 The third-batch S-class boats were fitted with either a Type 129AR or 138 ASDIC system and a Type 291 or 291W early-warning radar.Her first war patrol was in the North Atlantic, from 29 December 1943 to 12 January 1944.She returned to Lerwick, in Shetland, where a battery was replaced and a "singing" screw was repaired.