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Remedial solutions to control excessive propeller induced hull vibrations on a landing craft

Fan, Michael and Aktas, Batuhan and Shi, Weichao and Sasaki, Noriyuki and Fitzsimmons, Patrick and Atlar, Mehmet (2018) Remedial solutions to control excessive propeller induced hull vibrations on a landing craft. In: 14th International Naval Engineering Conference and Exhibition. IMarEST, London. (In Press)

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Abstract

Although landing craft are not sophisticated vessels, their functional/operational requirements often result in a hull shape which may encounter unusual hydrodynamic phenomena, requiring remedial attention. One such instance is discussed in this paper, which presents hull form solutions adopted to address excessive vibration experienced on-board an enhanced landing craft operating in the Arabian Gulf region. Through Computational Fluid Dynamics (CFD) simulations, the sources of excessive vibration experienced by this vessel were identified. The sources included the current bow design, which promoted aeration; an extensive flat bottom, which channelled the air to a shallow buttock-flow stern region; angled pram type stern fitted with blunt-ended appendages generated a non-uniform flow that was too severe for the existing propeller-hull clearances. The combination of these unfavourable flow conditions with the cavitating propellers resulted in undesirable Propeller-Hull Vortex Cavitation (PHVC) which manifested itself with excessive aft end vibrations and noise. To remedy the situation and to control the excessive vibrations, further CFD simulations guided the necessary hull form modifications. The identified countermeasures included anti-Propeller Hull Vortex (PHV) plates and streamlining of stern appendages. Subsequent sea trials showed horizontal vibration levels were reduced by 85%, which significantly improved the conditions on-board. This paper presents a technical summary of the above countermeasures, their implementations on the vessel, which included full-scale trials to measure the speed-power performance, hull vibrations and cavitation observations using a borescope system, and discussions of the results of these countermeasures. The paper concludes with an outline proposal for further design study, which could reduce on-board vibrations even further as well as providing other operational benefits regarding propulsive efficiency and manoeuvrability using the recently developed "Gate Rudder System®" as a novel Energy Saving Device (ESD).