1921 - 1925: X1 ClassX1 was a cruiser submarine. At the time of her completion in 1925 she was the largest submarine in the world at 363ft long and over 3000 tons.
She mounted 4 5.2 in guns in twin turrets and six torpedo tubes.
Her engines were very troublesome and she spent most of her life in dockyard hands before being placed in reserve.
Special provision had to be made for her in the London Treaty.