On Coupling of Hull Torsional Vibration and Horizontal Vibration of Superstructure of Car Ferries

Abstract

To make clear the nature of hull torsional vibration of car ferries, it is indispensable to investigate the coupling of hull torsional vibration and horizontal vibration of superstructure. On the above problem, the authors have carried out theoretical investigation under the assumption that the main hull and superstructure are uniform beams. Next, a comparison is made between the computed frequency or mode and experimental data of ship "A". The following results have been obtained. (1) In the case of uniform beams, 1-node hull torsional vibration couples with rotary vibration of superstructure. Therefore, the effect of coupling is maximum at ω_φ=ω_H. But, for 2-node vibration, the ratio of ω_φ/ω_H that induces the maximum effect of coupling is variable for the ratio of ω_φ/ω_B or ω_φ/ω_s and the effects decrease for increasing of nodes. (2) By the analysis of vibration for ship "A", the error between computed and measured frequencies is about 10%, and the effect of coupling on the frequency is about 20%. (3) For car ferries, transverse members between the wagon and bridge decks have great influence on the horizontal vibration of superstructure and torsional vibration.