1:600 line drawing of "H21" type © Andrew Arthur

The first batch of ten of these submarines were built by Vickers of Montreal, Canada, the parts being supplied by the Bethlehem Steel Company of the U.S.A. A second batch that had been ordered were to be impounded by the Americans until the end of the war, but they were eventually released on the U.S. entry into the war in 1917.
These boats were single-hulled and suffered from a low reserve of buoyancy.
H1 of this class crossed the Atlantic in 1915, this was a record length for any submarine of the time.
H11 and H12 were the only two boats of the second group to reach Britain and served with the 14th Flotilla stationed at Blyth. The remainder of group "2" were either ceded to Chile in exchange for ships which were being built in Britain at the outbreak of war or presented to the Royal Canadian Navy. Due to the U.S. witholding the second batch of 'H' class the British decided to go ahead and build their own. More or less the same design but somewhat longer and introducing the 21 inch torpedo tube in preference to the 18 inch. - the H21 class.
They were used as training boats, although they saw some operations in 1939 & 40.