'Drossel' was a U-boat wolfpack operation in the Atlantic (29 April/15 May 1943).

Created to attack the SL.128 convoy and then the HX.237 convoy, the wolfpack comprised U-89, U-221, U-230, U-332, U-406, U-436, U-439, U-447, U-456, U-600, U-607, U-659 and U-753, and for the loss of Korvettenkapitän Dietrich Lohmann’s U-89, Oberleutnant Eberhard Hüttemann’s U-332, Oberleutnant Helmut von Tippelskirch’s U-439, Kapitänleutnant Friedrich Bothe’s U-447, Kapitänleutnant Max-Martin Teichert’s U-456, Korvettenkapitän Hans Stock’s U-659 and Fregattenkapitän Alfred Manhardt von Mannstein’s U-753 sank four ships (totalling 25,962 tons) in its attacks on the two convoys.

On 3 May German air reconnaissance located two convoys, of which one (an LST convoy) was sighted briefly by U-89; in attempting to attack, U-659 and U-439 collided and sank.

On 5 May U-447 became involved in a gunfire engagement with a tank landing craft and shot down a barrage balloon, and U-456 failed to reach a firing position against a cruiser.

On 6 May German air reconnaissance located the SL.128 convoy of 48 ships escorted by one sloop and four corvettes.

During the morning of 7 May U-607 missed the convoy, U-456 was depth-charged and U-230, the contact-keeper, was bombed twice. U-447 was sunk in an attack by two Lockheed Hudson twin-engined maritime patrol bombers of the RAF’s No. 233 Squadron. U-436 and U-89 approached the convoy and made an attack at about mid-day, U-89 sinking the 3,803-ton Greek Lakonikos before bad visibility led to a loss of contact.

From 9 May the pack was deployed against the HX.237 convoy. This was also the target of the 'Rhein' wolfpack created out of the boats of the 'Amsel I' and 'Amsel II' which had not become involved in the attack on the ONS.5 convoy. The 'Rhein' wolfpack comprised U-709, U-569, U-525, U-468, U-448, U-752, U-466, U-454, U-359, U-186, U-403 and U-103.

The German effort was aided by the German naval radio intercept and decryption branch, the B-Dienst, which was able to report the target convoy’s evasive move to the south.

On 9 May, at the southern end of 'Rhein', U-359 located the HX.237 convoy of 46 ships supported by Lieutenant Commander E. H. Chavasse’s Canadian Escort Group C2 (the destroyer Broadway, the frigate Lagan, the corvettes Chambly, Drumheller, Morden and Primrose, and the rescue tug Vizalma). But the U-boat was immediately located by HF/DF while trying to report, and was compelled to dive.

A patrol line formed in front of the convoy was broken after a Fairey Swordfish anti-submarine aeroplane from the escort carrier Biter (part of the Captain E. M. C. Abel-Smith’s 5^th Support Group otherwise comprising the destroyers Inglefield, Obdurate, Opportune and Pathfinder), which had reinforced the Escort Group C2, arrived overhead and forced U-454, which was barring the way, to dive.

U-403 re-established contact on 10 May when shadowing the rescue tug, which has straggled. The U-boat was able to drive off a Fairey Swordfish single-engined biplane attack warplane with cannon fire, but was forced to dive by the approach of one of the 5^th Support Group’s three destroyers.

As the 'Rhein' wolfpack’s boats were not able to get ahead of the convoy, the German undertaking was now reliant on just the 'Drossel' wolfpack’s boats (U-456, U-230, U-607, U-436, U-89, U-600, U-221 and U-753) arriving from the east. The 'Rhein' wolfpack’s boats then combined with those of the 'Elbe' wolfpack' (made up of the boats involved in the attack on the ONS.5 convoy) to form the 'Elbe I' wolfpack (U-634, U-575, U-584, U-650, U-752, U-709, U-569, U-231, U-525, U-514, U-468 and U-267) and the 'Elbe II' wolfpack (U-103, U-621, U-448, U-466, U-223, U-454, U-504, U-402, U-377, U-359, U-107, U-383 and U-186) to operate against the southward evasive manoeuvre of the SC.129 convoy, which had been detected by the B-Dienst.

