 The Diadem class was designed to protect British merchant shipping from fast cruisers like the Russian Rurik and were smaller versions of the Powerful class . The ships had a length between perpendiculars of 435 feet ( 132 @.@ 6 m ) , a beam of 69 feet ( 21 @.@ 0 m ) and a draught of 25 feet 6 inches ( 7 @.@ 8 m ) . They displaced 11 @,@ 000 long tons ( 11 @,@ 000 t ) . The first batch of Diadems were powered by a pair of four @-@ cylinder triple @-@ expansion steam engines , each driving one shaft , which were designed to produce a total of 16 @,@ 500 indicated horsepower ( 12 @,@ 300 kW ) and a maximum speed of 20 @.@ 5 knots ( 38 @.@ 0 km / h ; 23 @.@ 6 mph ) using steam provided by 30 Belleville boilers . They carried a maximum of 1 @,@ 900 long tons ( 1 @,@ 930 t ) of coal and their hulls were sheathed with copper to reduce biofouling . Their complement numbered 677 officers and ratings . 
