The 'Battle of Pierres Noires' was a naval action within 'Dredger' off the coast of Brittany and involved four Canadian destroyers and one U-boat with four escorts near Brest in north-western France (5 July 1944).
The Canadian ships sank sank or damaged some of the escorts on the surface, but the U-boat was able to escape.
The port city of Brest had become an important German base for surface warships, and later U-boats, after the fall of France in June 1940, and its capture was one of the objectives of 'Overlord'. From 1943 the rapidly developing capabilities of the Allied anti-submarine campaign, however, meant that it had become increasingly essential for any U-boat departing the port to have surface escorts. The Kriegsmarine often used Vorpostenboot type escort trawlers for this task, and over time these boats developed a high reputation among the Allied navies. On 5 July 1944 the 12th Escort Group and 14th Escort Group were deployed to the Brest area as part of 'Dredger': of these two groups, the 12th Escort Group comprised four Canadian destroyers (Saskatchewan, Qu’Appelle, Skeena and Restigouche) and was tasked with intercepting any U-boats and their escorts, while the 14th Escort Group patrolled farther offshore to intercept any U-boats which managed to escape.
Oberleutnant Gerhard Palmgren’s U-741 of Korvettenkapitän Werner Winter’s 1st Unterseebootsflottille departed Brest on 5 July 1944 with four Vorpostenboot escort trawlers. Captain D. C. Wallace’s 12th Escort Group, with Qu’Appelle as the lead ship, detected the German force on radar and set off at 30 kt to effect an interception. The two sides engaged in the vicinity of the Pierres Noires lighthouse in the Iroise Sea late in the evening. The darkness, combined with uncertainty of the size of the German force and the close range at which the ships engaged each other, nullified some of the advantage in speed and protection enjoyed by the Canadian destroyers, and U-741 managed to escape. However, despite inflicting some damage to all of the Canadian vessels, one of the the Vorpostenboote, V-715, was sunk, another was heavily damaged, and the remainder withdrew to Brest. The Canadian destroyers withdrew to Portsmouth in southern England for repairs.
'Dredger' continue with further Allied naval infiltration into the Bay of Biscay, engaging the U-boats and their escorts in their home waters. U-741 was sunk in the English Channel on 15 August, a few days after US forces had laid siege to Brest. The last U-boat departed Brest on 4 September, just before the US forces took control of the city on 19 September. Skeena was later lost in a storm off Iceland on 25 October, the only ship of the 12th Escort Group not to survive the war.