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Abstract: The need of improving the energy demand in ship’s operation is driven by growing regulations regarding shipping emission. Therefore, this paper describes an approach to determine the optimal speed of a ship traveling a route with changing water depths. The power demand of vessels in shallow water is higher than in deep water at the same speed. Regarding the energy consumption, it is profitable to slow down in shallow water and compensate the lost time in deeper water. The presented tool enables to find the optimal speed for every water depth segment of a specific ship route by solving an optimization problem. The aim is to obtain the least energy consumption by retaining the same travel time. The approach is built on a fast and adaptable manoeuvring calculation, which is used to determine the fuel consumption of a ship traveling a specific ship route given by waypoints. The additional power demand due to shallow water calculation is obtained by the Schlichting method. The manoeuvring calculation is based on a body force model, that determines the ship’s motion from all forces acting on the ship. The approach is demonstrated by a specific use case and can results in fuel savings of up to 8% dependent on the reference case.