Damping and excitation in the torsional vibrations calculation of ship propulsion systems

Calculation of torsional vibrations is essential in the early phase of the design of any ship propulsion system, after selection of shafting diameters in accordance with the Classification Rules. Later on, during ship trials, the calculation shall be validated by measurements on board. The calculation results depend upon inertial moments of actual masses, stiffness of shafting components, damping in the actual shafting components, as well as the excitation forces and moments exerted by the propulsion engine(s) and the propeller. Inertial moments and stiffness can be determined with no ambiguities. However, this is not the case with either the damping, or the engine excitation. Actually, during validation on board the calculation supposed damping is the main influential factor to be verified. The aim of this paper is to present and compare several models to define and compare damping definitions in the the torsional vibrations calculation (Frahm's model, Archer's model, physical damping, magnification factor, etc.) in a systematic way, to enable designers to correctly apply the selected damping model. Further on the engine excitation may be expressed by means of cylinder pressures or tangential forces to the cranks. The essentials of the two models and the procedure to convert one excitation model into another one are also presented. The application of the presented damping and excitation models is presented on an actual ship propulsion system and conclusions drawn.