Pages 117-126
Many kinds of machinery, steel structure, vehicle and others have been freed from harmful and troublesome vibration with dynamic damper since the theory of the dynamic damper was established by Den Hartog at the early 1900's. This paper deals with the mechanical damper for reducing ship vibration. The authors devised three kinds of dynamic damper type here, namely beam spring type, rubber spring type and multi-beam spring type. The damper effect of these dynamic dampers were studied experimentally and theoretically. The authors obtained the following conclusion from this study. (1) The damper is the most effective when its natural frequency harmonizes with the ship's one, and larger is the weight ratio of dynamic damper to ship, more effective the damper is. (2) The large damping factor of dynamic damper installed on ship has effect to reduce the displacement amplitude of ship at resonance (3) The dynamic damper should be installed at the loop of mode shape, and the installation of the damper on the node is confirmed experimentally to be ineffective. (4) The ship model with two dampers has the three coupled natural frequencies, and the mode shape of ship at both the highest and lowest natural frequencies is two noded mode and the other is one noded mode. (5) The multi-beam spring type damper is found experimentally to behave as one damper on condition that its own many natural frequencies scatter in less than 5%. (6) The authors applied not only the coventional mass spring model but also the beam model with mass spring as calculation model, and they showed good agreement between measured and calculated frequencies of the ship model with the dynamic dampers.
