Fig. 3 presents results similar to the former, but in this case for the difference frequency of 1.5 rad / s. The observations are similar to the ones related to Fig. 2 , however with increased differences between numerical approximations. Hooft’s based method in particular over predicts the surge forces in the intermediate to large wavelength to depth ratio range, effect that increases with the decrease of the water depth. This is in part justified by the fact that second order wavelength cannot be assumed large any more compared to the body diameter: the wavelength is 1.7 m for deep water, 0.92 m for 55 cm waters and 0.75 m for 40 cm. Hooft’s approximation works much better for the heave forces, even though the results are marginally larger than the complete second order calculations. The Newman + Setdown method has similar behavior to the Hooft based method in the 40 and 55 cm water depths, but slightly underestimates the forces in the 300 cm water depth.