Vibration Analysis of a Marine Propulsion Shaft System with the Torsional-longitudinal Coupling Effect Induced by Propeller and Crankshaft

Due to the complicated structure and harsh working environment, the marine propulsion shaft suffers from excessive vibrations in torsional, longitudinal and their coupled vibration modes. The coupled torsional-longitudinal effect is mainly induced by two factors, namely the propeller additional water and the crankshaft structure. However, most of previous models were established with only one coupling factor, and consequently there is still a lack of a complete understanding for coupled torsional-longitudinal vibration. Hence, a comprehensive investigation is performed in this work to reveal deeper mechanisms of coupled torsional-longitudinal vibrations for marine propulsion shaft system. A discrete torsional-longitudinal vibration model is established for a real-life marine shaft. The coupling effects due to propeller additional water and dynamic characteristics of crankshaft are considered simultaneously to model the realistic vibration conditions. Then, a theoretical analysis is conducted on a simplified model to present a theoretical basis. Natural frequencies and forced steady-state responses are calculated numerically to analyze the influences of coupled torsional-longitudinal effect on the eigenvalue problem and vibration characteristics. Results show that, both the propeller additional water and the crankshaft structure could induce coupled torsional-longitudinal vibrations, and should be considered simultaneously in the model to achieve accurate vibration prediction and analysis. Besides, the coupling effect could induce a high amplitude beyond expectation that may even threaten the structure safe. The theoretical and numerical results in this study could provide some suggestions to designers and researchers attempting to obtain desirable vibration behaviors for marine propulsion shafts.