More than 90％ of fishing boats are small fishing boats smaller than 20 tons in Japan． Therefore ， even significant wave heights amund

The previous paper which we reported before described a kind of damping function in the equa − tion of single degree of freedom rolling motion which applicable to s 皿 all fishing vesselsc1) ． There is no problem in the application of the damping function to the free rol ！ motion in the ab −

The present paper proposes two estimation methods for estimating not only the coefficients of roll damping moment but also the coefficients of roll restoring moment without calculation of hydrostatics. The first method is an energy method which involves expanding Froude's method [5] to the case for which the restoring moment is nonlinear. The second method can identify the coefficients of arbitrary form of nonlinear damping and restoring moments by genetic algorithm. The advantage of these methods is that identification of a highly precise coefficient is very easy compared to other conventional methods. This advantage is most important in applications to practical problems. These methods are first applied to numerical simulation data and then to real free-decay data obtained from scale models.

Compared with other kind of ships ， small fishing boats tend to roll by relatively small wave ． Especially ， rolling motion with sudden change is very dangerous for crew working on the deck ． Therefore estimation for rolling motion is very i皿 portant ． However ， it is dangerous to predict rolling motlon recklessly without consideration for limitation of prediction models ． In many cases ， the prediction model for ship motiens in irregular seas is based upon probabilis −

This paper describes similarities between the narrow-band response of a resonant system excited by random inputs and low-dimensional chaos, with particular emphasis on the geometric characteristics of trajectories reconstructed in m-dimensional phase space from measured scalar time series data with a time-delay coordinate system. In this study, the time series data of ship roll angle in irregular waves were analyzed as an example of the narrow-band response of a resonant system. These time series data were measured by one of the authors in Tokyo Bay. The similarities between the narrow-band response of a resonant system excited by random inputs and low-dimensional chaos are verified by numerical simulation data.