The ship was assigned to the Portsmouth Reserve in January 1888 before beginning a major modernisation the following year. Her guns were replaced by four breech-loading 10-inch guns. To improve her defence against torpedo boats, her Nordenfelt guns were replaced by six quick-firing (QF) 6-pounder and eight QF 3-pounder Hotchkiss guns. "Thunderer"s machinery was replaced by inverted triple-expansion steam engines and cylindrical boilers. Their increased output of increased her speed to and their more economical consumption of coal allowed the coal storage to be reduced to .