The authors had measured the axial displacement of main shaft system of the Diesel ship. In the result, the resonant point of second mode axial vibration can be calculated by the free vibration theory, but that of first mode axial vibration cannot. It is estimated that the difference is caused by the over-estimation of thrust bearing rigidity. The first mode resonance is found in lower revolution of which resonance curve is very flat, because if the thrust force is smaller, the thrust bearing block has non-linear characteristics.

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