As the war progressed, and the threat of German u-boats became acute, Castle Harbour's role was expanded. She was refitted, armed with a 3-inch gun on the bow, four machine guns and sixteen depth charges, and commissioned as HMS Castle Harbour. Crewed mostly by local-service ratings, she was tasked with anti-submarine patrols within the reefline to prevent attacks like that carried out at Scapa Flow by the German u-boat U-47 on 14 October 1939.