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This study investigates a method of obtaining added power in weather from onboard shaft power measurements and hindcast weather data for the purpose of future weather routing. 6 months of voyage data (sample frequency 1Hz) has been obtained for a modern cargo vessel. Added power in weather is defined as the increase above calm water power for the same speed – measured by subtracting calm water power from total measured power experienced in weather. A calm water power model is generated from total power measurements when the significant wave height and wind speed are less than 0.5 m and 5 ms-1 respectively. Added power in weather is reported for discrete ship speed and heading bins. For head seas, a clear increase in added power for an increase in significant wave height is observed. For oblique seas the correlation between significant wave height and added power becomes less significant. It is identified that improvements to the calm water power model are required to reduce the uncertainty in the added power in weather prediction. Comparison with an existing added power in weather prediction by Kwon, (2008) identifies a weaker response to significant wave height for the ship in this study.