 In the latter stages of World War I the D class were used for training crews based at Portsmouth.During World War I the boats patrolled the North Sea and the Heligoland Bight, and protected cross channel troopships.During the war, four boats ("D2", "D3", "D5", and "D6") were lost, and the remainder ("D4", "D7", and "D8") were paid off in July 1919.