Vessel data: Harmonic motion

The harmonic motion data apply if the vessel's superimposed motion is RAOs + harmonics, allowing you to define a number of harmonic motions of the vessel.

These harmonic motions are in addition to any wave-generated motion resulting from the RAO data, so if you only want the wave-generated motion then you should set the number of harmonic motions to zero.

Each harmonic motion is a single-period sinusoidal motion of the vessel, defined by

The harmonic motion amplitudes (unlike the RAO responses of the vessel) are not relative to a wave amplitude – they are given directly in length units (for surge, sway and heave) or degrees (for roll, pitch and yaw). Similarly, the phases are not relative to the phase of a wave – they are the phase lags from the simulation time origin, $t{=}0$, until the maximum harmonic motion occurs. More precisely, the harmonic motion phase is given by \begin{equation} 360\, [ (t_\textrm{max}/p) \bmod 1] \end{equation} where $p$ is the period of the harmonic motion and $t_\textrm{max}$ is the simulation time at which you want the maximum of the motion to occur.

The whole of the harmonic motion is applied at the displacement RAO origin, subject to any Froude scaling on vessel length.

Warning: Harmonic motions can be used to model pre-calculated vessel slow drift. If you do so, and you move the vessel's initial position in the wave direction, or you change the data for the waves, then you will normally also have to adjust the phases of the slow drift. This is because such changes affect the simulation time at which a particular part of the wave train will reach the vessel and hence will also affect the simulation time at which maximum slow drift motion is achieved.