Vibration Calculation Based On A Combined Modelling Method And Vibration Reduction Of Ship Propulsion System

With main engine power increases and more requirements on cabin layout, shafting system vibration is drawing great attention. The accuracy of vibration calculation and improvements on vibration reduction are becoming more and more important. The thesis concerned with ship propulsion system vibration calculation improvement and vibration reduction method.In order to improve vibration calculation, a combined modelling method is put out in this thesis, which separates the ship propulsion system into discrete and continuous subsystem. The discrete subsystem refers to the diesel engine part of the actual ship, while the continuous one represents the shafts, propeller and flanges. Wave analysis and Multi-Degree of freedom(MDOF) dynamic analysis approach are used respectively in continuous and discrete subsystem to deduce the dynamic equation, and a global governing equation is obtained by combining the two subsystems through boundary conditions. To validate the effectiveness of the combined modelling method, numerical computations with different modelling methods are carried out on a propulsion shafting system of an engineering ship. It can be seen from the comparison of results that the combined method is more accurate to the modal vibration, with less time and computation resources especially in high frequencies. Moreover, the method put out in this paper can largely reduce the error caused by the node offset when dispersing the shaft into only one or two mass elements by using traditional method. This is helpful for accurate vibration calculation of propulsion shafting system with large length and low stiffness.The method is also applied on a complex branch system of ship propulsion system, longitudinal and bending vibration to validate the effectiveness and comprehensive applicability of this method, which is helpful for the reduction of the missing error of eigenvalue when using traditional Transfer-Matrix(TM) method.For the vibration reduction part, a simplified isolator model was put out to investigate the dynamic characteristic of the variable-rate spring isolator system and the analytic solution of the variable-rate spring vibration was deduced. The time-domain and spectral response of the variable-rate system under forced vibration together with its force transmission rates were simulated. The damping characteristics of variable-rate structure were concluded according to economy, damping efficiency and force transfer rate. It is shown that variable-rate spring is more effective for vibration reduction compared with the fixed-rate one at high frequencies under the same static mounted condition. This theory had been applied on the torsional and longitudinal vibration of ship propulsion system. And possible structures for this theory are investigated in this thesis. The conclusion is useful to improve the dynamic characteristics of the vibration system in engineering.