The present work identifies the nonlinear restoring characteristics and excitation force of ship roll motion in regular beam waves. The steady-state roll response data measured from a numerical simulation is utilized for the experiments of the applied method. Using the approach proposed by Jang (2014), the J-function and its one of zero-crossing times play a crucial role in the identification. The J-function is defined to be the sum of inertia force and nonlinear damping. For the zero-crossing time to be measured, system responses of angular displacement, velocity and acceleration need to be known and so does the numerical interpolation. Numerical simulations using the ship models are conducted to show the workability and validity of the methodology. These numerical simulations show that Jang's scheme is an effective and very simple method for the identification of the characteristics of ship roll motion and produces relatively reasonable solutions.

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