The problem is difficult because the free surface and body positions are not known a priori. The problem is formulated in terms of Taylor expansions about the mean position of the body and free surface. By assuming small wave steepness, only the zero, first and second order components of the expansion are kept, while the consequently small higher order terms are neglected. The zero order is related to the mean hydrostatic force which is balanced by the weight of the body, so it is usually not included in the problem. The first order problem is related to the harmonic incident wave with frequency ω j and the unknown is the first order velocity potential, φ(1) . The second order problem results from the interactions between pairs of incident harmonic waves with different frequencies, ω j , ω k (with ω j < ω k ) and the body. The aim is to calculate the second order velocity potential, which is decomposed into a sum and difference frequency components: