The 'Battle of Audierne Bay' was an engagement between Allied and German naval flotillas fought off the coast of Brittany (23 August 1944).
Three Allied warships, which had already established control off the coast of Brittany and were waiting off Audierne Bay to the south of the US-invested fortress of Brest, intercepted and sank eight German vessels of an armed convoy. This was the conclusion of 'Kinetic', an Allied undertaking to intercept shipping and hinder any German effort to support its forces besieged in Brest.
By 11 August, the German 'Lüttich' counter-offensive at Mortain, launched agaist the eastern flank of US 'Cobra' break-out from the Normandy lodgement within 'Overlord', had ground to a halt. To the east, US forces took Argentan on 13 August while British and Canadian forces closed on Falaise from the north, thus initiating the drive to encircle and destroy two German armies inside the Falaise pocket. As the siege of the Breton ports continued, the focus of the war was quickly shifting farther to the east.
'Kinetic' had been created by the Royal Navy’s headquarters with the object of eliminating the German navy all along the French Atlantic coast and its various German enclaves. Of three groups of ships involved in 'Kinetic', Force 27 under the command of Captain W. W. Davis, comprising the light cruiser Mauritius and the destroyers Ursa and Canadian Iroquois, departed Plymouth on 13 August to carry out a new patrol along the central section of the Bay of Biscay’s eastern coast. With the Germans under the command of Admiral Theodor Krancke’s Marinegruppenkommando 'West' active to evacuate by sea, 'Kinetic' had already scored two successes and was finalising its plans for a naval offensive in the Bay of Biscay to complete the success.
On the night of 22/23 August, operating on the basis of naval intelligence, Mauritius and her two destroyers were on patrol in Audierne Bay between Brest and Lorient when radar detected a large contact heading toward them. Davis ordered Iroquois, the ship closest to the contact, commanded by James Calcutt Hibbard to use her Type 293 radar and relay the information to the rest of the force. Hibbard placed so much confidence in the radar that he decided to direct the opening moves from his ship’s action information centre rather than the bridge, and then gave the order to illuminate the area with star shell. As a result, Force 27 was able to close undetected and launch a surprise attack on the convoy of three ships.
At 02.13 Iroquois scored the first hit on a Flak ship, which was soon seen to be on fire. A second Flak ship was quickly destroyed by the 6-in (152.4-mm) guns of Mauritius, while another was set on fire and driven onto the coastal shoals. Only 19 minutes after fire had been opened, all three German ships had been put out of action, with two sunk and the third aground and burning.
In a second action, just two hours later, Iroquois detected another convoy, this time of four ships, as it departed the harbour of Brest: the Germans vessels were one 'M' type minesweeper, two Flak ships and one converted Sperrbrecher mine-destructor ship. Using the radar on Iroquois once again, Force 27 stalked the convoy at long range until 04.08, when the German ships were illuminated with star shell. Upon opening fire, the British and Canadian ships quickly overwhelmed the convoy, sinking two vessels and causing two others to collide in the confusion and then catch fire as they headed for the shore, surviving crew members jumping over the side as they went. One of these vessels capsized and sank, while the other drove onto the rocks at full speed and exploded.
At dawn, Force 27 continued with another sweep around Audierne Bay in order to confirm the destruction of the German convoy. In the process a minesweeper was sighted, pounded with gunfire and driven onto a reef near Port Audierne. Iroquois finished off the minesweeper with a torpedo, while Ursa sent a boarding party onto the other grounded vessel to seize 11 prisoners. The other 150 or so survivors who were able to swim to shore were subsequently taken prisoner by the French resistance. This was the last part of the action, and the force consolidated and reviewed the damage its ships had caused.
The final tally for the night was eight ships destroyed: one minesweeper driven ashore and heavily damaged, and one Flak ship, five armed trawlers and one Sperrbrecher. Iroquois had fired 1,197 rounds of 4.7-in (119.4-mm) ammunition and 231 rounds of star shell. Davis in his report attributed the success of the night’s action to two principal causes, namely 'some lucky guesses, and the excellence of Iroquois' radar and plotting teams'. Although not known to Davis, it was actually 'Ultra' intelligence which should also be credited, for it was this which had put the ships in the right place at the right time.
In the last week of August there began another naval operation, 'Assault', during which the light cruiser Bellona patrolled along the Bay of Biscay’s coast from Belle Ile near Lorient to Arcachon Point to the south-west of Bordeaux. Continuing these sweeps through the remainder of the month, the Royal Navy and the Royal Canadian Navy patrolled off the coast. They sent landing parties ashore on the French mainland and outlying islands to secure them and ensured they were eliminated as potential bases of logistical supply to the Germans. The blockade of these Biscay ports, however, achieved impressive results independent of the campaign on land.
By the close of the 'Kinetic', the combined Allied naval and air offensive eventually resulted in the reorganisation, on 20 October, of the Kriegsmarine forces operating from the beleaguered fortress ports along the west coast of France into the smaller Marineoberkommando 'West'.