Comparison of Model-Scale and Full-Scale Planing Craft Accelerations in Waves

Abstract

We examine the degree to which acceleration data from model-scale testing agree with acceleration data from full-scale testing. We find strong evidence that the model testing fails to recreate a certain type of low-acceleration impact that is found in full-scale data. In addition, there is some indication that model-scale testing may underestimate the frequency with which full-scale planing craft experience extreme vertical accelerations. Finally, we present evidence that spectrum type plays only a negligible role in the distribution of vertical and longitudinal accelerations. 1. Introduction Planing hulls operating at sea experience frequent wave impacts. Extreme impacts can result in structural damage, equipment malfunction or failure, or personnel discomfort or injury, or may require reduced speed to avoid such results. The vertical accelerations measured during these wave slams are used as one of the key design factors for the assessment of onboard comfort level, operational limits, and structural design. However, there is only limited ability to predict the vertical accelerations a planing hull will experience. Computational fluid dynamics models show great promise, but they are still in the validation stages (Fu et al. 2013; Fu et al. 2014; Maousaviraad et al. 2015). Guidelines used in structural design are based on empirical relationships determined from model testing results. Yet, there is little research into the connections between the accelerations measured during model testing in tow-tank-generated waves and the actual accelerations of full-scale planing hulls operating in real seas. Full-scale measurements of accelerations are obtained under conditions that are hard to know precisely. The wave conditions are typically determined from buoys that are not necessarily located very close to the testing area. In addition, the information from the buoys is generally limited to a significant wave height and modal or average wave period. Researchers often do not have access to information about the frequency distribution of the waves or an estimate of the wave energy spectrum type. Researchers conducting model testing must, therefore, make assumptions about the irregular waves when deciding what waves to generate in the tank. In addition, many tow-tank tests use methods to propel the models that are very different from the self-propelled, nonconstrained motion of a full-scale planing craft. When vertical accelerations are investigated during model testing, little research is conducted on the longitudinal accelerations for the model.