engleski

A comparition 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 compute the added resistance in waves on the basis of regression formulas or well consolidated seakeeping theories. An approximate value of wind resistance is effectively determined by means of systematization of numerous model test results published by different researchers. In this respect, to make practical design predictions, several commercial codes are available on the market. In this paper attention is given to a recent Ro-Ro/pax ship operating in Mediterranean. The most popular prediction methods, both analytical and statistical, have been considered and compared. A critical analysis of the statistical formulation for the wind loads has been carried out and the conclusion is that such regressions should be updated with recent Ro-Ro profiles. As a result, the maximum operating conditions have been obtained for the ship by taking into account both involuntary and voluntary speed reduction that is required by comfort on board.

Added resistance; Seakeeping; Sea waves; Wind

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