Both the surge and heave QTF increase immensely for h /λj < 0.1–0.2 when calculated with the Hooft based method, the New- man + Setdown method, the no free surface forcing approximation and the complete formulation. These numerical predictions compare well with the experimental data, while Newman’s approximation very much under predicts the forces. Overall it can be concluded that second order potentials are important for the accurate predic- tion of difference frequency drift forces in limited water depth and, for the small difference frequency of 0.5 rad / s, the second order po- tential flow effects are dominated by the undisturbed incident wave component. The latter observation results from the fact that Hooft’s approximation and the Newman + Setdown method provide excel- lent predictions.