The Bayesian modelling procedure has been extended to estimate time-varying directional wave spectra. In the proposed method, the hull of a ship is regarded as a wave sensor and the time history data of ship motions is analyzed to evaluate cross spectra. In this concept, a special attention should be paid to changes of the directional wave spectrum because the ship is moving and the location at sea is changing continuously. Therefore, the extended Bayesian estimation is based on the cross spectra evaluated at different times and the fluctuation of wave spectrum is taken into account. In order to examine the reliability of the proposed method, data of onboard experiments were analyzed. Comparisons between the results of the proposed method and visual observations show good agreements.

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