Plymouth she averaged 15¾ knots, with one boiler out of her ten held in reserve. This was a fine performance in those days, but she never had the chance of repeating it, for the scare caused by the loss of the 'Captain' resulted in some 300 tons of extra ballast being stowed in her hold, which had the effect of completely ruining her exceptional qualities for speed, whether under steam or sail. She has been employed on various services since then, but is now quite obsolete.
By 1873 the engineer department at the Admiralty finally made up its mind that compound engines were as likely to be profitably employed on board the Queen’s ships as they long had been in the mercantile marine. The usual pressure of steam in the boilers of men-of-war had now settled at 60 lbs. on the square inch. Every ship as she was built was being fitted with compound engines, but there was a restless, uneasy feeling among the men who constructed them that they were very far from the end of the journey on which they had started when they first left the 'sweet simplicity' of simple engines and 30 lbs. pressure behind them. They little knew what was in store for them in the future.
Before that time arrived, however, two ships were added to the navy sufficiently remarkable both for hulls and machinery to demand particular notice. These were the 'Iris' and 'Mercury.' Entirely without defensive armour, these ships were intended to rely for safety on their exceptional speed. The 'Iris' was the first ship built in England in which soft steel was employed, and the first vessel of the Royal Navy wholly
