One of the authors presented the component-type mathematical model of hydrodynamic forces in steering motion on the basis of kinematic forces acting upon the both ends of the hull. The mathematical model can describe well the forces X , Y and N with large drift angles and tough turning motion. The rotative coefficients in the model are estimated from the static coefficients at oblique motion. The rotative coefficients thus derived are, however, somewhat different from the ones delivered from the experimental data of turning motion. In order to improve this point, the authors will now break up the hydrodynamic forces into the following components, i. e., ideal flow force, viscous force, induced drag, cross flow drag, cross flow lift and frictional resistance in the normal sense. Then a simplified vortex theory is called upon to evaluate the ideal flow force, viscous lift and induced drag. As the results, the present mathematical model can describe the hydrodynamic forces acting upon a hull in turning/drifting motion with a fair accuracy. The model also provides a sensible insight on the mechanism of hydrodynamic forces appearing in steering motion.