[C320-list] Engine/Shaft Noise and Vibration

[Irving Grunes]

Has your prop been balanced?
Irv #851

On Sat, Feb 19, 2011 at 9:40 AM, Dennis Harris <dharris02 at suddenlink.net>wrote:

> 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
>