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.
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 .
