Torsional-axial coupling in the line shafting vibrations in merchant ocean-going ships

Mutual interaction of the two major vibration modes, i.e. torsional and axial, resulting from propeller action in the solid elastic shaft of the merchant ocean going ships is analyzed in this thesis. This type of vibration interaction can be described as torsionally-induced propeller thrust fluctuation. Generated longitudinal vibrations due to the thrust variation, can be the cause of excessive ship hull vibrations in modern merchant ocean going ships. The aim of this study is twofold; i.e. to show how the design of the propulsion line shafting affects the coupling phenomenon between torsional and axial vibrations, and to correlate line shafting design with potential structural problems of the hull.;The line shafting design is first addressed using conventional approach which yields information about torsional and axial eigenvalues and stresses. It provides no details regarding propeller coupling. The method used in the analysis is transfer matrix method, which suits well the rotor systems on modern ocean going ships. Vibration coupling computation is then performed using finite element method to demonstrate the importance of obtaining torsionally induced axial force. It is shown that dynamic response of the ship hull will be greatly affected by longitudinal force generated by propeller coupling of torsional and axial vibrations.