by a Vickers Wellington twin-engined aeroplane, fitted with ASV.Mk IV radar and a Leigh Light, of No. 172 Squadron; and Kapitänleutnant Heinz Wolf’s U-465 was damaged by Consolidated Catalina of No. 210 Squadron.
The increasing number of night attacks by British aircraft with radar that could not be detected by the U-boats' Metox warners and the shooting down, on 4 May 1943, of a Consolidated Catalina twin-engined flying boat of the RAF’s No. 5 Squadron by Korvettenkapitän Heinrich Heinsohn’s U-438, which suffered only minor damage in the action, then paved the way to changed German tactics: the U-boats now proceeded submerged at night and charged their batteries by day, when they stood a better chance of defeating attacking aircraft with anti-aircraft fire.