Natural frequency of the rubber-mounted engine is decided being affected by weight and position of center of gravity of engine itself and stiffness and number of mounting rubbers as well.
0.5th or 1 order vibration generates in addition to main vibration of 3rd order in 4-stroke inline six-cylinder engine depending on shaft arrangement.
The engine will vibrate seriously if the abovementioned natural frequency coincides with these order vibration. 0.5th order vibration can be made little by adjusting the engine minimizing deviation in combustion among cylinders and balance weights fitted on the crankshaft makes 1st order vibration.
The biggest vibration of 4-stroke inline six cylinder engine is 3rd order and transmissibility of this vibration to the engine foundation of the ship is reduced by mounting the engine on rubber blocks.
It is necessary to lower the resonance frequency of mounting rubber and the engine, anti-vibration engine mount is possible where the rated speed of the engine is 370 rpm or more in selection of mounting rubber in connection with engine mass. Fig.12 shows the arrangement of LH31 (1800PS/370rpm) mounted on anti-vibration rubber.
