Mode Coupling in Torsional and Longitudinal Shafting Vibrations

The effect of propeller coupling on the torsional and longitudinal vibration of marine propulsion shafting is studied. Recent research on the nature and computation of propeller added mass and damping is reviewed. It is now possible to estimate the inertia coupling and velocity coupling characteristics which couple the torsional and longitudinal vibration of a marine propeller. Regression equations suitable for estimating the torsional and longitudinal added mass and damping of 4-, 5-, 6-, and 7-bladed Wageningen B-Series propellers are presented in the Appendix. The torsional and longitudinal modeling of a typical marine propulsion plant is reviewed. The numerical techniques and computer programs used in solving the free vibration problem for natural frequencies and mode shapes and the forced vibration problem for vibratory response are introduced. Results for a realistic numerical example are presented to compare the natural frequencies, mode shapes, and vibratory response which are obtained when the propeller coupling is neglected with those obtained with the propeller coupling included. In general, the natural frequencies are shown to change by less than 2 percent. The modes shapes can show significant change. Vibratory response can be either increased or decreased. Some cases show that neglecting the propeller coupling can result in more than a 50 percent underprediction of the vibratory response. The coupling effects are greatest when a torsional natural frequency obtained with the propeller coupling neglected and a longitudinal natural frequency obtained with the propeller coupling neglected converge.