The results in Fig. 10 , namely Coh 1, clearly indicate that the energy present in frequencies above around 2.5 rad / s result from a linear pro- cess between the wave elevation and the wave exciting forces, while the same process is not linear below 2.5 rad / s. Below 2.5 rad / s the correlation between the numerical second order surge forces and the total experimental forces increase, while the correlation with linear predicted forces vanishes. The correlation of second order spectra is higher than the linear correlation, meaning that in that region mostly the quadratic forces are responsible for the energy captured by the Fourier analysis.