'Donau' was a U-boat wolfpack two-element operation in the North Atlantic Ocean against the ONS.7, HX.238 and SC.130 convoys (15/26 May 1943).
On 11/12 May three wolfpacks had been established to the south-east of Cape Farewell, on the southern tip of Greenland, with U-304, U-645, U-952 and U-418 ('Isar' wolfpack), U-202, U-664 and U-91 ('Lech' wolfpack) and U-258, U-381, U-954 and U-92 ('Inn' wolfpack).
Among the U-boats proceeding to other proposed packs, Oberleutnant Karl-Heinz Nagel’s U-640 spotted the ONS.7 convoy of 40 merchant ships supported by Lieutenant Commander J. Jackson’s British Escort Group B5 (the destroyer Volunteer, the frigate Swale, the corvettes Buttercup, Godetia, Lavender and Saxifrage, and the rescue ship Copeland) during the night of 11/12 May. Although repeatedly driven off, the boat managed to retain contact until 13 May, allowing U-760, U-636, U-340, U-731 and U-657 to be deployed as the 'Iller' wolfpack. U-640 was then bombed and sunk by a Consolidated Catalina twin-engined flying boat of the US Navy’s VP-84 squadron.
In an effort to regain contact with this important convoy, on 15/16 May the U-boat command created the two 'Donau' wolfpacks with U-657, U-760, U-636, U-340, U-731, U-304, U-645, U-952, U-418, U-258 and U-381 as 'Donau I', and U-954, U-92, U-202, U-664, U-91, U-707, U-413, U-952, U-264, U-378 and U-218 as 'Donau II' from the original wolfpacks supplemented by the 'Nahe' wolfpack just moving out after replenishment.
The ONS.7 convoy was off the northern end of the German disposition, and during the night of 16/17 May Korvettenkapitän Heinrich Göllnitz’s U-657 attacked and sank the 5,196-ton British Aymeric, but was itself sunk by Swale.
On 17 and 19 May Oberleutnant Heinrich Wulff’s U-646 and Oberleutnant Hermann Rossmann’s U-273, while on passage to their positions, were sunk off Iceland by Lockheed Hudson twin-engined maritime reconnaissance bombers of the RAF’s No. 269 Squadron.
As a result of the efforts of their B-Dienst naval radio intercept and decrypting service, on 17/18 May the Germans were able to plot the positions of the evasive HX.238 and SC.130 convoys and the U-boats were therefore shifted and their line extended at the southern end by the addition of the newly gathered 'Oder' wolfpack comprising U-221, U-666, U-558, U-752, U-336, U-642, U-603 and U-228.
However, the HX.238 convoy of 45 ships, supported by Lieutenant Commander E. E. G. Boak’s Canadian Escort Group C3 (the destroyers Skeena and British Burnham, and the corvettes Bittersweet, Eyebright, and Canadian La Malbaie and Pictou and, within the convoy, the escort carrier Fencer on transfer to the UK) passed through the patrol line without being detected.
Bound from Sydney, Cape Breton island, to Liverpool, the 37 ships of the following SC.130 slow convoy departed Halifax on 11 May and were met by Commander P. W. Gretton’s British Escort Group B7 (the destroyer leader Duncan, the destroyer Vidette, the frigate Tay, the corvettes Snowflake, Sunflower, Pink, Loosestrife, and Canadian Kitchener, and the convoy rescue ship Zamalek). The convoy was spotted and reported during the night of 18/19 May by Oberleutnant Heinz Koch’s U-304, allowing U-645 and U-952 to come up. However, the Germans lost contact in the morning as the convoy executed a sharp change in course. Air escort, provided by Consolidated Liberator four-engined long-range maritime reconnaissance bombers of the RAF’s No. 120 Squadron frustrated the efforts of the U-boats: in its first approach, one Liberator attacked U-731, which was undamaged, and forced five other boats to submerge. One of these, U-952, was then depth-charged and badly damaged by Tay, Kapitänleutnant Wilhelm-Heinrich Graf Pückler und Limpurg’s U-381 was lost to an unknown cause, and U-636 was located by Snowflake and attacked with the help of Duncan as the two ships covered the area several times with depth charges but failed to damage the boat. Two other boats managed to escape Pink and Sunflower on the surface.
At 12.00 the 1st Support Group (frigates Wear, Jed and Spey, and Canadian sloop Sennen) arrived from behind the German patrol line and spotted two U-boats. Of these, Kapitänleutnant Odo Loewe’s U-954 fired torpedoes while diving but was sunk by 'Hedgehog' bombs from Jed and Sennen. Duncan evaded an attack by U-707, which was damaged. A second Liberator of No. 120 Squadron, in co-operation with Vidette, compelled six boats to submerge, and two more aircraft of the same unit forced down another four and two boats, respectively, three of the six being bombed. Before dusk, Jed and Spey had driven off the last contact-keeper, and only U-92 could mount an attack, which was unsuccessful.
In the morning the Germans broke off the operation, and a Liberator sank Kapitänleutnant Wilhelm von Mässenhausen’s U-258.
The 'Mosel' wolfpack was created out of the remaining boats (U-552, U-264, U-378, U-607, U-221, U-666, U-752, U-558, U-336, U-650, U-642, U-603, U-228, U-575, U-621, U-441, U-305, U-569, U-468, U-231 and U-218) on 19 May to operate against the HX.239 convoy. The convoy reached Liverpool on 26 May after losing only one ship.
It is generally agreed that this period marked the turning point in the 'Battle of the Atlantic' from German to Allied superiority.