Speed optimization of a container ship on a given route considering voluntary speed loss and emissions

•The influence of voluntary ship speed loss is considered in the ship speed optimization.•The main engine fuel consumption, ship operating costs and greenhouse gas emissions are calculated.•The speed optimization results with and without voluntary speed loss are compared.•The influence of time window on the speed optimization results is analyzed. This study performs a speed optimization of a 4250 TEU container ship in which the influence of sea states and voluntary speed loss on ship sailing are considered. A speed optimization model is established for a single voyage, wherein the objectives are minimizing the main engine fuel consumption, and the ship operating costs. Based on the verified speed optimization model, ship speed optimizations with and without voluntary speed loss are performed. The research results show that the optimization results of the model with voluntary speed loss are significantly different from those without voluntary speed loss. To make the optimization results more accurate and more in line with actual navigation conditions, it is necessary to consider voluntary speed loss in the optimization model. In addition, the ship speed optimization is extremely sensitive to the time window. A larger time window constraint can provide more potential for speed reduction and increasing ship energy efficiency, which is consistent with the theory of slow steaming for optimizing energy efficiency. The established model can provide reasonable speed optimization decisions for ship operations.