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

Abstract

The paper deals with the coupled effect between a fore-and-aft vibration of super-structure and a longitudinal vibration of ship's hull. Characteristics of the coupled vibration were discussed by using the two-beam model. The present theory was found to explain well the coupled vibration of actual ship. Main conclusions are as follows : 1) Coupled effect depends on two parameters. One is a mass ratio of superstructure to ship's hull. The other is an uncoupled natural frequency ratio. 2) Natural frequencies of the coupled vibration obey the law of so-called "spring-off". : When uncoupled natural frequencies coincide with each other, coupled ones spring off in the largest amount. 3) Coupled natural frequency is estimated by multiplying the uncoupled one by the correction factor.