The German wolf-packs return to the North Atlantic armed with "Gnat" acoustic torpedoes. Their plan is to use the new torpedoes to sinking the escorts first, then attack the unprotected convoy. On September 20, 20 U-boats attack Slow Convoy ONS 18 using the new "Gnat" acoustic torpedoes. Six of 63 merchant ships are sunk as well as 3 escorts. HMCS St. Croix is hit by 3 acoustic torpedoes. Royal Navy frigate Itchen picks up 87 survivors. Itchen is also torpedoed a few days later and only one crewman from the St. Croix survives the double attack. Fortunately the Allies scientists have anticipated the introduction of acoustic torpedoes and soon put into service 'Growler' noisemakers, towed astern to attract the Gnat away from the vessel. The U-boats do not repeat their successes.