22. U-20 was ordered to reposition after it left port
U-20, under Walther Schwieger, departed on patrol with specific orders to proceed up the North Sea, around Scotland, and down to the Mersey bar, a sandbank which restricted access to the estuary and the Port of Liverpool. When ships arrived at Liverpool they had to wait until high tide to enter the port, and many Captains altered their speed accordingly as they entered the approaches, to ensure they arrived at a time they could enter the Mersey without having to wait. SM U-20 was to sit outside the bar, where as ships slowed to begin their journey up the river he would have easy pickings with his torpedoes.
After he was underway, he received orders directing him to the south across the Irish Sea to the southern approaches of the British Channel. The radio messages sent to U-20 were intercepted and decoded by the Admiralty, in an area which Churchill had dubbed Room 40. The British were aware of the original mission of the submarine and the German decision to reposition to an area which would be traversed by the liners, including Arabic and Lusitania, but the warnings sent to the ships were limited to vague reports of submarine activity. Why more specific warnings were not sent, nor escorts provided, remains a mystery.