The Royal Navy needed a northern base large enough for the Grand Fleet, but Rosyth and Invergordon could not accommodate the entire force without disrupting civilian facilities. Scapa Flow had the necessary water area, yet almost none of the construction required to make it a secure operational base. Jellicoe feared submarine or destroyer attack and kept much of the fleet patrolling off Britain's west coast until the anchorage was safer. More than sixty blockships, together with booms, nets, minefields, artillery, and concrete barriers, turned geographical space into a defended fleet base. Copenhagen would have offered a stronger blockade position against both the Baltic and North Sea, but Scapa Flow was the practical alternative available to Britain.
Chapter · 16:44-19:36
Making Scapa Flow safe for the Grand Fleet
Jellicoe would not treat the poorly defended anchorage as a proper base until blockships, nets, mines, and artillery made it secure enough for blockade operations.