 The Napier Lion VIIB engine was judged to be incapable of further development, and the S.6 was to use the new 1,900 hp (1,417 kW) Rolls-Royce R engine.With the problems of cooling attendant on such a high power output, the S.6 had surface radiators built into the floats as well as the wings, and Mitchell even arranged for airflow through the wing internal structure.Two aircraft (serial numbers "N247" and "N248") were built at Woolston and operated by the RAF High Speed Flight, delivered in August 1929.