During the early design stage, it is essential to produce quick and reliable results. Regarding the calculation of forces acting on a rudder in the propeller slipstream, potential flow solvers are often used for this purpose due to their low computational effort. The drawback of these solvers is that viscous effects as drag or flow separation cannot be calculated. To overcome this drawback, the coupling of an inviscid three dimensional boundary element method with a viscous two dimensional boundary layer method is presented. The inflow from the propeller and the propeller rudder interaction is calculated using a lifting line approach. The inviscid calculation of the rudder forces is done with a boundary element method and the viscous rudder forces are calculated with the boundary layer method. Two different approaches are presented for the coupling. The results calculated with the implemented methods are compared to reverse open water model tests.