Accurate estimation of not only hydrodynamic forces acting on a ship hull, but also interaction among ship hull and propeller, rudder is essential for the mathematical model of ship maneuverability. In the present paper, the Navier-Stokes code NEPTUNE developed at Ship Research Institute has been extended to compute flows around a ship with a rudder in a maneuvering motion. Brief description of the numerical method is given followed by the comparisons of the computational results with experimental values. Since both the hydrodynamic forces and the interaction between ship hull and rudder show good agreement between the simulations and the measurements, the practical usefulness of the present method has been demonstrated