As general conclusions from the study, it is possible to say that Newman’s approximation (without the setdown correction) for the difference frequency drift forces is valid in deep water, slightly under- estimates surge forces in shallow water and it is not valid for heave forces in shallow water. Hooft’s based solution predicts correctly the difference frequency forces in shallow water at the small difference frequency range and it consists on a practical good solution. In prac- tice, the free surface forcing effect seems to be negligible; therefore there is no need to solve the complete second order problem. The for- mer conclusions have in mind that, for many practical applications, the objective of calculating difference frequency drift forces is to pre- dict the floating structure horizontal slow drift motions, which are characterized by low natural frequencies; therefore, inaccuracies in the prediction of second-order drift forces for intermediate to large difference frequencies will not affect the system’s motion response.