 In 1955 Crabb took frogman Sydney Knowles with him to investigate the hull of a Soviet "Sverdlov-class cruiser" to evaluate its superior manoeuvrability.According to Knowles, they found a circular opening at the ship's bow and inside it a large propeller that could be directed to give thrust to the bow.That same year, March 1955, Crabb was made to retire due to his age, but a year later he was recruited by MI6.