A simple procedure for predicting stability derivatives of a ship equipped with a propeller and rudder from those for the bare hull was examined. A procedure of this kind is very important as it enables us to easily examine an influence of rudder area on stability derivatives and scale effect in maneuvering characteristics. This paper indicates that the lateral force exerted by rudder deflection on a hull must be taken into consideration when making a presumption of this kind, and the influence of a propeller operating on stability derivatives is, in comparison with the influence of the rudder force, not so negligibly small that it cannot be disregarded. Further, through the above mentioned studying process, lateral force acting on the propeller installed on the obliquely moving ship was directly measured, and it was made clear that the force delivered to the propeller itself (the so-called fin effect of the propeller) would not be a significant component of the lateral forces produced by the action of the propeller on the hull and propeller.