'Dernge' was an Allied intensive anti-submarine air successor to 'Enclose II' in the Bay of Biscay at a time when the U-boat campaign was at its height and boats were trying to run across the bay on the surface at high speed to get as rapidly as possible into the Atlantic, out of the range of UK-based anti-submarine aircraft (13 April/6 June 1943).
As a result of the change in the German tactics resulting from 'Enclose II', there was a major increase in day sightings of U-boats, resulting in many attacks. On 26 April the outbound U-566 was damaged by a Vickers Wellington of the RAF’s No. 172 Squadron, and on 29 April the outbound U-437 was damaged by a Wellington of the same unit and Oberleutnant Eberhard Hüttemann’s U-332 was sunk by a Consolidated Liberator of the RCAF’s No. 224 Squadron. In May there were 98 sightings and 64 attacks. On 1 May the returning U-415 was damaged in attacks by a Wellington of No. 172 Squadron with Leigh Light, a Short Sunderland of No. 461 Squadron RAAF and an Armstrong Whitworth Whitley of the RAF’s No. 612 Squadron. On 2 May Kapitänleutnant Heinz Wolf’s outbound U-465 was sunk by a Sunderland of the RAF’s No. 461 Squadron, on 5 May outbound U-663 was sunk by a Handley Page Halifax of the RAF’s No. 58 Squadron and on the following day the Korvettenkapitän Heinrich Schmid’s outbound U-214 was damaged by an Armstrong Whitworth Whitley of the RAF’s No. 10 OTU.
On 15 May Korvettenkapitän Leo Wolfbauer’s outbound tanker U-463 was sunk by a Halifax of the RAF’s No. 58 Squadron and the outbound U-591 was damaged by a Whitley of No. 10 Squadron. On 16 May the Italian submarine Enrico Tazzoli was sunk by a Halifax of No. 58 Squadron and on 24 May the outbound U-523 was damaged by a Whitley of No. 10 OTU. On 31 May Oberleutnant Gustav Borchardt’s outbound U-563 was sunk by a Halifax of No. 58 Squadron, Sunderland of No. 228 Squadron and Sunderland of the RAAF’s No. 10 Squadron, and on the same day the returning U-621 was damaged by a Liberator of the RAF’s No. 224 Squadron.
On 1 June Oberleutnant Gerhard Lange’s returning U-418 was sunk by rocket fire from a Bristol Beaufighter of the RAF’s No. 236 Squadron. On 24 May U-441 was used for the first time as an anti-aircraft trap: a Sunderland was damaged and then shot down by a fighter, but the boat also suffered many killed and considerable damage. In addition U-666, U-394, U-648, U-662 and U-459 each shot down one aeroplane.
Their experience in 'Derange' led the Germans to develop new tactics of massed 'wolfpack' transits of the Bay of Biscay for more concentrated anti-aorcraft firepower.
'Derange' was resumed on 2 August in the waters north-west of Cape Finisterre, but achieved essentially nothing as the Germans had temporarily halted the outward movement of their U-boats, and ordered their inbound U-boats to concentrate and move through neutral Spanish waters, into which Allied aircraft were not permitted to penetrate. The Germans also deployed the Heinkel He 177 aircraft of the II/Kampfgeschwader 40 on long-range flights to and from Cape Finisterre as a means of providing the U-boats with cover between their assembly area in neutral waters and their bases on the west coast of occupied France. Before the end of 'Derange' on 22 August, the Allies nonetheless lost 17 anti-submarine aircraft and six long-range fighters, most of them to the heavily armed Heinkel He 177 long-range patrol bombers of Major Martin Kästner’s II/Kampfgeschwader 40.