Torsional-longitudinal shock responses of a ship propulsion shaft system

A torsional-longitudinal shock finite element model for a diesel engine's propulsion shaft system was established considering torsional-longitudinal coupled effects caused by the diesel engine's crankshaft, and added hydrodynamic inertia and damping of a ship's propeller. The free vibration and torsional-longitudinal shock responses of the coupled system were studied and compared with those of the uncoupled system to analyze influences of torsional-longitudinal coupled effects on the free vibration and shock responses of the propulsion shaft system. Results showed that the torsional-longitudinal coupled effects have little influence on the natural frequencies of the propulsion shaft system, but they obviously influence the shock responses of the propulsion shaft system; under the torsional-longitudinal coupled effects, either a single longitudinal shock or a single torsional one can simultaneously excite longitudinal vibration and torsional one to cause larger fluctuations of displacements and torques, and threaten the operation security of the shaft system, so enough attentions must be paid to torsional-longitudinal shock responses of a ship propulsion shaft system in its design stage.