'Stürmer' (i) was a U-boat wolfpack operation, in tandem with 'Dränger' and 'Raubgraf', in the Atlantic against the HX.229 and SC.122 convoys (11/20 March 1943).

The wolfpack comprised U-134, U-190, U-229, U-305, U-338, U-384, U-439, U-526, U-527, U-530, U-598, U-618, U-631, U-641, U-642, U-665 and U-666, and for the loss of Oberleutnant Hans-Achim von Rosenberg-Gruszcynski’s U-384 sank 10 ships (60,364 tons) and damaged one 5,234-ton ship in attacks on the SC.122 and HX.229 convoys, which were a pair of British transatlantic convoys involved in the war’s largest convoy battle with U-boat wolfpacks between 16 and 19 March 1943.

During March 1943 the Battle of the Atlantic was reaching its crescendo, after a lull in the winter of 1942/43, as the German navy renewed its U-boat offensive on the North Atlantic convoy routes with increased success. March saw a series of fierce convoy battles and was, for the Allies, the crisis point of the whole campaign. The largest of these battles developed around the HX.229 and SC.122 convoys, which were both attacked in mid-Atlantic, and which coalesced into a single widespread action involving more than 100 ships and escorts, and nearly 41 U-boats of the 'Raubgraf', 'Stürmer' and 'Dränger' wolfpacks with 12, 18 and 11 U-boats respectively.

The SC.122 slow eastbound convoy comprised 60 ships bound from New York rather than Sydney, Cape Breton, as the had done in an earlier period and would once more do so at a later period, and departed on 5 March. The convoy was initially shielded by one destroyer and five corvettes of the Western Local Escort Force. On 6 March, off Cape Cod, two ships turned back to New York as a result of heavy weather, and on 8 March another six turned back, in this instance Halifax, Nova Scotia. The convoy continued, changing escorts off Cape Race on 13 March, when the ships of the Western Local Escort Force were replaced by Commander R. C. Boyle’s British Escort Group B5 from St John’s, Newfoundland. The new escort comprised eight warships, with Boyle leading in the destroyer Havelock, and otherwise including the US destroyer Upshur, British frigate Swale, and British corvettes Buttercup, Godetia, Lavender, Saxifrage and Pimpernel, together with Zamalek as rescue vessel.

HX.229 was also an eastbound convoy, and departed New York on 8 March with 40 ships supported by warships of the Western Local Escort Force; another 34 merchant ships, which should have been included in HX.229 but had been delayed as a result of the port congestion in New York, departed on the following day as the HX.229A convoy. The first few days of the convoy were uneventful. The warships of the Western Local Escort Force left the HX.229 convoy on 14 March as responsibility for it was assumed by Commander G. J. Luther’s British Escort Group B4 from St John’s, comprising four destroyers and one corvette. Luther led in Volunteer, and the group’s other ships were the destroyers Beverley, Mansfield and Witherington, and corvette Anemone, although Witherington had to detach on 15 March and be replaced by the corvette Pennywort for the crossing.

In the path of the convoys were three wolfpacks. The 'Raubgraf' wolfpack of eight boats was already established and had just been involved in a battle with the HX.228 convoy, and this wolfpack was now sent to establish a patrol line in the area to the east of Newfoundland, at the western edge of the mid-Atlantic 'air gap' over which no Allied aircraft possessed the range to operate. The 'Stürmer' (i) wolfpack was a new group of 18 boats, and this was directed to form in the middle of the 'air gap'; this wolfpack was formed of boats from the 'Westmark' wolfpack, which had previously engaged the SC.121 convoy. The third wolfpack was the 'Dränger' group of 11 boats, and formed in the area to the east of the 'Stürmer' (i) wolfpack; some of this wolfpack’s boats were from the 'Neuland' wolfpack, which had also been in the battle with the HX.228 convoy, and the rest were newcomers.

The B-Dienst naval signals intercept and analysis service had warned that an eastbound convoy was imminent, and by 20.00 on 13 March had a location for the SC.122 convoy. Grossadmiral Karl Dönitz, commander-in-chief of the German navy but still in control of the U-boat arm via his deputy, Konteradmiral Eberhard Godt in the capacity of the service’s operations chief, directed the 'Raubgraf' pack to intercept, forming a new line farther to the west. A westerly gale increased the speed of the SC.122 convoy, however, so this passed through the patrol area of the 'Raubgraf' pack on the morning of 15 March, just 24 hours before the patrol line was formed.

The HX.229 convoy was following a similar course, and passed through the patrol line of the 'Raubgraf' wolfpack during the night of 15/16 March without being sighted, largely as a result of the adverse weather. However, on the morning of 16 March Kapitänleutnant Gerhard Feiler’s U-653,which had left the 'Raubgraf' wolfpack to return to base with mechanical problems, sighted the HX.229 convoy heading to the east, and reported this sighting. Dönitz immediately ordered the 'Raubgraf' wolfpack to pursue and intercept, while the 'Stürmer' (i) and 'Dränger' wolfpacks were ordered farther to the west to form a line ahead of the convoy. Dönitz saw this as an major opportunity to attack an eastbound convoy, laden with war matériel bound for the UK and with the full width of the 'air gap' still to cross.

