The propeller, shafts and bearings ,it transmits power

                                       The torsional vibration analysis of marine
power transmission  is a challenging
subject  and it has been investigated
since 1950s.Two distinct types of system vibration that exists in marine diesel
engines are (1)Machinery Vibration caused due to relative motion of moving
parts at certain frequency and (2) Hull vibration caused due to propagation of
Hull structure. Torsional vibration are an example of Machinery Vibrations and
caused by the twisting of the crank usually along the whole propulsion shaft
system including propeller shaft,engine crankshaft, engine gearbox, flexible
coupling and along the intermediate shafts. Pulsing torque at the ends of crank
usually creates twisting along the shafts, which results in torisonal vibration
at high frequencies.

                                      
Transmission system in a ship is made of propeller, shafts and bearings
,it transmits power from engine to the propeller. The power transmission system
is a system with large number of concentrated masses. Such systems can be
studied using the approach of lumped-parameter methods. Holzer method is a
lumped-parameter method for the torsional vibration analysis of shafting
systems, which consist of multiple lumped (concentrated) rigid masses. The
segments of shaft between the lumped masses are assumed to be massless and are of
uniform stiffness.

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Modeling
method:

The
power transmission system of a 6 cylinder slow speed marine diesel engine
studied in this paper consist of Crank shaft,propeller shaft,propeller and the
intermediate shaft.In propotion with keeping the vibration characteristics
unchanged,the above stated system is simplified to a lumped parameter model
with 12 inertias and 11 elastic shafts as shown in figure 1.The input
parameters of the lumped system are shown in Table 1.The torsional vibration
characteristics of the lumped parameter model will be equal to that of real
system.In the Simplified model the crank shaft is modeled by first nine
elements,shaftline is represented by tenth and eleventh element and the
propeller by twelth element.This type of slow speed marine diesel engine
usually have a short shaft line. To carry out natural frequency calculation,a
matlab program is developed based on the following flowchart as shown in figure
2.