While L Class submarines were under construction, Vickers received an order, in January 1917, to build 12 boats to the American H Class design. Twenty boats of this class had been contracted from the Bethlehem Steel Works, USA in November 1914 for the Royal Navy. Fourteen had been delivered (H1 to H12, H14 and H15) - whilst the other six had been forfeited to Chile as compensation for warships seized in 1914. H14 and H15 were transferred to the Royal Canadian Navy in April 1919.
The British H boats, known as the H21 Class, were a modification of the American design, and accommodated a heavier torpedo armament (21-inch bow tubes replacing 18-inch