Thus U-436 located the HX.237 convoy during the evening of 11 May. Kapitänleutnant Heinz-Ehlert Clausen’s U-403 and Kapitänleutnant Max-Martin Teichert’s U-456 sank the 7,138-ton British Fort Concord, which was straggling, but U-753 missed the convoy. After dawn on 12 May the U-boats were attacked by Biter's Swordfish aircraft: U-230 shot down one Swordfish, but the boats were then forced to dive as the British escorts arrived on the scene. U-89 was sunk by Broadway, called in by a Swordfish of No. 811 Squadron and the frigate Lagan.

Damaged and rendered incapable of submerging by the hit of a homing torpedo dropped by a Consolidated Liberator four-engined patrol bomber of the RAF’s No. 86 Squadron on 12 May, U-456 was sunk while attempting to do so when the destroyers Pathfinder and Opportune arrived. The same Swordfish then led Pathfinder to U-603, which was slightly damaged in the attack which then developed.

A returning boat, U-628, maintained contact until the morning of 13 May while, from the rear, Kapitänleutnant Hans-Hartwig Trojer’s U-221 and Oberleutnant Rudolf Batz’s U-603 each sank a straggler in the form of the 9,432-ton Norwegian Sandanger and 4,819-ton Norwegian Brand respectively.

During the morning of 13 May Short Sunderland four-engined flying boats of the RAF’s No. 423 Squadron provided additional air escort, and one of these flying boats vectored in the frigate Lagan and corvette Drumheller to attack and sink von Mannstein’s U-753. The corvettes Chambly and Morden drove the other U-boats away from the convoy.

At this stage the U-boat command appreciated that continued attacks would result in heavy losses for no appreciable success and ordered the termination of the undertaking. At the same time Admiral Sir Max Horton, the commander-in-chief of the Western Approaches Command, ordered the 5^th Support Group to make for the SC.129 convoy, which had been reported by U-504 during the evening of 11 May. Though the convoy escort managed to drive away this contact-keeper, which had been located by HF/DF, U-402 attacked the convoy, which comprised 26 ships supported by Commander D. G. F. W. MacIntyre’s British Escort Group B2 (the destroyers Hesperus, Whitehall and Vanessa, and the corvettes Campanula, Clematis, Gentian, Heather and Sweetbriar).

Before the fall of night, Korvettenkapitän Siegfried Freiherr von Fostner’s U-402 sank the 4,545-ton British Antigone and 3,082-ton Norwegian Grado in a submerged attack.

During the night of 11/12 May U-383 and U-359 were driven off by Whitehall and Clematis, and Hesperus became involved in a classic engagement with U-223: the U-boat was depth-charged and forced to the surface, missed the destroyer with a salvo of five torpedoes and then, despite the destroyer’s use of depth charges set to explode just beneath the surface, gunfire and a cautious ramming, ddid not sink and managed to escape.

In the morning of 12 May Hesperus sank Korvettenkapitän Siegfried Hesemann’s U-186, the contact-keeper, which had been located by HF/DF, but in the course of the day 11 other U-boats establish contact, though they were again located by HF/DF. By dusk all the U-boats had been driven off by Hesperus, Whitehall, Sweetbriar, Clematis and Heather, and then lost the convoy after it had made a sharp turn during the night.

From a time early on 13 May air escort was once more provided by Swordfish from Biter, which had meanwhile reached the convoy, and in the afternoon Consolidated Liberator long-range maritime patrol bombers of the RAF’s No. 86 Squadron also arrived over the convoy.

On the following day one of the squadron’s Liberator aircraft attacked U-403, but the boat escaped without damage. On 15 May Korvettenkapitän Ralf von Jessen’s U-266 was sunk by a Handley Page Halifax four-engined medium-range maritime patrol bomber of the RAF’s No. 58 Squadron to the rear of the convoy, and at this stage Grossadmiral Karl Dönitz, commander-in-chief of the German navy but still in operational command of the U-boat arm, ordered the U-boats to break off their attack.

During the boat’s return to base, Oberleutnant Wolf Jeschonnek’s U-607 sank the 5,589-ton Irish Irish Oak, but the ON.184 convoy, supported by the British Escort Group B4, was not located by any of the boats.