**7. Conclusions**

This study conducted a FMEA to evaluate the safety and reliability of a fuel cell-based hybrid power system for ships. Unlike diesel engines that are mainly used as propulsion power sources in conventional ships, new FMEA evaluation criteria and items are needed to apply fuel cell-based hybrid power sources to ships. In the RPN evaluation currently applied to shipbuilding in shipyards, existing RPN evaluations, the evaluation items and criteria are vaguely established; therefore, results for the same evaluation would differ vastly between evaluators. Accordingly, for the FMEA of this study, the evaluation was performed using several external evaluators who applied reestablished evaluation criteria that mitigate RPN evaluation problems. To analyze the concordance of the reestablished evaluation items, a research hypothesis was established, and the significance probabilities and Kendall's concordance coefficient were calculated using SPSS. The concordance coefficient was 0.906 for S, 0.844 for O, and 0.861 for D. The results indicate that each evaluator applied essentially the same criteria when evaluating the samples, demonstrating that the reliability of the evaluation results was high. The criteria used to establish countermeasures for each failure mode were set based on the D results of the evaluation.

Although having the same evaluation configuration for each hybrid power system is essential to establish countermeasures, each system contains different devices and characteristics, therefore, the number and type of evaluation items also differ. Since it is difficult to apply relative criteria, this study instead used absolute RPN values to set the criteria for establishing countermeasures: a RPN of 100 or more and an S, O, or D of 8 or more.

For the FMEA of this study the power generation system of the hybrid power system (i.e., the failure mode and failure effect of the power source) was evaluated. However, future research must conduct FMEA for the entire set of systems including the power generation, power distribution, output performance verification, and control and management systems of hybrid power systems. Future studies must also perform FMEA for different system operation modes (e.g., single and hybrid operation) to identify hazards that may arise in the systems of actual ships during operation. However, in spite of these limitations, the results of this study showed significant results as an evaluation to confirm the stability and reliability for applying a fuel–cell based hybrid power source to several ships.

**Author Contributions:** Conceptualization, J.K. and H.J.; Methodology, J.K., H.J. and K.P.; Formal analysis, H.J.; Software, J.K. and H.J.; Writing-original draft preparation, H.J.; Writing-review and editing, J.K., H.J. and K.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
