The German "blockade" of Great Britain followed on the British Government's declaration of the closing of the North Channel of the Irish Sea. The German Admiraltv extended its "war zone" beyond the Orkney Islands to include Kirkwall Harbour and the Shetland Islands.
The picture illustrates a squadron of four British battleships at sea, engaged in one of those periodical sweeps down the North Sea which were a feature of Royal Naval strategy during the war. Lord Jellicoe noted a number of these sweeps during the two years of his command, frequently undertaken by the entire Fleet. At first, owing to the undefended condition of Scapa Flow, Admiral Jellicoe took the Fleet to sea very frequently, with the view that the danger from submarines was better countered in the open, with the ships under steam, and therefore capable of skilful manoeuvring at the hands of their well-trained officers and crews.
Later, when the defences of Scapa Flow and the other bases were complete, it was not necessary to go out so often, but throughout the whole period of the war the North Sea was swept by repealed cruises of the Grand Fleet, and by more numerous sweeps of the lighter vessels. In this way a large amount of sea space was efficiently covered, and the chances of the German Fleet, if within that area, of not being brought to action were few.
A typical chart of one of these sweeps was included by Lord Jellicoe in his book, and shows that the Grand Fleet went within sixty miles of Heligoland. Admiral Jellicoe flew his flag on HMS Iron Duke during his period of command.
Battleships of the "Iron Duke" class during a "sweep" of the North Sea.