During 1939-1945, the yard's main activities were in building U-boats and repairing damaged warships. On 18 December 1939, 12 out of 22 RAF's Wellington bombers were shot down in an air battle over the naval base. Personnel were often assigned to organizing naval facilities in occupied countries, e.g., in the ports of Lorient, Brest and St. Nazaire. At the war's end there were about 17,000 workers.