The Reynolds-averaged Navier-Stokes (RANS) method, along with the Fluent software package, was used to study the steady and unsteady interaction of propellers and rudders with additional thrust fins. The sliding mesh model was employed to simulate unsteady interactions between the blades, the rudder and the thrust fins. Based on the numerical results, the pressure distribution on the propeller and the efficiency of the fins were calculated as a function of the attack angle. The RANS results were compared with results calculated by the potential method. It was found that the results for the potential method and the RANS method have good consistency, but they yield maximum efficiencies for the fins, and thus corresponding attack angles, that are not identical.