Although Scapa Flow was nearly landlocked with narrow entrance channels providing protection from strong tides and North Atlantic swells, the fleet required protection from attack whilst at anchor and so the harbour was fortified. The lack of infrastructure at the newly selected Fleet Anchorage was a logistical nightmare for the Admiralty which had to organise supplies and the transport of stores, men and equipment not just for the fleet but also for all of the defences that were to be built around the coast.
Defence against submarines was provided by boom nets strung across the entrances into Scapa Flow and by hydrophone listening stations where observers would listen for the sounds of approaching underwater craft. Booms were constructed of heavy duty chain and supported by wooden floats. This chain could impede any hostile vessel attempting to enter the harbour on the surface slowing it down and allowing shore based gun batteries to open fire on it. An anti-submarine net was often suspended below it to counter submerged craft. The boom could be opened and closed by a pair of boom defence drifters (requisitioned fishing boats with a pair of outriggers mounted on the bow to lift the boom net) to allow the passage of friendly vessels.
Clestrain Sound, at the western entrance to Scapa Flow was too fast flowing for boom defences so a complex underwater scaffolding arrangement known as the Clestrain Hurdles was installed there. A magnetic induction loop was also laid on the seabed outside of each entrance. This would register changes in the electrical field when the metal hull of a submerged or surface craft passed over it. This change would alert observers monitoring the magnetic field at shore station of a vessels presence. The Royal Navy also operated an examination service during both World Wars. This was effectively a traffic control system which stopped and checked all civilian traffic attempting to enter Scapa Flow, only admitting those with a permit during the hours of daylight.
The coast artillery batteries supported this service providing firepower if it was required to halt a ship. The examination service ran in conjunction with Port War Signal Stations which were a vital link that enabled ship to shore communications. Through these stations military and civil shipping could request permission for the booms to be opened to allow them to enter or leave Scapa Flow. Without them ships also ran the risk of coming under fire from the coast artillery batteries or from sailing into one of the controlled minefields which would be deactivated to allow friendly ships to pass through unharmed.
The greatest threat was thought to be from the German surface fleet and so 12 pounder, 4.7 inch and 6 inch naval ships guns were emplaced at the entrances to Scapa Flow in coast artillery batteries which were manned by naval ratings and Royal Marines. The defences of each entrance into Scapa Flow were divided into Fire Commands. These Commands had a single commander who was responsible for coordinating all of the gun batteries that were defending each entrance. This system allowed the guns to be directed onto specific targets making the defences far more effective than if each gun battery were working independently.
The batteries normally consisted of two ex-naval guns, however there are a number of exceptions to the rule where more were thought necessary. The guns were directed from a Battery Observation Post where men would calculate the bearing and range to a target and transfer the data to the gun crews. To facilitate defence against night intruders, each battery was equipped with searchlights housed in corrugated iron and concrete emplacements. These searchlights could illuminate an area of water at night and give warning of an approaching vessel. They could also be used to track a target so that it could be fired upon.
The coast artillery batteries were often situated in remote locations and the conditions in which these men had to live and work were incredibly basic. Indeed some of the accommodation was reminiscent of a Napoleonic warships mess deck with hammocks slung from the ceilings and bench seating to eat at below. The job of keeping watch for intruders both day and night and through adverse weather conditions would have been both gruelling and monotonous for the men stationed at these sites.
(c) Gavin Lindsay