To improve the safety and efficiency of trans-ocean voyage, authors developed a new onboard weather routing system (so called SORAS). The system utilizes weather forecasting data to evaluate seakeeping performance and to generate optimized route plan with respect to fuel consumption and sailing time. The system can provide decision support for navigator in real time. For this feature, onboard wave measurement system and hull stress monitoring system are integrated to provide real time wave information and actual hull stress and bow acceleration. The optimal route depends on not only weather condition but also ship’s propulsion performance. We performed a simulation study to determine the accuracy limit of mathematical model for propulsion performance. To evaluate the system, we compared calculation results with actual voyage data. The estimation results of speed reduction and fuel consumption showed good coincidence with measurement results. The wave bending moment was estimated on the forecasted wave condition. The results were compared with measured wave bending moment. For optimal route, it was confirmed that the efficiency of optimal route is superior to the efficiency of the actual route which planned by captains or officers, and the improvement of efficiency would be significant.