 Unlike her half @-@ sisters , the entire side of Northumberland 's hull was not covered with wrought iron armour . To compensate for the additional weight of her armament , only her battery was protected above the main deck . The ship was fitted with a complete waterline armour belt that tapered from 4 @.@ 5 inches ( 114 mm ) at the ends to 5 @.@ 5 inches ( 140 mm ) amidships . The armour extended 5 feet 9 inches ( 1 @.@ 8 m ) below the waterline . The sides of the battery were 184 @.@ 5 feet ( 56 @.@ 2 m ) long and it was protected by 5 @.@ 5 @-@ inch armour on all sides , including transverse bulkheads fore and aft of the guns . The two forward chase guns on the upper deck were also protected by armoured bulkheads , but the stern chase guns on that same deck were entirely unprotected . The armour was backed by 10 inches ( 254 mm ) of teak . The ship also had a conning tower protected by 4 @.@ 5 @-@ inch armour plates . 
