Attainable ship speed in a seaway

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.