Ship motions and wave-induced loads on high speed catamarans

Advancement in the design of large high-speed ferries demands comprehensive knowledge of ship motions and wave-induced loads to optimise their structural integrity. This investigation focused on the fluid structure-interaction problem experimentally to obtain such information. Motions and loads were investigated by using two different high-speed catamaran models and a full scale ship. Firstly, a hydroelastic segmented model (HSM) of the INCAT 112 m class wavepiercer catamaran with centrebow was designed. It was tested in a towing tank for a range of head seas conditions to determine the motion responses, vertical bending moment (VBM) and slam loads. A second catamaran model was designed and tested in oblique seas in a model test basin (MTB) to examine the motions and asymmetric wave-induced loads. Thirdly, full scale measurements were performed during the delivery voyage of the INCAT 112 m Hull 064 from Hobart in Australia to Hakodate in Japan, to measure the motions and structural load responses. The structural dynamic behaviour of the full scale vessel was replicated by the HSM model and the slamming and subsequent whipping behaviour were successfully modelled. As wave energy was consumed in the structural vibration of the model, the heave and pitch transfer function peaks reduced significantly when compared to the rigid configuration. The HSM test results also showed a strong non-linear motion response, with respect to wave height for this type of vessel, mainly due to the influence of the centrebow. The peak values for the VBM and slam loads were confirmed to be proportional to the square of the wave height for large waves. The MTB test model, which was not fitted with a centrebow, provided linear pitch, heave and roll motions for varying wave heights. The pitch connecting moment was found to be the dominant of the asymmetric wave loads and was linear with respect to wave height. During the full scale measurement programme, slamming phenomena were clearly recorded whilst crossing the Bass Strait. A reconstruction of the event, from the measured data, indicated that a slam event with subsequent whipping occurred with a bow down trim. Spectral analysis was used to detect the response frequencies of the VBM and machinery, with the frequency of the longitudinal mode increasing as the displacement reduced. These experimental programmes, encompassing model and full scale measurements, have revealed valuable insights into the motion and structural dynamic behaviour of large high-speed catamarans. A comprehensive set of motion transfer functions, VBM and slam loads coefficients have been obtained, thus providing designers with important slam and wave load knowledge to aid the improved structural optimisation of these vessels.