'Ungestüm' was a U-boat wolfpack operation in the Atlantic against the HX.218, ONS.152 and, in tandem with 'Spitz', ONS.154 convoys (9/26 December 1942).

The wolfpack comprised U-336, U-373, U-435, U-445, U-455, U-524, U-569, U-591, U-604, U-615 and U-628, and for the loss of none of its own number sank one 3,066-ton ship of the ONS.152 convoy.

From 9 December the 'Büffel' wolfpack (U-373, U-445 and U-663) was deployed to the south of Greenland along the anticipated course of the HX.218 convoy, which was supported by the British Escort Group B3. During 13 December U-373 sighted and reported the HX.218 convoy of 54 ships, but was driven off four hours later by two destroyers of the escort. U-663 was depth-charged by a destroyer during the night 13/14 of December and had to turn for base after suffering damage.

In the hope that the convoy would adhere to the course predicted by the B-Dienst signals intelligence service, the 'Ungestüm' wolfpack was redeployed from its current area into the area to the south of Iceland on 13 December to intercept the convoy. On 14 December the strong Allied air escort forced U-373 and U-445 to submerge, and on the following day the HX.218 convoy passed to the south of the 'Ungestüm' wolfpack’s patrol line. U-373 then spotted and reported the ONS.152 convoy of 22 ships, supported by the Canadian Escort Group C3, on 16 December. U-445, U-524 and U-615 came up, but the U-boats then lost contact in the deteriorating visibility and a worsening of the weather toward hurricane force on 18 December.

The U-boats continued to search until 22 December, but sighted only independents and scattered ships, Kapitänleutnant Hans-Jürgen Zetzsche’s U-591 sinking the 3,066-ton British Montreal City on 21 December.

The 10 boats of the 'Ungestüm' wolfpack, supplemented by U-373, established a new patrol line on 25/26 December. It was on 24 December that the 'Spitz' wolfpack (U-123, U-203, U-225, U-260, U-356, U-406, U-440, U-659, U-662 and U-664) was established from boats newly arrived in the eastern part of the North Atlantic.

On 26 December, somewhat to the south of the patrol line, U-664 spotted and reported the ONS.154 convoy of 45 ships and the HF/DF-equipped rescue ship Toward, supported by Commander G. S. Windeyer’s Canadian Escort Group C1 (destroyer St Laurent and corvettes Battleford, Chilliwack, Napanee, Orillia and Shediac). As well as the 'Spitz' wolfpack, the 'Ungestüm' wolfpack, stationed a little farther to the west, was then deployed against the convoy. During the afternoon of 26 December U-662 established contact with the convoy and during the night of 26/27 December Oberleutnant Günther Ruppelt’s U-356 made two approaches and sank three British ships in the form of the 5,952-ton Empire Union, 5,224-ton King Edward and 2,473-ton Melrose Abbey, and torpedoed and damaged the 7,051-ton Dutch Soekaboemi, which was later despatched by Kapitänleutnant Klaus Hartmann’s U-441. This boat also attacked but missed St Laurent and one other vessel. After competing its second attack, U-356 was itself sunk by St Laurent, Battleford, Chilliwack and Napanee.

Oberleutnant Wolfgang Leimkühler’s U-225 regained contact in the afternoon of 27 December, and during the following night torpedoed and damaged the 7,087-ton British tanker Scottish Heather, which was taken in tow by Chilliwack. The Germans yet again re-established contact after a nine-hour gap, when U-260 sighted the convoy and called up U-123, U-203, U-225, U-336, U-406, U-440, U-591, U-615 and U-664 during the daylight hours and then U-435, U-628 and U-662 in the dark hours. During the night of 28/29 December the U-boats made a series of attacks. Following misses by U-203 and U-435, Zetsche’s U-591 torpedoed and damaged the 5,701-ton Norwegian Norse King, which was later sunk by Kapitänleutnant Siegfried Strelow’s U-435; Leimkühler’s U-225 sank the 5,273-ton British Melmore Head and torpedoed and damaged the 5,598-ton British Ville de Rouen, which was later sunk by Korvettenkapitän Wolfgang Hermann’s U-662; Kapitänleutnant Hubertus Purkhold’s U-260 sank the 4,893-ton British Empire Wagtail; and after a miss by U-203, Kapitänleutnant Horst Dieterichs’s U-406 torpedoed and damaged three ships, the 3,385-ton British Baron Cochrane later sunk by Oberleutnant Horst von Schroeter’s U-123, the 5,029-ton Lynton Grange by Oberleutnant Heinrich Hasenschar’s U-628, and the 4,871-ton British Zarian by Zetsche’s U-591. At 24.00 Leimkühler’s U-225 damaged the 4,919-ton Belgian tanker Président Francqui, which was then sunk by U-225 and Kapitänleutnant Hans Hunger’s U-336, and also the 7,068-ton British Empire Shackleton, which was sunk by Kapitänleutnant Karl-Heinz Moehle’s U-123 and Strelow’s U-435. Other attacks by U-203, U-435, U-628 (twice) and U-664 on the escorts and merchant vessels were all are unsuccessful. The British destroyers Meteor and Milne arrived at daybreak on 29 December to strengthen the escort and drove off U-260, U-591 and Strelow’s U-455.

In the evening of the same day U-225 and U-615 made several attacks on the 2,456-ton auxiliary warship Fidelity, but this French-manned ship managed to avoid the attacks. On the following day, however, she was sunk by Strelow’s U-435. U-455 maintained contact with the convoy until the night of 30/31 December, but could bring up no more of the boats, all of which then broke away to rendezvous with U-117 for replenishment.