[C320-list] Engine/Shaft Noise and Vibration

[Dennis Harris]

Just curious if other C320 owners experience this.  I get a low level 
noise/vibration at certain speeds, and always have the fear that, despite 
what the mechanic said, the shaft may be striking the hull, but the 
frequency doesn't seem right.  The boat is a 2000 model (#694) with Yanmar 
3GM30F engine and transmission with 2.21 reduction.  The sound is a little 
like a very low level "whump, whump, whump".  The frequency increases with 
engine speed, but at higher speeds, the noise is largely overridden by 
engine noise and vibration does seem to increase significantly, so it may be 
noise more than vibration.  The cutless bearing is snug and there is no 
movement in the shaft.  However, in last year's haul out, on the forward 
edge of shaft strut, a small crack had come in hull fairing material.  The 
three remaining sides around the strut were solid and there was no leaking 
into the boat.  The four strut bolts were tight.  In previous haul outs, 
this area was fine, and the noise/vibration that I have mentioned has always 
been present through out the 10+ years that I've owned the boat.  There have 
been no issues (abnormal, frequent wear) at the packing gland. At ~2200 rpm, 
the intermittent sound occurs roughly 100 times/minute.  With the 2.21 
transmission reduction, this means that the sound occurs at a frequency of 
once per 10 shaft rotations, so in my mind, I have attributed the 
sound/vibration to be a harmonic of the combined system (hull, engine, 
shaft, etc.) and suspect that it's something not to worry about. 
But....what if?

Do others experience this?  Is it a problem that needs to be addressed, and 
if so, how?

Thanks,

Dennis Harris
C320 #694, Washington, NC