He joined the Navy at the start of World War II and served as a stoker aboard the destroyers Zulu and Lookout, taking part in trans-Atlantic convoys and in Operation Pedestal, the relief of Malta.
[1] 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.
On 17 December 1942, during one such attack, two of the Italian frogmen (Lt. Visintini and Petty Officer Magro) died, probably killed by small explosive charges thrown from harbor-defence patrol boats, a tactic said to have been introduced by Crabb.
In July 1943 Knowles took part in the recovery of the body of Free Polish Commander General Władysław Sikorski, after his aircraft crashed off Gibraltar.
Crabb disappeared after a dive on the Sverdlov's sister ship, Ordzhonikidze in Portsmouth harbour in 1956, but 14 months later a body in a frogman suit was found floating off Pilsey Island.
[1] [3] Knowles was played by Michael Craig in the fictionalised 1958 film The Silent Enemy about World War II divers in Gibraltar.