The author showed in the former report a method of calculating the steered motion of a ship under the action of external forces such as wind and/or flow. In such calculation, since the propeller experiences very wide range of loads and accordingly the rudder relative speed varies much, it seemed to be necessary to express the propeller and rudder forces separately from the hull forces. Including the above requirement, the author proposed here a new mathematical model for such practical calculation of steered motion, and applied it to the analysis of a self-propelled model test data in the rotating-arm tank at Ship Research Institute. The propeller force was described as the function of advance speed, and a mean effective rudder speed was assumed to be a geometrically composed function of propeller revolution and wake. An effective attack angle of the rudder including the above rudder speed and hull-interaction coefficient was applied, which coefficient had been decided from the analysis of total longitudinal forces as a function of drift angle, turning rate and rudder angle. A method of analysing the rudder force derivatives with relation to both the measured side force and moment at the same time by a least-squares method, taking the rudder moment upon ship into account, was attempted. From the analysis of the new mathematical model applied to a ocean-going car ferry model tested in the rotating arm tank, the following conclusions were obtained : (1) Hull and rudder derivatives could be derived separately from a self-propelled restrained model ship. (2) Accordingly an arbitrary propulsion point of the model ship could be chosen. (3) The effect of interaction between hull, propeller and rudder, besides the well-known propulsion factors, could be approximated in the form of effective rudder angle, including two simple coefficients which represent the effect of hull and propeller upon rudder.