The Sikorsky (model S-58) CH-34A/CH-34C, essentially a lengthened and more powerful version of the Sikorsky (model S-55) H-19 Chickasaw cargo helicopter, could carry 12 to 16 troops. It could carry eight litters in the MedEvac role. The U. S. Marine Corps version, UH-34, was the primary Marine utility/assault helicopter used in Vietnam. The Sikorsky (model S-58) was licensed and built in Great Britain as the "Wessex". The Choctaw had a single four-bladed main rotor and a four-bladed metal tail rotor. The CH-34C was powered by a single Curtis-Wright R-1820-84C 1425 hp piston engine and had a speed of 123 mph (107 knots).