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
