A COMPARISON OF DIFFERENT METHODS FOR ADDED RESISTANCE PREDICTION

Added resistance prediction represents an important challenge for ship-owners due to its economic implications in terms of choice of engines, fuel consumption and route-time evaluation. Design offices should consider seriously this problem already in the early stages of the design. Usually, the performance evaluation of a ship in a seaway is primary based on the calm water resistance without properly considering the weather conditions prevailing on the operating route. Even if the calm water resistance is used as a first estimation of the power required, an allowance is added to this value of the resistance to consider the effect of the environment. The designers consider this effect on over-powering by application of a standard “sea margin factor”, independently from actual behavior of the designed ship in a seaway. This value of the sea margin is usually stated at the design stage by the ship owner or ship designer (often 15–30 % of the ship calm water power), based on tradition or experience of similar ships sailing on the same route. However, reliable evaluation of added resistance in a confused sea allows to quantifying the real overload on the propeller and the engine power needed to achieve the required service speed. A more accurate value of sea margin can be obtained through towing tank tests or theoretical methods that