This paper discusses the results of a study to numerically determine the speed-power curve of a ship for several loading conditions. Therefore the RANS solver PARNASSOS coupled to the BEM code PROCAL has been applied to a single screw container vessel. Loading conditions have been varied: two different draughts and a large range of speeds have been computed. In a first step, the resistance curves have been computed and compared to model test results. The computed results show a difference contained within 4.5% with the experimental data, noting that the model was tested with rudder but the calculation was done without. In a second step, self propulsion was simulated by a RANS-BEM coupling and results have also been compared to model test results. A reasonable agreement with experimental data for moderate speeds is found but larger deviations, up to 10%, are obtained for the higher speeds. The absence of the rudder can explain a part of this difference. However, further research should be done to understand all details.