The purpose of the work was to the investigate the interaction between ship hull with rudder, propeller, rudder bulb and attached rudder thrust fins in relation to the effect on the flow field in order to provide a complete numerical study method on the energy improvement on the fully appended ship quantified by CFD. To investigate the scale effect on energy saving effect, two model scales and ship full scale were analyzed. The numerical study method used is not only suitable for rudder bulb and rudder fin type energy saving devices of the present work but also could be used for performance prediction of other pre-propeller and post-propeller energy saving devices. The numerical study method could provide reliable theoretic basis and technique support for the final design and model test.