Sailing performances of sail equipped ships are generally provided by the balance of forces between sail and hull. However, there are few works on the prediction of sailing performance particularly with a large ship. The authors have been trying to establish the predicting technique based upon hydrodynamic and aerodynamic researches with a new sail training ship "Nippon-maru". Then we find the suitable mathematical model which can describe the sailing performance well. In this model, hydrodynamic forces acting on hull and rudder are described by a modular type mathematical model which is similar to MMG's, and the aerodynamic forces are added in it. The predicted sailing performances are well coincide with observed data in actual navigation.
