Recently, in the upward tendency of tonnage and output power of the ship, vibration has become a serious problem such as tube damage for boiler generating bank, especially for the auxiliary boiler in diesel tanker.
This paper describes the results of experimental investigations with miniature model and the theoretical analyses on the method to estimate the natural frequency of the tube with complicated shape and also describes the new method for modulation of natural frequency, changing tube shape.
The following results are obtained.
The natural frequency of tube can be modulated far higher than resonance frequency with hull vibration when the tube bent slightly convex at the centre of support points. Inversely, when the tube bent concave, the natural frequency decreases. The reasons are as follows.
In the case of convex shape, the node of vibration occurs at this convex point. Therefore frequency increases. But in the case of concaved shape, node of vibration does not occur and the frequency decreases.
Through our experiments, we have established the method of frequency estimation from the model test simulated to actual tube and also we have suceeded in the theoretical analyses on the vibration of the tube with complex shape by means of matrix treatment, assuming that such complicated tube is the plain structure consisted of straight elements. The results calculated from this equations by means of electronic computer have been found to agree with the results of mesurement with good accuracy.

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