Catalina 30 TRBS #2889 "Northern Light"

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 raise two issues that have already been documented on the list over the years:

1. the engine shakes prominently and everything rattles if idle speed is a bit too low, and
2. despite careful engine/shaft alignment the propeller shaft exhibits mechanical resonance at certain RPMs (slow beat frequency (approximately three second period) and possible shaft contact with the narrow shaft tunnel.)

Both issues must be managed.

• The idle shake is the easier one and is managed by raising idle speed a bit. With the addition of pedestal mounted engine controls that have an idle limit, it's easy to adjust the idle speed at the engine and stay out of the shake range.
• The shaft resonance occurs (I think) because of the inevitable slight engine/shaft misalignment that dynamically occurs with the more compliant engine mounts. 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). Both fixes would serve to lower the mechanical resonant frequency somewhere below the idle frequency which is to say "the system will still resonate (like all systems do) 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.