Under steady state condition, unstable torsional vibration normally does not occur in shafting systems using 4-stroke diesel engine due to hysteresis damping of shafting system and relative damping of standard fitted damper. However, the unstable torsional vibration occurs on marine propulsion shafting systems due to slippage of a multi-friction clutch installed between increasing gear box and shaft generator. To identify this unstable vibration and make proper counter measure, the simulation for transient torsional vbration using the Newmark method is introduced in this paper. The mechanism of this unstable vibration is verified by vibration and noise measurements of the shafting system. The results of theoretical investigation and actual measurement, can be summarized as follows: 1) Unstable torsional vibration on ship propulsion shafting system not reported up to now, occurred at frequency of node 1/3rd order torsional natural frequency. 2) The reason for this unstable vibration could be unexpected resonance with node I torsional natural frequency and 2nd order of shaft generator idle running speed by means of thrust reversal and friction torque from clutch. Some additional measurement with rubber coupling disengaged showed that unstable torsional vibration did not occur, which means shaft generator idle running speed is the key for unstable vibration. 3) To protect coupling and gear box from unstable torsional vibration, a barred speed range between 600 and 700 rpm is quickly by-passed by dint of engine governor. 4) For transient vibration analysis full-model is used instead of equivalent mass elastic data previously used, which can increase accuracy and reliability of results for both, steady state and transient condition.