In this thesis, a numerical approach based on a potential flow method has been developed in order to simulate unsteady rotor/stator interaction, and to predict the unsteady performance of a propeller and its rudder. The method is first developed and tested in two-dimensions by using a boundary element method in which a front hydrofoil is moving downward, while a back hydrofoil is stationary. The wakes of the two hydrofoils are modeled by continuous dipole sheets and determined in time by applying a force free-condition on each wake surface. The wake/hydrofoil interaction is de-singularized by applying a numerical fence on the surface of the back hydrofoil. The viscous wake/hydrofoil interaction is considered by employing a viscous wake vorticity model on the rotor's wake surface. The present method is validated by comparison with analytical solutions, experimental data and by using the results from a commercial Reynolds Averaged Navier-Stokes (RANS) solver for the same set-up and conditions. The numerical approach is further extended to three-dimensions to predict the mutual interaction between a propeller and rudder. A fully unsteady wake alignment algorithm is implemented into a Vortex Lattice Method to simulate the unsteady propeller flow. The interaction between propeller and rudder is investigated in a fully unsteady manner, where a panel method is used to solve the flow around the rudder, and a vortex lattice method is used to solve the flow around the propeller. The interaction between a propeller and its rudder is considered in an iterative manner by solving the propeller and the rudder problems separately and by including the unsteady effects of one component on the other. The effect of the unsteady propeller-rudder interaction on the mean and on the unsteady propeller/rudder performance, including sheet cavitation on the rudder, is studied. / text