A steam launch (length 30 feet, beam 6 feet 6 inches, draught 2 feet 6 inches; trial speed 8 and a half miles per hour) was fitted with a twin cylinder inverted-vertical simple marine engine operating on steam supplied at 70 pounds per square inch. Armed with a spar torpedo, and built for an unnamed foreign government under the superintendence of Mr Hunter Davidson of the United States Navy by Messrs. Yarrow and Hedley, Poplar.