Stock engine mount. Very non-compliant!
The new engine mounts are Vetus K50 just as other C-30 owners have used. The Vetus mounts make a huge difference in engine noise transmitted to the hull. Being much more mechanically compliant than the stock mounts, they do bring two issues that have already been documented on the list over the years:
Both issues much be managed. The idle shake is the easier one and is managed by raising idle speed a bit. The shaft resonance occurs (I think) because of the inevitable slight engine/shaft misalignment that dynamically occurs with the more compliant engine mounts. To manage it, I'll first check my cutlass bearing, but I suspect that minor engine mount adjustments (while motoring at certain speeds) will be more significant in mitigating or eliminating the resonance. Beyond those fixes, it might also be possible to add significant static mass to the engine (for instance across the two rear mounts straddling the transmssion,) or by adding a mass (more significant mass than a new zinc) to the propeller shaft. Both fixes would serve to lower mechanical resonant frequency somewhere below the idle frequency which is to say "the system will still resonate but not at normal any operating condition."
I did try out the concept of a "PTS-like" disk that I made out of 1/4" polyethelene installed between the transmission flange and shaft flange using nuts as standoffs.
Here's the installed view. But, it didn't really make a difference in the resonance. Perhaps a real PTS coupler would.
One year later...with the addition of pedestal mounted engine controls and not yet having an idle detent arrangement, it seems just as easy to manually adjust the throttle as necessary to keep the engine RPM out of the resonant range.