**4. Conclusions**

In this study, motion analyses for a newly developed lightweight light buoy in waves were performed to predict the buoy's motion performance and check the effect of the developed appendages on the performance. First, free decay tests using CFD including benchmark cases for the validation were carried out to estimate the viscous damping coefficients, which cannot be obtained by potential-based simulations. The results for the validation were in good agreemen<sup>t</sup> with those of the reference. Second, potential-based simulations with and without considering the viscous damping coefficients were performed, and the results were compared. From the results, it was confirmed that viscous damping should be considered when the frequency of an incoming wave is close to the natural frequency of the buoy. Finally, motion simulations in regular waves using CFD were carried out to compare the RAOs and maximums of motions with those obtained from a potential-based simulation considering viscous damping. The RAOs of both results near the natural frequencies of the buoys were expected to be reduced by adopting the appendages. Although the quantitative superiority of the appendages was different, the qualitative superiority was similar for both results. The present methods and results would be useful not only for a buoy but for other cylindrical floating structures, such as spars and wave energy converters. Motion simulations in irregular waves using CFD will be carried out to compare its RAO and motion response with those by potential-based simulation.

**Author Contributions:** Conceptualization, B.-H.S. and S.-M.J.; methodology, S.-M.J. and C.-Y.L..; validation, B.-H.S.; formal analysis, B.-H.S.; investigation, S.-M.J. and C.-Y.L.; resources, S.-M.J.; writing—original draft preparation, S.-M.J.; writing—review and editing, S.-M.J. and C.-Y.L.; visualization, B.-H.S.; supervision, S.-M.J. and C.-Y.L.; funding acquisition, S.-M.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by research fund from Chosun University (K207177002). **Conflicts of Interest:** The authors declare no conflict of interest.
