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Abstract: During the early ship design stage there is a demand for calculation methods with a short response time, to be able to calculate the effects of design decisions. Hydrodynamic design tools based on potential flow theory fulfill these requirements. In the past a lot of work has been done to develop analytical methods with a high degree of mathematical modeling. This was necessary due to the lack of computer power. Nowadays these theories can be used as very fast analysis tools during the early design stage. The unsteady lifting line theory of Zwick(1962) is an example for such a method. The method can be used to calculate the slipstream of a propeller in behind condition. The mathematical description of the theory has been implemented in Fortran and has been integrated in the ship design framework E4.Within this framework the tool now can be used to calculate unsteady interaction between wake field, propeller and rudder.