Harmonic Engine Vibration

[Frank Intino]

I recently purchased a 1985 CAT 30. When the engine runs, in gear, there is a very noticeable harmonic vibration about 1600 RPM's. The harmonic frequency increases with engine speed above 1600 RPM's. The cutlass bearing looked fine prior to launching and the sound appears to come from the area of the strut. Anyone have any ideas? Possibilities I have considered: prop cavitation, slightly bent shaft, bent strut, zinc placement, crooked stuffing box, shaft touching hull where it exits. Please help with any advice.

 

[LaDonna Bubak - Planet Catalina]

Could it be the engine mounts?

This *might* be something to consider.LaDonna

 

[Jeff Walker]

What kind of prop do you have?

2 or 3 blade, 15X10 or smaller? Is there sufficient clearance between the blade ends and the hull (1 inch recommended)? I get a little vibration at about 1800 rpm but it goes away at 2000. Jeff

 

[Ron Doescher]

harmonics

Every moving part in the "real world" (engine, drive chain, prop turning/dragging in water, the passage of the hull through water, etc.) can (and does to some degree) cause some vibration. Every physical structure has a natural resonant frequency. When the vibration has a frequency that is a sub-harmonic of a body's natural resonant frequency, that amplifies the vibration. Since a boat is a physical structure with many cavities, each with their own resonant frequency and all coupled together, these amplified vibrations travel all over your boat & set up other sympathetic vibrations elsewhere. That coupling process even works from one body in water (or air) to another.So, designers try to stop or reduce these vibrations at their source. That's usually in the form of vibration-damped engine mounts tuned (just any mount will not do! --and worn-out mounts can be worse than none.) to provide the most damping for the expected vibration frequencies, acoustic absorbtion linings around the engine compartment, etc.

 

[Pat DiFelice]

Check engine alignment

Last year I bought a 89 C-30 On survey I was told you will be needing a cutlass bearing, but you'll be fine for now. This spring I replaced the cutlass bearing, and while having the prop shaft out I decided to change the stuffing box hose. I noticed that the prop shaft was wearing a groove into the yoke were the stuffing box hose connects to. (the shaft to engine alignment was out.) I had to align the engine so the shaft would exit the yoke without touching the sides and then align the shaft to engine. Only then did I notice now how much smoother the drive train feels and sounds. That was my experience, and hopefully you my have more thoughts. Good luck