Simplified method of torsional vibration calculation of marine power transmission system

• The calculations methods of torsional vibrations of marine power transmission system are presented.
• Two simplified methods of natural and forced torsional vibrations frequencies and modes are developed.
• The methods were verified by calculations and measurements made by independent researchers.
• The results of conformities are good. The damping coefficient was corrected based on measurements.
• Presented analysis method is much faster and easier to perform by engineers.

The paper presents two estimation methods to calculate the natural torsional vibration mode of marine power transmission system. Additionally, the method of forced vibration analysis (torsional stress level amplitude estimation) is discussed. A typical propulsion system of merchant ship is made up of slow speed main engine connected directly to the propeller by a relatively short shaft line. All classification societies require calculation of the propulsion system operating parameters, but they have no simplified formulas. Torsional vibrations of the marine power transmission system are usually most dangerous for the shaft line and the crankshaft. Numerical algorithms based mostly on Finite Element Method (FEM) are unobtainable and not easy to use by the ship crew and unspecialized engineers. Estimation method of the torsional vibration of the system was being investigated by the authors. Modelling method of more difficult propulsion system parameters like propeller added water polar moment of inertia was presented. Short description of the advantages and disadvantages of the undercritical and overcritical propulsion system was introduced. A discussion about the calculation results was included in the final part of the paper. The presented calculation method was verified by comparison with the detailed FEM calculation and measurements on real ships.