The first harmonics of the motion components of a boating structure are often of interest, because in many cases a very realistic mathematical model of the motions in a seaway can be obtained by making use of a superposition of these components at each of a range of frequencies; motions in the so-called frequency domain will be considered here. In many cases the ship motions have mainly a linear behavior. This means that, at each frequency, the ratios between the motion amplitudes and the wave amplitudes and also the phase shifts between the motions and the waves are constant. Doubling the input (wave) amplitude results in a doubled output amplitude, while the phase shifts between output and input does not change.
As a consequence of the linear theory, the resulting motions in irregular waves can be obtained by adding together results from regular waves of di¤erent amplitudes, frequencies and possibly propagation directions. With known wave energy spectra and the calculated frequency characteristics of the responses of the ship, the response spectra and the statistics of these responses can be found.
Kinetics
A rigid body’s equation of motions with respect to an earth-bound coordinate system follow from Newton’s second law. The vector equations for the translations of and the rotations about the center of gravity are respectively given by: