Ordered under the Naval Defence Act of 1889, which established the "Two-Power Standard", the class was contemporary with the first torpedo boat destroyers. With a length overall of , a beam of and a displacement of 1,070 tons, these torpedo gunboats were not small ships by the standard of the time; they were larger than the majority of World War I destroyers. Halcyon was engined by Hawthorn Leslie and Company with two sets of vertical triple-expansion steam engines, two locomotive-type boilers, and twin screws. Halcyon produced , nearly twice the power of the rest of her class. She was capable of 19 or . She carried between 100 and 160 tons of coal and was manned by 120 sailors and officers.