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Published June 9, 2004 | Submitted
Report Open

Wave-induced oscillations of small moored vessels


The general objective of this research is to investigate the motion of small boats moored to fixed or floating platforms in a standing wave environment. The study is directed toward an understanding of the problems of mooring small craft in marinas and toward providing information that will assist in the planning and operation of marinas. This report deals with the first phase of the experimental study concerning the surge motions of a simply moored body in a standing wave system. The body is a rectangular parallelpiped moored to a fixed support by means of a linear spring. In general it can be stated that the inviscid theory proposed by Wilson (5) and Kilner (7) adequately describes the surge motion of this body for standing waves ranging from shallow-water to deep-water waves and for ratios of body length to wave length from 0.1 to 1.5. Agreement between the experimental data and the theoretical response curves is better for certain ranges of the ratio of the natural period of the body to the wave period than for others. This is attributed to the effect of wave generation by the body on its motion. The response curves become more selective with respect to frequency as the distance of the body from a reflecting surface increases. Therefore, coupling this with viscous effects it is possible to reduce the effect of resonance considerably simply by choosing the proper body location in its standing wave environment for a particular natural frequency. The coefficient of virtual mass of the body in surge (ratio of width to length, 1:4) determined from simple free oscillations was found to correlate best with the ratio of draft to beam. For a variation of draft to beam from 0.25 to 0.95 the coefficient of virtual mass varied from approximately 1.1 to 1.25. This study emphasizes the need for more field information on the characteristics of small craft, such as the elastic characteristics of the mooring system, natural frequencies of moored boats, and the relative importance of viscous effects upon boat motions.

Additional Information

© 1965 W. M. Keck Laboratory of Hydraulics and Water Resources. California Institute of Technology. The author would like to express his deep appreciation to Professor Vito A. Vanoni for his assistance and valuable criticism throughout the course of this investigation. The construction and the maintenance of the wave basin and associated instrumentation was carried out by Elton Daly, Laboratory and Shop Supervisor and Robert Greenway, Laboratory Mechanic. A good deal of the system design can be attributed to the ingenuity of Elton Daly. The assistance of Carl Eastvedt, Senior Photographer and Robert W. Wilson, Laboratory Specialist, is appreciated. The drawings presented in this report were done by James Murray, Laboratory Technician. The writer wishes to thank Patricia Rankin for her assistance in typing and assembling this report. Some of the initial experimental work on the virtual mass coefficient and the damping coefficient was done by Loh-Nien Fan, Research Assistant. This project is supported by the U. S. Army Corps of Engineers under Contract DA-22-079-CIVENG-64-11.

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