 He joined the Navy at the start of World War II and served as a stoker aboard the destroyers and , taking part in trans-Atlantic convoys and in Operation Pedestal, the relief of Malta.In 1942 Knowles joined the Underwater Working Party, under Lieutenant Commander Lionel Crabb, at Gibraltar.Knowles was one of a group of underwater guard divers who checked for limpet mines in Gibraltar Harbour during the period of Italian frogman and manned torpedo attacks.