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The influence of rudder size and rudder angular on maneuvering performance of a typical large tanker (the 250.000 DWT type) was examined by digital computer motion simulations of turning and Z-maneuvers under open, deep water conditions, The mathematical model is based on hydrodynamic data obtained from captive model tests carried o“t in the rotating-arm facility. Ship response in turning and particularly in Z-maneuvers is improved by increasing the rudder angular velocity. The improvement is more significant at lower rudder angular rates and higher ship speeds. At rates greater than the present standard of 2.3 degrees per second, the improvement due to increased rate is less noticeable than at rates lower than standard. At ship speeds less than 16 knots the improvement is proportionately reduced until at modest maneuvering speeds such as 4 to 8 knots it becomes relatively insignificant. Increasing the rudder size from its design value has relatively little effect on turning performance but somewhat improves Z- maneuver response. In particular, the lateral overshoot of the ship trajectory is reduced with an increase in rudder size. The effect of the rudder rate shown in this study is for a typical 250,000 DWT tanker at the fully loaded condition and is largely affected by ship size and speed.