The possibility of decreasing the number of crew for maintenance on board depends on how the failure of machines can be decreased. The authors think that scheduled preventive maintenance at port is the only way to achieve it because of the difficulty of sharp improvement of the reliability. In this paper, to know the differences of characteristics of maintenance of each ship, Hazard Plotting Method is practiced. And simulation on the basis of preventive maintenance is practiced for application to back-up system of ships. Furthermore, from the results of the simulation, economical profitability of preventive maintenace is shown.
