Influence of wave variability on ship response during deterministically repeated seakeeping tests at forward speed

In order to validate numerical results or verify design choices using experiments, knowledge about the experimental variability is essential. This variability was evaluated for seakeeping tests at forward speed with a model in a steep wave condition over the long axis of a basin and in a less steep oblique wave condition, in a commonly applied test procedure. The incoming wave and response variability was evaluated using deterministic repeat tests. The results for incoming waves at some distance before the model have been published already; the present study discusses the model responses. Overall time trace similarity as well as the amplitude and timing variability of individual wave crests and response peaks were studied, after assessing the input uncertainties. The response variability increases with distance from the wave generator for the wave crest height and (relative) ship motion peaks. The variability of the impact loads on a deck structure is large with a lot of scatter. Small wave-induced currents may build up differences in wave propagation speed between the repeat runs, which means that the seakeeping variability partly depends on previous wave conditions. A proportional relation could be identified between most response peaks and the corresponding incoming wave peaks. The timing variability of the response peaks follows from that of the incoming wave crests. Unfortunately, there is no direct relation between the response amplitude variability and that of the corresponding wave crest. The presented results can be used as reference for the typical variability of free-sailing seakeeping experiments.