The destroyers Mermaid and Cheerful (length 210 feet, beam 21 feet, draught 8 feet 2 inches; displacement 320 tons), were fitted with twin 6,000 indicated horsepower 19-inch by 29-inch by 46-inch by 18-inch stroke inverted-vertical triple-expansion direct-acting marine engines operating at 390 revolutions per minute with steam supplied by four Thornycroft water-tube boilers with 13,200 square feet of heating surface. The high and intermediate-pressure cylinders were fitted with double-ported slide valves. The engines had double-bar link-motion valve gear, with forged steel links, and gunmetal excentric straps, lined with white lead. The pistons were made of forged steel, and the crankshafts were made of Vickers steel. The bed-plate frames were manganese bronze.