Under the command of Vice-Admiral Commanding, Channel Fleet, Sir Lewis Bayly, the 5th Battle Squadron spent 31 December participating in gunnery exercises off the Isle of Portland, supported by the light cruisers and . The squadron received no escort of destroyers for the operation. After the exercises, that night the fleet remained at sea on patrol even though submarine activity had been reported in the area. Visibility that night was good, though the sea was rough enough to make detection of a submarine difficult. Bayly suspected no danger from submarines, and so steamed his ships in line ahead formation at a speed of . "Formidable" was the last battleship in the line, followed only by the two cruisers. Unknown to the British, the German submarine stalked the squadron while it was exercising all afternoon, trying to find a suitable attack position.