Experimental analysis of the wet flexural mode response of an NPL 6A hydroelastic segmented model

Abstract

This paper investigates the effects of stiffness and mass distribution on the longitudinal flexural natural frequency response of an NPL 6A segmented monohull model in still water at zero speed. Experimental tests were undertaken in order to establish the parameters that affect the whipping frequency at model-scale so as to replicate the vibratory response of a full-scale vessel subject to slamming. The model was cut into two halves at the longitudinal centre of buoyancy and connected by a backbone beam with an elastic hinge joining the two segments. Wet vibration tests conducted on the model showed significant influences on the flexural natural frequency response through variations in stiffness and ballast mass distribution. The whipping frequency was predicted with a two degree of freedom theoretical model using an added mass approximation to provide good correlation with measured experimental data. Damping ratios of the wet transient response are also presented with respect to variations in the condition of the void separating the two model halves.

Keywords:

• Slamming
• Whipping
• flexural mode response
• natural frequency
• hydroelasticity
• NPL 6A
• segmented model
• added mass
• viscous damping ratio

Additional information

Notes on contributors

J Lavroff

Jason Lavroff is currently undertaking a PhD at the University of Tasmania in the area of ship hydrodynamics, with a primary focus on slamming and whipping responses of high-speed catamarans. He completed his bachelor degree in mechanical engineering with honours at Monash University while specialising in the field of fluid mechanics. Jason has worked as a mechanical engineer, and has assisted with several teaching roles both at the University of Tasmania and within private industry.

M R Davis

Michael Davis is a professor of engineering at the University of Tasmania. Mike completed his bachelors degree in aeronautics and doctorate in sound and vibration at the University of Southampton. He subsequently worked at the de Havilland Aircraft Company and the Royal Aircraft Establishment, Farnborough, before joining academic staff in the Department of Fluid Mechanics and Thermodynamics at the University of New South Wales.

Mike has also spent periods in visiting positions at the University of Cambridge and the Massachusetts Institute of Technology. He is currently working in the area of ship dynamics and propulsion, with particular reference to high-speed ship motion and wave loads, and has published and acted as consultant in heat transfer, fluid dynamics, vibration and noise.

Mike is a Fellow of Engineers Australia, and a former President of the Tasmania Division and Chair of the Mechanical Engineering College. He is also a Fellow of the Royal Aeronautical Society and member of Australian Council of Professions.

D S Holloway

Dr Damien Holloway is a lecturer at the University of Tasmania in the areas of stress, structural analysis and dynamics. He completed a BE(Hons) in civil engineering at the University of Tasmania in 1992, graduating with a University Medal, followed at the same institution by a PhD in ship hydrodynamics, completed in 1998. He has held postdoctoral positions in both mathematics (free surface groundwater problems) and engineering (ship hydroelasticity), and worked as a consulting civil/hydraulic engineer.

G Thomas

Dr Giles Thomas is a senior lecturer at the Australian Maritime College (AMC). He received his PhD in 2003 from the University of Tasmania for his work on the slamming of large high speed catamarans. He is also currently a chief investigator on a collaborative project with Incat and Revolution Design, investigating asymmetric and non-linear loads on high-speed catamarans.