In order to guarantee the safety of vessels, it is very important to estimate roll motion adequately. However, it is very complicated to calculate roll motion theoretically, because of significant viscous effects on roll damping. It is well known that there is a prediction method of the roll damping proposed by Ikeda et al.¹⁾²⁾. It is developed with theoretical and experimental backgrounds for periodical roll motion. Therefore, it is difficult to apply it to estimation of transitional and non-periodical roll motions (i.e. roll motion in irregular waves, broaching to capsize etc.). In the previous studies²⁾³⁾⁴⁾, it is pointed out that the flow-memory effects and the effects of transient motion on the bilge keel component of roll damping are necessary to consider for time domain simulations of non-periodical roll motion.

In this study, the bilge-keel component of roll damping is focused, because the component is generally most part of total roll damping. In order to estimate bilge-keel component of non-periodic roll damping for time domain, the flow-memory effects are investigated by numerically to understand its mechanism. Moreover, based on the result, the roll damping time domain prediction method including the effects of transient motion, which is proposed by Katayama et al.⁵⁾ based on Ikeda's method¹⁾ is improved.