Herein, an analysis method for discussing the navigation safety of ships is presented considering both the average (steady) sailing condition and the fluctuation components, based on the two-time scale method (Yasukawa, 2006; Skejic and Faltinsen, 2008; Yasukawa and Nakayama, 2009), where low-frequency maneuvering motions and high-frequency wave-induced motions are treated separately. Furthermore, the statistical method of those fluctuating components by the short-term prediction method is described. Along with a calculation example for a pure car carrier (PCC), the navigation limit is discussed. The limiting environmental condition for the safe navigation (navigation limit) of the PCC was wind speed is 24.5 m/s and significant wave height 7 m. This result is different from the analysis result of the navigation limit excluding the fluctuation components. Therefore, it is necessary to consider the fluctuation components to discuss the navigation limit more correctly.

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