"Monmouth" was designed to displace . The ship had an overall length of , a beam of and a deep draught of . She was powered by two 4-cylinder triple-expansion steam engines, each driving one shaft, which produced a total of designed to give a maximum speed of . "Monmouth", however, was one of three of the "Monmouth"s that failed to meet her designed speed. The engines were powered by 31 Belleville boilers. She carried a maximum of of coal and her complement consisted of 678 officers and enlisted men.