There are two important points worthy of special attention:

1) The calculations of added mass and damping should be done using the encounter frequency. Further, the wave number should be based on encounter frequency. This is because the radiated waves which cause the damping and contribute to the added mass are generated by the motion of the vessel which is assumed to occur at the encounter frequency and not the natural frequency of the waves causing the excitation.

2) There is an error in the equation for F_S quoted by Sutherland (1987) - Equation 4.7 and Bishop and Price (1978) second equation, Appendix I; the bracketed numerator in the integral should be a minus sign for F_S, the plus sign should be kept for Y_S. The correct equations are given in the original derivation by Ursell (1949) and are also correct in the work of de Jong (1973). Interestingly, the original Sutherland Seakeeper code is correct.