Prime Minister David Lloyd George was aware that Britain’s manpower resources were dwindling. He had prioritised shipbuilding, tanks and aircraft production before army demands. The army wanted 1.25 million new entrants. Lloyd George was only prepared to provide 0.37 million personnel. He proposed that the army in France stood on the defensive until US forces were built up for a successful offensive later in 1918.
In January 1918, the United States began to transport their troops across the Atlantic in escorted convoys. Half were delivered by American ships directly to Bay of Biscay ports and half by British ships to Liverpool, mostly through the Irish Sea.
The German Imperial Navy were aware of these developments and targeted the troop convoys, but with little success. By May 1918, the Admiralty had made the Straits of Dover impassable for submarines. This forced the U-boats on a six-day detour around Scotland to reach attack zones south of County Cork and a six-day journey back.
Every few days, high priority south-north ocean convoys passed through the Irish Sea. They preferred to make the last stage of the journey to Liverpool at night and in blackout conditions. This was designed to foil attacking U-boats. However, this also made it difficult for west-east traffic with several collisions and near misses. In March, the London North Western Railway Company’s SS Rathmore collided with another ship and had to be towed back to Dublin; 700 were on board and 5 died. Night crossings were subsequently rescheduled to be undertaken during the day.
The Admiralty had a reasonable idea of the position of each U-boat at sea by using a network of direction finders to pick up their frequent wireless signals. This information was collated in London and relayed to the local command centres for action. More locally, from May 1918, Holyhead set up Irish Sea hunting patrols, using nine older destroyers and Airship reconnaissance patrols.
The approach appeared to work until the sinking of the unescorted mailboat RMS Leinster on the morning of 10 October, 20 miles out from Dublin, with the loss of over 500 lives. Bad weather had halted patrolling. Four days later, the SS Dundalk was sunk off Anglesey.
This suggests that the U-boats, with their limited resources towards the end of the war, had concentrated their attention elsewhere for the previous few months. There is some evidence to suggest that they targeted troop transports from May to September with several sinkings of returning ships with no troops on board.