Coupled Vibration of Fore and Aft Vibration of Superstructure and Vertical Hull Vibration

Abstract

This paper deals with the coupled effect between a fore-and-aft vibration of superstructure and a vertical vibration of ship's hull. Characteristics of the coupled vibration were discussed by using the simple model, namely, one beam and mass-spring model. Main conclusion are as follows : 1) Coupled effect depends on four parameters. One is position of superstructure. Second is a ratio of distance between fore and aft wall of superstructure to ship's hull length. Third is a mass ratio of superstructure to ship's hull. The last is an uncoupled natural frequency ratio. 2) Natural frequencies of the coupled vibration obey the law of so-called "spring-off". But the law of "spring-off" for vertical vibration is rather complicated compared with that for longitudinal vibration. 3) When uncoupled natural frequencies coincide with each oter, coupled ones spring off in the largest amount.