The commandos were aided in their planning for the operation by Captain Bill Pritchard 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 disable 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.