From the previous graphs it is evident that the quadratic slow drift force is responsible for the experimental forces measured in the low frequency range, both in surge and heave, since the magnitude and trend of the numerical simulations follow closely the experimental forces. The coherence spectrum provides a way to evaluate this aspect. In its original form, the coherence spectrum of two time series is calculated as the ratio between the cross spectrum of two data series, and the product of the auto spectrum of each series: