Czesław Antony Marchaj (9 July 1918 – 21 July 2015), often known in the West as C.A. Marchaj or Tony Marchaj, was a Polish-British yachtsman whose published scientific studies of the aerodynamics and hydrodynamics of sailing boats have been influential on yacht, sail and rig designers. He was the author of "Sailing Theory and Practice" and approximately 60 other publications on sailing. He was a member of the Royal Institute of Naval Architects (RINA), and he was awarded the Silver Medal of The International Sailing Federation (ISAF).