The ability to accurately predict the powering performance of a ship when travelling in waves is of high importance for the design of new ships. Almost a century of experience exists regarding how to predict the mean resistance increase in waves compared to calm water. Despite this, improvements in numerical models are still in high demand. Traditionally, the mean increase together with the calm water resistance and propeller open water curves are used to determine the powering performance. This thesis argues that, to achieve better predictions, a more holistic approach can be taken. A RANS based numerical approach to predicting the performance of a self propelled ship in waves is presented. The model is supported by a review of previous literature as well as new experiments to determine what phenomena need to be modelled. It is concluded that the surge force amplitude in waves is something that is not well studied but that has an impact on the propeller performance. The experiments show that this is likely to be harder to predict than the mean increase. Furthermore, the inclusion of RPM control in the model is seen as important to make it better suited for predicting the performance. In developing the numerical model, it is shown that the amplitude and phase of the viscous surge force are affected to some extent by the way the RANS equations are solved numerically. Recommendations on the choice of schemes are given based on several comparative studies where a limited TVD scheme is found to give the best representation of the flow. Furthermore, detailed analysis on how the boundary layer is affected by the passing waves is presented. A framework for coupling the RANS solver with a simplified propeller model is presented. This is a powerful tool that allows for a broad range of present and future studies regarding propeller modelling and RPM control for self propelled simulations in waves. The implementation of Blade Element Momentum theory in the framework is outlined and a correction able to achieve a satisfactory run time coupling in terms of identifying the propeller induced velocities from the total wake is presented. The coupled solver is found to be a computationally efficient tool for studying ship performance in waves. It is applied to study the propulsive performance of the KCS in unsteady inflow conditions. Reasonable agreement with experiments is found both for resistance and for propeller performance. Overall, the findings and methods presented here represent a contribution towards better predictions of the performance of self propelled ships in waves.