The Grand Fleet sortied on 18 August to ambush the High Seas Fleet while it advanced into the southern North Sea but miscommunications and mistakes prevented Jellicoe from intercepting the German fleet before it returned to port. Two light cruisers were sunk by German U-boats during the operation, prompting Jellicoe to decide to not risk the major units of the fleet south of 55° 30' North due to the prevalence of German submarines and mines. The Admiralty concurred and stipulated that the Grand Fleet would not sortie unless the German fleet was attempting an invasion of Britain or that it could be forced into an engagement at a disadvantage. When Stanley was promoted to rear-admiral on 26 April 1917, he was replaced by Captain Walter Ellerton.