The tide of the battle of the Atlantic turned in May 1943 when the Allies destroyed forty-one U-boats and Germany withdrew its forces from the North Atlantic. Allied codebreaking and advances in radar, along with growing numbers of very long-range aircraft, escort carriers, and support groups had proved decisive in defeating the wolf packs. However, Grand Admiral Karl Doenitz, Commander-in-Chief of the German Navy, viewed this defeat as merely a temporary setback, soon to be avenged by technical advances and improved weapons. In September 1943 the U-boats returned to the convoy lanes of the North Atlantic, equipped with the Zaunkoenig acoustic torpedo which homed on to the sound of a propeller, to spearhead a fall offensive against Allied shipping.
Doenitz formed a patrol line of twenty U-boats deployed astride the great-circle route to intercept transatlantic convoys. They did not have long to wait: two westbound convoys, ONS-18 and ON-202, steamed towards the pack from Great Britain following similar routes. Allied intelligence failed to reveal the presence of the pack in time to divert ONS-18 and ON-202 clear of the danger, but on 19 September Liberator A of 10 Squadron, RCAF, en route from Iceland to Gander, attacked and destroyed U-341 ahead of the convoys. U-270 sighted the silhouettes of the merchant ships in the early hours of 20 September and closed to attack. The U-boat fired an acoustic torpedo that struck the stern of HMS Lagan, disabling the frigate. HMCS Gatineau pounced on the enemy with an accurate depth charge attack, but U-270 escaped destruction. As dawn approached, U-238 penetrated the screen at periscope depth and sank two merchant ships with standard torpedoes.
Allied shore authorities ordered ONS-18 and ON-202 to merge to form a single convoy of sixty-five merchant ships with an escort of twelve warships, including HMCS Gatineau, Drumheller, and Kamloops. The support group EG 9, including HMCS St.Croix, Chambly, Sackville, Morden, and HMS Itchen, reinforced the escort. The large screen held the pack at bay during the daylight hours of the twentieth but later that night, in a battle with the escorts well astern of the convoy, U-305 sank St.Croix with an acoustic torpedo and U-952 destroyed HMS Polyanthus. Only 82 survivors were rescued by HMS Itchen from the two warships. In the meantime, a British Liberator and HMCS Drumheller attacked U-338. The U-boat was never heard from again. During 21 September fog enveloped the battle but the escort kept seven U-boats well away from the merchant ships, and in the process of doing so HMS Keppel rammed and sank U-229.
The fog lifted during the following afternoon and very long-range Liberator aircraft of 10 Squadron, RCAF, made their appearance, over 800 miles from their base in Newfoundland. One of these, L/10, attacked U-270 on the surface but had an engine shot out by flak, and was forced to return to base. Meanwhile, Liberator X/10 wounded U-377's captain with machine-gun fire. Both U-boats withdrew from the battle but ten others were still in pursuit during the night of 22-3 September. The escort had intercepted and forced off several U-boats when U-666 torpedoed HMS Itchen. Only three survived--including two of the 82 rescued from St.Croix and Polyanthus.
Although four escorts had been torpedoed the pack had been unable to menace the ships in the convoy since the attack on the first night. However, during the early hours of 23 September U-238 penetrated the screen for the second time and sank three merchant ships, escaping unscathed. Shortly before dawn, U-952 torpedoed another merchant ship but, despite these final successes, the battle had run its course. During the following day the surface escort and aircraft frustrated the remaining shadowers, leading Doenitz to call off the hunt. The U-boats had destroyed three escorts--not including Lagan which was towed into port but declared a total loss--and six merchant ships in return for three U-boats lost and several damaged.
The submarine commanders claimed to have sunk twelve escorts and seven merchant vessels; Doenitz thought that these losses showed that convoy operations again were feasible. But German success was an illusion and the element of surprise was missing. ONS-18/ON-202 would be the last major convoy battle in the northern Atlantic routes. The acoustic torpedo had been anticipated and within five days the RCN had developed CAT gear: a noise-making decoy towed astern that, like the British foxer, drew homing torpedoes away from the escort's propeller. Allied air power, support groups, and codebreaking continued to thwart the U-boats, and by November 1943 the offensive upon which the German Navy pinned its hopes had collapsed.