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UBC Theses and Dissertations

Small craft motion in reflected long-crested seas Kennedy, Andrew B.


This thesis attempts to provide an analysis of several aspects of moored small craft motion, concentrating on vessel response to wave action and wave reflection, and briefly examining the effects of irregular waves. Experiments were performed in the National Research Council multidirectional wave basin in Ottawa to determine the response of a moored small craft model in long-crested waves. The SELSPOT imaging system was used to measure vessel motions. With several exceptions, model response was found to be highly linear. The most notable of these exceptions were the resonance peak in surge, and all yaw motions. The modes of sway and roll also exhibited nonlinearities but, surprisingly, these were fairly A numerical program was undertaken to attempt to correlate and extend the results of the experiments. Linear diffraction theory as represented by the programs FACGEN3 and WELSAS3 was used to calculate vessel movements for various wave periods, incident angles and mooring conditions. Various perturbations were made towboat properties to find the sensitivity of response. Response in head seas generally agreed with experimental results with the notable exception of the surge resonance peak, which was predicted to be much too high. However, the addition of an extra 5%damping lowered this peak to experimental levels without substantially affecting another modes of motion. Response in beam seas was not as well predicted. For both sway and roll, long wavelength response was predicted to be much too low. This was thought to be due to poor representation of the boat hull by Green's functions and neglect of viscous effects. Calculations performed with a widened hull showed better agreement with experiment. Expressions were developed to relate vessel motion in a partially reflected and unreflected wave field and were found to agree well with experiment. The effect of irregular waves on wave criteria developed for monochromatic waves was also considered. For both cases, recommendations were made toward extending existing wave criteria in these directions.

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