Calculating second order forces in the time domain using the dou- ble summation procedure is a highly inefficient method from the nu- merical point of view. There are methods to reduce the computational burden significantly, such as the one applied in the present study which is based on the inverse of the Fourier transform. It consists of calculating the second order spectrum by equating the contributions from each combination of wave frequencies in a geometric manner. A time series can then be obtained by using the inverse Fourier trans- form. The method has been described in Duncan and Drake [ 18 ] for the sum frequency component of the second order wave elevation and in Agarwal and Manuel [ 19 ] for the complete second order wave model of both the wave elevation and kinematics, including the sum and difference components. These references present the method de- tails. Although the method was developed to calculate the second order wave elevation, the same procedure can be applied to obtain the second order force spectrum.