For the Churchill barriers
Quote:
Work began on the barriers in May 1940, and the channels were effectively blocked by the end of 1942. By 1944 they had been so built up that locals were using a road built over them to drive between the islands – and they are still used as roads today
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The extra defences would have started early 1940. Mines and nets would have been placed quite quickly with extra block ships in Hoy Sound between the islands of Graemsay and Hoy. I cant find specific dates but by mid 1940, most should be in place.