Self-excited vibration of an impeller blade of the waterjet propulsion shaft in a naval vessel caused by flow-induced thrust load

Recently, one of the blades of an impeller installed in the water jet propulsion shaft of a particular naval vessel was fractured. Macro and microscopic investigation of the fracture surface indicated that the fracture was caused by mechanical fatigue. Therefore, root cause analysis is performed related to the vibration fatigue. Measurement of the accelerations on the thrust bearing housing showed that very high vibration occurred at the high-speed range over 600 rpm, and that the frequency of vibration did not vary according to the variation in rpm. Since dynamic thrust load is applied on the impeller blade, it can be assumed that the dynamic thrust load is represented as a nonlinear function of the ship speed and the rotation speed of the impeller. When the equation of motion of the shaft is derived with this assumption, it can be known that the shaft system can be unstable at certain condition where the self-exciting vibration occurs. In this research, numerical analysis and experiments are conducted to verify the assumption that the root cause of the impeller's fracture is self-exciting vibration from the increasing flow-induced thrust load of the impeller.