As the name implies, double mounting is an isolation practice in which two mounts separated by an auxiliary mass is used at each mounting location. Figure 1 depicts the schematic of such mounting system. As in any isolation application, the goal is to isolate the base from the vibration of the machine caused by the excitation force F, i.e., lowering the force transmitted to the base Ft , while avoiding excessive vibration of the mass (bouncing) due to shock excitation at the base (x_base) common in vehicular (including luxury watercrafts) applications.

To have the same static deflection as that of single mount, the mounts used in a double mounting application are normally twice as stiff at the ones that would have been used in an equivalent single mounting application; note that the 2 similar mounts in series provide the same stiffness as that of a single mount twice as soft.

Considering that the dynamics associated with the inertia forces are negligible at low frequencies, isolation effectiveness of double mounting at low frequencies is very much the same as that of single mounting. At higher frequencies the auxiliary mass M2 affects the dynamics of the double mount system, enhancing its isolation capabilities.


Figure 1 Double mounting system

Figure 2 shows the magnitude of the frequency response functions mapping the vibration excitation force (F) and shock excitation (x_base) to the transmitted force Ft and mass displacement x, in a double mount system with two similar mounts. Clear from Figure 2, at high frequencies, double mounting has a dramatic effect in lowering the transmission of vibration (and noise). This high frequency effectiveness increases with increase in the auxiliary mass (M2 in Figure 1). Unfortunately, this enhancement in high frequency effectiveness is at the expese of either deteriorationg or not affecting the low frequency isolation effectivcenss of double mounting, compared to that of single mounting. The low frequency deterioration in isolation effectiveness is mainly due to the addition of the 2nd mass (the auxiliary mass) which introduces a 2nd resonance in the system.


Figure 2 FRFs mapping the vibration (F) and shock (x_base) excitations to the transmitted force (Ft) and displacement of the mass (x) for varying auxiliary mass in a double mounting system.

Enhaced high frequency vibration isolation, provided by double mounting, will benefit the mid-range and high frequency noise abatement aspect of the isolation. The transmission of low frequency vibration and noise to the living quarters of the vessel are the same or worse than such transmissions in single mounting applications.

Clear from Figure 2, the added effectiveness of the double mounting is realized over the frequencies higher than the 2nd resonant frequency of the system. To extend this effectivenss to lower and lower frequencies, the 2nd mass (auxiliary mass) should get larger and larger. Figure 3 depicts the ratio of the two natural frequencies vs. the size of the 2nd mass m2 (as a fraction of the main mass, m1). Evident from Figure 3, lower 2nd natural frequencies require excessively large auxiliary masses. In other words, extending the benefits of double mounting to lower frequencies requires unacceptably large auxiliary masses. For example, reducing the natural frequency ratio by a factor of 2 from 6 to 3, requires the auxiliary mass to increase by a factor of 10 from 10% of the main mass to 100% of the main mass.


Figure 3 Natural frequency ratio


Lastly, a word or two about realization of double mounting: although double mounting is rather straightforward to realize while the watercraft is being built, it is a challenge to retrofit an existing single mount system to a double mount system.

Home
Raising an existing diesel generator, to slide a large mass under it, in the confines of an engine room, could be difficult in some retrofits. Besides, the height of the isolation mechanism (two rubber mounts and a mass in between) is hard to accommodate in most applications.