Question:

What is the difference between the Zero-Up-Crossing period (PERZUC) and the period between maxima (PERMAX)?

Answer:

Think of the vessel's response as a sine-wave, oscillating with some amplitude about a mean-zero line over time. The zero line is the vessel's position at rest. The Zero-Up-Crossing period is the time (in seconds) between successive points at which the vessel's response curve passes up through the zero line. The period between maxima is simply the time between successive peaks in the response curve (i.e. the tops of the sine wave). With perfect sinusoidal motion, these periods are always the same. However, in irregular waves, these periods will differ from each other and differ over time, depending on the waves that the vessel encounters. When responses in irregular waves are considered, the average periods are computed.


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