The influence of the second order potential is small for the 300 cm water depth and 0.5 rad / s difference of frequency, and thus the results from all methods are very similar, both in surge and heave, and correlate well with the experimental data. The exception is the heave force for depth ratios smaller than 0.1 ( h /λj < 0.1), where numerical prediction by all methods but Newman’s approximation increase sharply. This is due to second order potential flow effects which become important for shallow water.