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. Construction was speeded up by using engines, main motors and other fittings obtained from America. The Vickers-built first-of-class boat, 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 480bhp. Also of American design, their main motors, powered by 120 battery cells, produced 620bhp for one hour and had a continuous rating of 320bhp.
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 of 13 knots surfaced and 10.5 knots submerged was obviously optimistic. In service, actual speeds of 11.5 knots surfaced and 9 knots submerged were recorded.
