'Stier' (i) was a U-boat wolfpack operation, in tandem with 'Vorwärts', in the Atlantic against the ON.127 convoy (29 August/2 September 1942).
The wolfpack comprised U-96, U-380, U-404, U-584, U-594 and U-608, and for the loss of none of its own number sank seven ships (58,544 tons) and damaged three others of the ON.127 convoy, as well as the 415-ton Delães, a neutral Portuguese three-masted schooner which happened to be in the area.
The German attack on the ON.127 convoy was the only North Atlantic battle in which all of the U-boats involved launched torpedoes.
The ON.127 convoy of 34 ships departed Liverpool on 4 September, and was joined at mid-day on the following day by Lieutenant Commander A. H. Dobson’s Canadian Escort Group C4 (destroyers Ottawa and St Croix, and corvettes Amherst, Arvida, Celandine and Sherbrooke). All of the warships carried radar, but none had HF/DF equipment.
The 'Vorwärts' and 'Stier' (i) wolfpacks were forming in an area some 500 miles (800 km) to the west of Ireland as the convoy sailed, and U-91, U-92, U-96, U-211, U-218, U-380, U-404, U-407, U-411, U-584, U-594, U-608 and U-659 established a patrol line across the convoy’s path just beyond the range of land-based aircraft.
U-584 sighted and reported the convoy on 9 September, but lost contact during the evening of the same day. Oberleutnant Hans-Jürgen Hellriegel’s U-96 re-established contact on 10 September, and sank the 6,313-ton Norwegian tanker Svene and 4,241-ton Belgian Elisabeth van Belgie, and also damaged the 12,190-ton British tanker F. J. Wolfe in a single submerged daylight attack. Sherbrooke fell back to aid the torpedoed ships while an unsuccessful search for the U-boat was undertaken by St Croix, Ottawa and Celandine. F. J. Wolfe was able to regain her position in the convoy.
Ottawa now patrolled astern of the convoy after St Croix and Celandine had returned to their normal patrol stations. A co-ordinated night attack on the convoy began with Kapitänleutnant Hans Stock’s U-659 torpedoing and damaging the 8,029-ton British tanker Empire Oil during the evening of 10 September. St Croix had made sonar contact immediately before the attack, and Celandine, Ottawa and St Croix searched for U-659 after the attack before the two destroyers fell back to rescue 23 of the tanker’s 41-man crew.
Kapitänleutnant Otto von Bülow’s U-404 torpedoed and damaged the 7,417-ton Norwegian tanker Marit II, U-608 missed, Kapitänleutnant Richard Becker’s U-218 torpedoed and damaged the 7,361-ton Norwegian tanker Fjordaas, and U-92 and U-594 missed before Ottawa, St Croix and Celandine rejoined the convoy. Sherbrooke remained astern of the convoy aiding the ships torpedoed by U-96, and rescued all but one of the crew of Svene and Elisabeth van Belgie before they sank. The other escorts counterattacked, depth-charge damage compelling U-218 and U-659 to return to port. Both Marit II and Fjordaas were able to regain their stations in the convoy, but Empire Oil was later sunk astern of the convoy by Kapitänleutnant Joachim Deecke’s U-584.
None of the escorts' radar equipments was serviceable on 11 September, when U-584 sank the 4,884-ton Norwegian Hindanger in a submerged daylight attack while St Croix was investigating a visual sighting some 6 miles (10 km) away, and Amherst dropped back to rescue all but one of Hindanger's 41-man crew. The appearance overhead of a Consolidated Liberator long-range patrol bomber of the RAF’s No. 120 Squadron now prevented further daylight attacks on 11 September, but Hellriegel’s U-96 sank the 415-ton Delães with gunfire in the vicinity of the convoy.
During the night which followed, the Germans made a number of co-ordinated night attacks. U-380 missed with a salvo of four torpedoes, Kapitänleutnant Karl Hause’s U-211 torpedoed and damaged the 13,797-ton British whale factory ship Hektoria and 6,849-ton British Empire Moonbeam, U-92 missed Ottawa with four torpedoes, and von Bülow’s U-404 torpedoed and damaged the 9,272-ton Norwegian tanker Daghild before Amherst and Sherbrooke rejoined the convoy. Daghild was able to maintain station in the convoy and Arvida rescued all but four of the 140 crewmen from Hektoria and Empire Moonbeam before those ships were sunk astern of the convoy by Oberleutnant Rolf Struckmeier’s U-608.
Excellent visibility on 12 September allowed a close forward screen of four escorts to discourage U-boats sighted at distances as great as 7 miles (11.25 km). U-407 and U-594 attacked during the following night but missed. Then Oberleutnant Friedrich Mumm’s U-594 sank the 6,131-ton Panamanian straggler Stone Street as the convoy arrived within range of RCAF Canadian Boeing Canso flying boats operating from Botland, Newfoundland, on 13 September. The escort was reinforced at dusk by the Canadian destroyer Annapolis and British destroyer Witch of the Newfoundland-based Western Local Escort Force. Kapitänleutnant Heinz Walkerling’s U-91 and U-411 made unsuccessful torpedo attacks, but U-91 then torpedoed Ottawa in the pre-dawn hours of 14 September, the destroyer sinking with 114 of her 181-man crew.
The convoy’s surviving merchant ships reached New York City on 20 September.