On the Vibration of Bottom of Ships

Abstract

According to the results of forced vibration tests on actual ships, singular phenomena are found in the higher modes of hull vibration. For instance, the stern vibrations, jump of frequency curves and irregularity of mode curves, etc. occur frequently. The most important cause of them are seemed to be the effect of coupled vibration of the ship's bottoms with the hull. For the purpose of investigating the ship's bottom vibration, the authors made the forced vibration tests, using the vibration exciter, on seven ships which include three cargo vessels, two bulk carriers and two oil tankers. Furthermore, they manufactured the methacrylate box girder of hollow section partitioned by bulkheads into 13 subdivisions, of which bottom panels vibrate locally, and made the forced vibration tests on water by magnetoelectric vibration exciter. What clarified from these model experiments and vibration tests on ships are summarized as follows. (1) Ship's bottom panels show remarkably large amplitudes compared with those, measured on upper deck. (2) The vibration of ship's bottoms have a decidedly impoortant influence upon ship's hull frequencies in the higher modes, which are lowered some extent than expected by ordinary calculations of hull natural frequencies. (3) The experiments on the box girder of which bottom panels vibrate locally show that the peak amplitudes of each resonance diminishes gradually when resonant frequencies approach to the natural frequency of bottom panel, and stern vibrations arise when exciting frequencies exceed the resonance frequency of bottom panels.These phenomena simulate the behavior of ship's vibrations in the higher modes very well.