Reliable evaluation of added resistance in a storm conditions allows to quantifying the real overload on the propeller and the engine power needed to achieve the required service speed. The added resistance of a ship in a seaway has been studied for several decades. The essential concern of ship designer and operator lies in how a vessel will perform at sea; to wit, in her sustained sea speed under probable environmental conditions. However, even today the added resistance often is accounted for by increasing engine power between 15 and 30 percent to calm-water resistance. New computational methods of ship motion prediction have raised the prospect of new and advanced added resistance prediction techniques.
This paper tends to review the present day methods of added resistance estimation and their development through years. Some of these methods are very simple and based only on main ship characteristics and the others are quite demanding in computational sense. The results of their practical application to a modern ro-ro ship are presented.
