This class were the first six production boats for use in the Far East. They were slightly larger and faster than Oberon and had a “net cutter” bow as in Otway and Oxley. They too failed to reach their designed diving depth, but at first were otherwise satisfactory. With age, however, their engines became unreliable at high power and had to be derated
to 80% reducing their maximum speed by two knots. With age, too, their external fuel tanks began to leak. This was put right by replacing their external tanks with welded instead of riveted joints. By 1939 these submarines were definitely past their prime but were considered fully operational.