These were initially powered by a 65 horsepower Rolls-Royce Continental A65 air-cooled flat four, but this produced insufficient power, due to the shorter-stroke, higher-RPM mid-20th century engine, driving a small-diameter modern propeller, being inefficient at the low airspeed achieved by the Boxkite, which originally used the slower-revving Gnome Omega 50 hp seven-cylinder rotary engine.