Forth Defences, Outer, Submarine Boom

A chart on a War Office file (The National Archives WO 78/5179) dated August 1916 shows the location of the anti-submarine and anti-destroyer booms and nets in the Forth. There are four lines of defence, the outer (easternmost) two being associated with the 'Outer' line of gun batteries in Leith and Kinghorn and on the island of Inchkeith.

The outermost defence comprises two lengths of boom between Inchkeith and the southern shore of the Forth, at Leith. The northern section is about 1.5km long, from NT 2948 8170 to NT 2891 8024 blocking water about 5m deep at high water. There is a gap of about 600m to the southern barrier: this is the South Channel, which is about 14m deep at high water. The southern barrier runs for about 2km from NT 29097965 to NT 2757 7828, blocking water 5-6m deep at high water, stopping about 800m from the Leith sea wall (an area where the water is only 2-3m deep.

The next line of defence, about 3km to the west, but also associated with the 'Outer' defences of the Forth, was a complete barrier across the estuary, running from Black Rocks on Burntisland Sands (NT 24734 85457), with a dog-leg to the east (to the shallows at West Gunnet NT 26418 82122), to Granton Harbour (NT 24151 77629). Entrances through the boom are marked in the northern section - 500ft wide at the North Channel of the estuary (48m deep), 250ft wide in the southern sector, south of the South Channel and 500ft wide just north of West Gunnet, where the water is 11m deep.

The map describes the booms as being supported on 'pile dolphins' which are bundles of piles set into the sea-bottom, supporting 5-inch hawsers and 3-inch submarine nets.

Information from HS/RCAHMS World War One Audit Project (GJB) 14 August 2013.