1917 - 1945: H Class
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 were delivered (H1 to H12 , H14 and H15 ) - the other six were 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 tubes) which increased the overall length of the class by over 21 feet. Using engines, main motors and other fittings obtained from America speeded-up construction, and the Vickers-built first-of-class H21 was completed in January 1918 - 11 months after being laid down.
In June 1917, further orders were given to five other yards for 22 additional H21 submarines (H33 to H54), with their engines and motors being made in England to the American H Class design. Ten of these boats were subsequently cancelled when it was decided, in October 1917, to construct 12 new R Class submarines.
The H21 Class were the first Royal Navy twin-shafted single-hulled submarines, and their American-designed eight-cylinder vertical single armature diesel engines produced a total of 480 bhp. Also of American design, their main motors, powered by 120 battery cells, produced 620 bhp for one hour and had a continuous rating of 320 bhp.
As the H21 Class employed the same engines and main motors as the American H boats, but had an increase in displacement of over 70 tons, quoting the American design speeds -13 knots surfaced and 10.5 knots submerged - was obviously optimistic. In service, speeds of 11.5 knots surfaced and 9 knots submerged were recorded.
Although built for service in the First World War, the only H21 Class submarines to be lost on active service were, ironically, during the Second World War - over 20 years later. The H49 was depth charged off the Dutch coast in October 1940 and the Barrow built H31 was lost, presumed mined, in the Bay of Biscay on Christmas Eve 1941.