The Hull Vibration Tests by Means of Vibration Exciter and the Study of Some Formulae for Calculation of Natural Hull Frequencies

Abstract

Calculation of natural frequency of vessel's hull is a matter of vital importance from the viewpoint of eliminating hull vibration. Although numbers of calculation formulae have been made public in the past, it seems that they have not sufficient accuracy for use in ship designing, it thus still remains as essential as ever to conduct vibration measurement on actual vessels. In view of this, we, the writers and their colleagues, designed and manufactured a hull vibration exciter and a marine vibrograph of cross hinge type associated with wire strain gauge with which we have been conducting forced vibration tests on vessel's hull with the intention of making experimental researches. In this treatise are given an outline of the vibration tests with the results thereof as well as the study of several formulae for calculating naturalf requency. The outcome may be summarized as below. 1. Comparison was made between our actually measured values with those obtained from the natural frequency calculation formula formulated by Messrs. Tomita, Kumai and Hirowatari, and as a result it was found that, although with regard to the fundamental vibrations the above values turned out to be comparatively similar, they showed considerable difference with regard to vibrations of higher order. 2. For large type supertankers, ranging from 40,000 to 50,000dwt., the capacity of the vibration exciter used by ourselves has proved to be sufficient. However, in order to know exactly the mode of vibrations, higher order vibrations and coupled vibrations, it is essential to make simultaneous measurement at least at more than ten points. 3. A specific phenomenon was observed in the forced vibration test conducted on actual vessels by means of vibration exciter, namely, coupled vibration appeared to the area comprising the vibration exciter and the structural portion of where the exciter was fitted, whereby the result of measurement was affected, and therefore, particular attention should be given to the structure of the fitting portion of the vibration exciter.