<p>Recently, the flexible coupling connecting a diesel engine and reduction gear fractured in a specific naval vessel of ROK navy. Usually, the flexible coupling and mount are applied to the main engine and reduction gear to reduce the vibration transmitted to the hull and resulting underwater radiation noise. When the shaft system is flexible by applying this kind of flexible coupling and mount, lateral-torsional coupled vibration and self-excited vibration can occur. In addition, since the dynamic motion between engine and reduction gear installed in a ship under the rough sailing condition can be much larger than that of land-based equipment, this dynamic motion should be considered to ensure sufficient safety factor of the shafting system in ship application. In this research, root cause analysis suggests that the fracture resulted from self-excited vibration due to lateral-torsional coupled vibrations. Moreover, the measured load applied to the coupling was significantly high, indicating that an inadequate safety factor might be a primary cause of the fracture. Consequently, this study verifies the root causes of the coupling fracture through experimental and numerical analysis and proposes corrective actions.</p>