The present paper relates to experimental researches on rudder performance utilizing the freerunning, self-propelled models. The variation of rudder performance caused by the difference of ship form ( C_b ) is attributed to not only the change of slip stream of screw propeller but also the change of ship turning motion. The rectification factor of screw propeller may be regarded as the function of the ship form and rudder area ratio. Alternative steering tests have been carried out for three different type of ships and exchanging the balance ratio. As the result, it is considered that the optimum balance ratio for single rudder single screw ship exists between 0.250 and 0.270, and the smaller C_b is, the smaller balance ratio must be applied.