Defending the Home Fleet.

During both world wars the Royal Navy's home fleet was based in Orkney, in the huge natural harbour of Scapa Flow.

The main entrances to the anchorage, Hoxa, Switha and Hoy Sounds and were defended by booms with steel nets and controlled minefields and indicator loops to detect and destroy intruders. Nineteen coastal gun batteries, with guns and searchlights were in operation by June 1940, with more to follow. Many of these coastal batteries remain prominent in the landscape today, despite a massive tidy up of wartime relics.

This is part of the large battery at Hoxa Head on South Ronaldsay which protected the boom across the main entrance of Hoxa Sound. The two emplacements in the photograph each housed a rapid firing 6lb gun, each with its own two-level control tower. The lower levels controlled the searchlights and the upper levels directed the guns.