Question:

RMS, average, significant, maximum, extreme: Which amplitude should I use?

Answer:

SEAKEEPING displays a short list of amplitudes in the response statistics tables. This includes, by default, the Root-Mean-Square (RMS), Average, Significant (Average 1/3rd), Average (1/10th), and the average maximum in 100, 1000, and 10000 samples. There is also the option to define extreme amplitudes using confidence and time interval parameters.

All of these amplitudes have one thing in common: they are derived from the statistical distribution of the vessel's response in irregular waves. Because we deal with responses in irregular waves as a distribution, we must use statistical measures of this distribution to quantify the "extreme" or "worst-case" scenario. As we move further up the distribution, we observe responses that occur less often (are less probable) but occur with a much larger magnitude. It is desireable to select a magnitude that is both extreme and statistically unlikely. It is up to the engineer to decide where to draw the line.

This process should be based on some type of regulatory guidelines and/or engineering judgement. Typically, users will opt to use the Average Maximum in 1000 samples as a starting point, with some opting to use a custom amplitude derived using a certain level of confidence over a time interval.


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