John Rushworth Jellicoe, admiral of the Grand Fleet, was perpetually nervous about the possibility of submarine or destroyer attacks on Scapa Flow. Whilst the fleet spent almost the first year of the war patrolling the west coast of the British Isles, their base at Scapa was defensively reinforced, beginning with over sixty blockships sunk in the many entrance channels between the southern islands to enable the use of submarine nets and booms. These blocked approaches were backed by minefields, artillery, and concrete barriers.