Boom Defence Nets
During both World Wars the main entrances into Scapa Flow were protected by a series of block ships and anti-submarine boom nets which were strung from shore to shore. These anti-shipping booms and anti-submarine boom nets were the first defences put into place during the second World War. The theory was that any fast moving destroyer would suffer extensive damage if it hit a boom, giving the coastal defences time to fire at the vessel.
Work began in June 1938 and the work progressed under the supervision of the boom defence officer who was based in Rysa Lodge in Hoy. By the end of the year the Hoxa booms were in place. When the war began on the 3rd of September 1939, the boom defences in Scapa Flow were the only ones that were near to being ready.
Three main booms were in place. A net ran from the shore at Hoxa across to Flotta, Innan Neb in Flotta had a net running across to Hackness at South Walls, Hoy and a further net ran from Houton Head in Orphir across to Scad Head on Hoy.
When the war began the Switha and Houton booms were also in place. Gates in the booms were opened and closed by Boom vessels which were converted drifters.
HMS Pomona established at Lyness was the Boom Defence Command and was responsible for maintain these nets and booms.