In both coherence spectra versions, the closer the result is to 1 the better the correlation between the two time series. Fig. 10 presents the coherence spectra for the surge and heave forces (left and right graphs respectively). Coh 1 (black dots) stands for the experimental data; therefore, the two time series consist on the experimental wave elevation and wave exciting force. Coh 2 results are calculated from the experimental force time series and numerical time series, where different Coh 2 results stand for the: complete second order solution (black line), no free surface forcing approximation (green dashed line), Newman’s approximation (red dotted line), Hooft based method (blue dashed line) and linear exciting forces (red dashed line).