High Quality Content by WIKIPEDIA articles! HM Submarine X1 was conceived and designed as a submersible commerce raider for the Royal Navy; at the time of her launching she was the largest submarine in the world. The idea of a submarine cruiser had been proposed as early as 1915, but was not put into practice until 1921. X1, which was based on the uncompleted German U-173 class of 2,000-ton "U-Cruisers", was laid down on 2 November 1921 at the Naval Dockyard Chatham and completed on 23 September 1925, commissioning in December 1925.