The torque rating needed from sail control servos depends very much on the sail area. As a guide, my Saphir, with 3,700 cm^2 sail area sails nicely in all but the strongest winds on about 12 kg/cm torque (a GWS S04 with 5xNiMh cells). The Hitec 815bb mentioned has about double that but weighs about 50% more than the GWS.