On the second-order slow drift force spectrum

In this paper, it is shown that the slow drift force spectrum of a floating body, obtained from the exact quadratic transfer function, is flat in the low-frequency range of interest and can be written in the form SF( )=SF(0)+o( 2), where SF(0) can be computed from the known drift force coefficient in harmonic waves and the wave energy spectrum. It is also shown that a special and normally used form of Newman's approximation for the exact quadratic transfer function has an error of the form [1+0( 2) at low frequencies.