The 'Raubgraf' wolfpack came up with the HX.229 convoy on the evening of 16 March and attacked during the night of 16/17 March, sinking three ships. Another five followed on the morning of 17 March for a total of eight sinkings in eight hours. The escort was reported to be weak, as two of the warships had dropped back to rescue survivors. The escorts chased three contacts during the night, but with no result. During the rest of the day, boats of the 'Stürmer' (i) wolfpack started to arrive, one of them being attacked by a destroyer but again without success.

Meanwhile, at the north-eastern end of the 'Stürmer' (i) wolfpack’s line, Kapitänleutnant Manfred Kinzel’s U-338 had sighted the SC.122 convoy as it steamed to the east, about 120 miles (195 km) from the position of the HX.229 convoy. After sending a sighting report U-338 attacked, sinking the 7,886-ton Dutch Alderamin, 4,071-ton Panamanian Granville, 5,072-ton British King Gruffydd and 4,898-ton British Kingsbury in quick succession and damaging the 7,134-ton British Fort Cedar Lake, which was sunk later in the day by Oberleutnant Hans-Jürgen Haupt’s U-665. The U-boat then survived a fierce counterattack by the escorts. The HX.229 convoy lost two more ships during the day. Two boats of the 'Stürmer' (i) wolfpack were able to penetrate the convoy’s defences at about 12.00 on 17 March, but the escorts were able to fend off any further attacks, assisted by brief visits from very-long-range aircraft flying to the limits of their operational radius. The SC.122 convoy was also able to resist further attacks until the evening.

During the night of 17/18 March the attack on both convoys, now just 70 miles (110 km) apart, continued. After 24.00 Kapitänleutnant Rudolf Bahr’s U-305 sank the 8,789-ton British Port Auckland and 4,256-ton British Zouave. The HX.229 convoy escaped further losses this night. The escort of the HX.229 convoy suffered a blow as Mansfield was forced to detach during the night of 17/18 March. Help was on the way to the Allies, however, in the form of the destroyer Highlander, under Commander E. C. L. Day. Arriving on 18 March Day, as a senior and more experienced officer, Day assumed command of the British Escort Group B4 for the rest of the engagement. Also en route from the Hvalfjörður in south-western Iceland were the British destroyer Vimy and US destroyer Babbitt for the HX.229 convoy, and the US Coast Guard cutter Ingham for the SC.122 convoy. These had been dispatched on the morning of 18 March and arrived on the following day.

During the afternoon of 18 March Kapitänleutnant Hans-Hartwig Trojer’s U-221 succeeded in sinking two of the HX.229 convoy’s ships, namely the 8,293-ton British Canadian Star and 7,191-ton US Walter Q. Gresham, but the convoy then suffered no further losses. Highlander joined the convoy that afternoon, and was an especially welcome addition as the escort had by this time been reduced to five ships.

During the night of 18/19 March the two convoys were running in tandem, though sailing independently. All attacks on the convoys were repelled during this night, and six firm contacts were attacked, but little damage was inflicted. One ship was lost by the HX.229 convoy. A US vessel which had broken away to proceed independently, this 5,848-ton Matthew Luckenbach ran into the fighting round the SC.122 convoy and was torpedoed by U-527, to be sunk later on 19 March by Kapitänleutnant Werner Pietzsch’s U-523. A straggler from the SC.122 convoy, the 5,754-ton British Clarissa Radcliffe, was hit by Kapitänleutnant Heinrich Schmid’s U-663, and disappeared without trace.

On 19 March the escorts were reinforced by the arrival of Vimy and Babbitt for the HX.229 convoy, and of Ingham for the SC.122 convoy. The HX.229 convoy was also joined by the British corvette Abelia, detached from another convoy. Also on 19 March von Rosenberg-Gruszcynski’s U-384 was attacked and sunk to the north of the SC.122 convoy by a Boeing Fortress long-range patrol bomber of the RAF’s No. 206 Squadron.

There were no further losses on 19 March and, faced by stiffening resistance, and sensing nothing further would be achieved without disproportionate losses, Dönitz called off the assault as the convoys continued to the east. Further changes to the escort occurred on 20 March as reinforcement arrived in the form of the British destroyer Sherbrooke, while Upshur and Ingham were detached.

The local escort groups met the convoys on 23 March. The HX.229 convoy, with 27 ships surviving, reached Liverpool on 23 March, while the SC.122 convoy, with 42 ships surviving, reached the same port later the same day. The double battle had involved 90 merchant ships and 16 escorting warships, though not all of the latter were present at the same time: 22 merchant ships had been sunk, in the form of 13 and nine from the HX.229 and SC.122 convoys respectively, amounting to 146,000 tons. More than 300 merchant seaman lost their lives. Some 38 U-boats had been involved (though again, not all had been in contact throughout), and one had been lost with its entire crew. This was the largest convoy battle of the Atlantic campaign, and was, for the Allies, the crisis point of the whole campaign as so many ships had been lost in return for only one U-boat sunk.