 He invented the "Hezlet Rail", a bar and strap to secure the watchkeeper to the X-craft's casing in bad weather.In May 1943, Hezlet was appointed as the Lieutenant in Command of HMS "Thrasher" (N 37).From May to August 1943 she took part in exercises in Scottish waters with one-man Welman submarines and X-class "midget" submarines, undergoing alterations to her casing to allow her to operate with those craft.