Part of the Powerful-class consisting of the Powerful and Terrible, preceded by the Edgar-class and succeeded by the Diadem-class. Especially designed to act against commerce raiders as the armoured cruisers of the French navy and the Russian Rurik. The design dimensions were limited by the at that time available shipyards large enough to built and maintain both ships. Laid down by Vickers Limited, Barrow-in-Furness, England in 1894, launched on 24 July 1895, renamed Impregnable as training school in November 1919 and sold to be broken up on 31 August 1929.
Displacement 14.400 (deep load) tons and as dimensions 152,4 (between perpendiculars)-164,o x 21,6 x 8,2 metres or 500-538 x 71 x 27 feet. The machinery consisted of 2 vertical triple expansion steam engines and 48 Belleville water tube boilers supplying via 2 shafts 25.000 shp allowing a speed of 22 9design) knots and with 14 knots a range of 7.000 nautical miles. Speed during 4 hours trial with forced draught and a horsepower of 25.866 ihp was 21,18 knots in bad weather conditions and during 30 hours natural draught with 18.433 ihp 20,6 knots. Crew numbered 894 men. Armour consisted of a 5,1cm/2”-15,2cm/6” thick deck and further more 15,2cm/6” thick barbettes and gunshields. The armament consisted of 2-23,4cm/9,2” Mk VIII breech loading guns, 12-15,2cm/6” quick firing guns, 16-7,6cm/3”/12pd guns, 12 4,7cm/3pd guns and 4 submerged torpedo tubes.