 Thus the seven vessels ordered from Yarrow were built to a distinct design ("Yarrow S class"); similarly the five ordered from Thornycroft were the "Thornycroft S class".Most of the Admiralty S class had Brown-Curtis single-reduction geared turbines; seven vessels ("Steadfast", "Sterling", "Stonehenge", "Stormcloud", "Tilbury", "Tintagel", and "Strenuous") had Parsons Impulse-Reaction turbines, instead.Three Yarrow boilers (maximum pressure ) were fitted in all except the vessels built by Whites (which had White-Forster boilers).