 The Commandos were aided in their planning for the operation by Captain Bill <unk> of the Royal Engineers , who had pre @-@ war experience as an apprentice in the Great Western Railway dockyards and whose father was the dock master of Cardiff Docks . In 1940 while part of the British Expeditionary Force in France , his duties had included determining how to <unk> the French dockyards if they were captured . One of the dockyards he had studied was St Nazaire , and he had submitted a report detailing how to put the dock out of action . 
