The effects of tunnel height and centre bow length on motions and slam loads in large wave piercing catamarans

An above water centre bow in wave piercing catamarans provides additional reserve buoyancy for minimising deck diving in following seas. The centre bow entry in waves, however, contributes to the severity of the wet-deck arch slam loads and slam induced bending moments. In this thesis, the effects of the centre bow length and wet-deck height/tunnel clearance on the motions and slam loads in large wave piercing catamarans are investigated through model tests to establish a framework for the preliminary design analysis. The model tests were performed in regular waves in head seas using a 2.5 m hydroelastic segmented catamaran model designed with an adjustable wet-deck and a changeable centre bow segment. Five different centre bow and wet-deck configurations were considered and over 500 towing tests were performed at two model speeds, in three wave heights and for various wave encounter frequencies. The catamaran model was comprehensively instrumented to measure the pitch and heave, centre bow loads, centre bow accelerations, wet-deck arch slam pressures and the vertical bending moments at two segment cuts located in each port and starboard demihull. Motion analyses showed that both heave and pitch increased over a wide range of encounter wave frequency as the wet-deck height of the catamaran model increased. Increasing the length of the centre bow showed an increase in the pitch but a decrease in the heave for a particular range of encounter wave frequency. The vertical motions along the model length indicated that the position of minimum vertical displacements and accelerations were aft of the LCG, between 20% and 38% of the overall length from the transom. The increase in wet-deck and consequently the archways clearance also resulted in an increase in relative vertical displacement in the centre bow area. This indicated that although the wet deck height had been increased, the consequent increase in motion still caused slamming to occur. In dynamic analyses, the maximum force acting on the centre bow segment during a slam event was decomposed into a bow entry force and a slam force. It was found that the slamming force, the centre bow entry force and slam induced bending moment increase as the centre bow length increases. Increasing the wet-deck height reduced the maximum slam load and pressure in moderate waves, but not in large waves. A correlation analysis between the slam force and slam pressure showed that the slam loads increase in the longer centre bows because of the increase in the impact area. The location of maximum pressures along the centre bow length was more related to the encounter wave frequency rather than the centre bow configurations. The distribution of the peak pressures within the centre bow archways showed that the inboard peak pressures were higher than the top arch and outboard peak pressures. The slam occurrence and severity for different centre bow and wet-deck configurations were analysed by considering the centre bow immersion depth and relative velocity at slam using the undisturbed water profile. It was found that the undisturbed immersion depth for severe slam loads was greater than the immersion depth at the wet-deck level. Therefore, the cross structure between the demihulls and the centre bow can be modified to reduce the slamming pressure. The results obtained by the comparison of the various centre bow and wet-deck configurations demonstrate the significance of considering the effect of the centre bow and archway slamming in structural design and suggest that the class rules currently available for wave piercing catamarans with a flat deck structure could be modified accordingly. Amongst the tested centre bow configurations, the shortest centre bow was the best design for slam load alleviation. Ideally, a trade-off amongst the centre bow buoyancy in waves, slam load and the catamaran motions should be made to optimise the centre bow design according to the vessel‚ÄövÑvºs operating conditions.