At 13:05 the U-boat estimated that it was now about eight miles ahead of the convoy and submerged to use her hydrophones, proceeding dead slow at a depth of . On detecting propeller noise, the U-boat altered course to approach the convoy. After half an hour the hydrophone operator heard faint propeller noises approaching. The U-boat altered course again and after 15 minutes the operator again reported oncoming propeller noises. The captain ordered the U-boat to rise to periscope depth. Unknown to "U-353", this was the destroyer , about two miles ahead of Convoy SC 104, which had gained a firm Asdic (sonar) contact, and increased speed to 15 knots to intercept the U-boat.