PREDICTING METHOD OF NATURAL FREQUENCY FOR SHIP’S OVERALL VERTICAL VIBRATION

At ship design stage, empirical formulas are generally used to predict the overall vertical vibration natural frequency of ship to avoid the harmful resonance against main excitation. Nowadays, with the development of modern large-scale ships, the existing empirical formulas are becoming unpractical, some corrections shall be applied on these classical formulas or a new formula is to be derived for the prediction of vertical vibration frequencies of ship. In this paper, a new empirical formula is given for predicting the natural frequency of ship overall vertical vibration. Based on the Timoshenko beam theory, the formula is derived by introducing by shear coefficient to correction shear uniform distribution hypothesis for a thin-walled box beam with free-free boundary condition. This new formula is obtained by statistical analysis for large amounts of measured natural frequency results of the ship overall vertical vibration. The proposed prediction method in this paper was used to predict the natural frequencies of six ships. The predicted natural frequencies are consistent with that by measurements. The comparison with measurements show that the formula proposed in this article is more feasible to use, and also provide a new method to the prediction of natural frequency of the ship overall vertical vibration.