[C320-list] Engine vibration at low RPM. 3GM30

[John Meyers]

Not sure what Brahmany will say, but from this array of discussions I
wondered what my shaft was doing. I had not notice any vibration or had any
adjustments made. So right after I launched  a couple weeks ago this year I
put the boat in the slip and put it in forward at about 1500 rpm. I laid a
wrench (could have use a screw driver too) on top of the spinning shaft and
there was some (minor?) vibration. When I held the wrench on the side of
the shaft there was no vibration compared to the top. So I assume that
there is some misalignment in one direction. Now I know that just after
launch the boat needs to settle in and come back to floating shape. So
hopefully this weekend when I go back out all vibration will be gone.  My
cutlass bearing appears to be still in good shape.

John Meyers
Wind Chime #406
Muskegon, MI



On Fri, May 15, 2015 at 8:36 AM, Marek Fluder <marekf at gmail.com> wrote:

> Brahmany,
> Can you elaborate on the "screwdriver trick"?
> Sounds like some interesting diagnostic.
>
> Marek
> #1028
>
> On Thu, May 14, 2015 at 5:49 PM, Brahmany Kite <brahmanykite at gmail.com>
> wrote:
>
> > The shaft contacting the stern tube is a source of vibration also as
> there
> > is not much clearance. Especially if there is misalihnment. On one boat i
> > found the gland body pushed too far on astern through the hose and
> bumping
> > on the inboard end of the stern tube. Yanmar mounts are soft as you know
> > and small clearances dont help. Under load the engine will twist slightly
> > sometimes allowing shaft contact as well. The old screwdriver end held to
> > the ear trick will help find the source. Just a thought.
> > On 14/05/2015 10:28 PM, "Graeme Clark" <cg at skyflyer.co.uk> wrote:
> >
> > > Jeff
> > >
> > > New mounts fitted as the  rear starboard original had become disbonded
> > due
> > > to a diesel leak onto the rubber. They were 18 years old original to
> the
> > > boat, anyway!  All the correct spec mounts in the correct place!
> > >
> > > BUT -  they aren't Yanmar mounts. They are made by a  company called
> R&D
> > > as a direct replacement and I chose them because the design protects
> > > against a  similar problem as they are shielded and the rubber is
> > inserted
> > > into the mount not bonded to it.
> > >
> > > There is a page on the Yanmar site about using non-Yanmar mounts and it
> > > does mention that “other makes’ can be stiffer which increase vibration
> > at
> > > lower rpms.
> > >
> > > The tacho is spot on, calibration, (I have a laser tachometer that I
> used
> > > on the crank pulley to check). So at 1000 rpm its in fine form, at 800
> > its
> > > shuddering away!
> > >
> > > One thing someone else suggested was to check that with higher
> amplitude
> > > vibrations it is possible that some part of the engine actually touches
> > the
> > > boat; I’ll have to check next time I am on board.
> > >
> > > So really my question is whether it is normal for the engine to shake
> > > around a lot at low revs, rather than for the boat itself to shudder.
> > >
> > > The other thing I am unsure of is how the governor works - mine
> maintains
> > > a set RPM when the boat is out of gear or if the boat is in gear, but
> > when
> > > you transfer from neutral to forward gear, obviously the engine comes
> > under
> > > load, and the rpm drops requiring more throttle.
> > >
> > > I don't know if this is expected behaviour or not?
> > >
> > > Thanks
> > >
> > > Graeme
> > >
> > >
> > >
> > >
> > > On 13 May 2015, at 23:24, Graeme Clark <cg at skyflyer.co.uk> wrote:
> > >
> > > >> Hi,
> > > >
> > > > It depends.  Do you feel boat vibration, or are you just seeing the
> > > engine
> > > > "rocking" in its mounts?  If you feel the boat shuddering, then this
> is
> > > not
> > > > normal, and *usually* indicates that the idle is too low.  If the
> > engine
> > > > "looks" like it's vibrating a lot in its mounts, that's probably ok
> as
> > > long
> > > > as the prop shaft isn't bouncing all over. Some shaft movement is to
> be
> > > > expected at idle however.
> > > >
> > > > The engine is supposed to oscillate (rotate/rock side to side) in
> line
> > > with
> > > > the prop shaft.  (imagine the prop shaft extending through the engine
> > and
> > > > out the front and attached to fixed mounts at both ends. The engine
> > will
> > > > pivot side to side around this shaft, more so when at a low idle, and
> > > when
> > > > revved up, it should appear to be much more stationary).  The Yanmar
> > > rocks
> > > > quite a bit at idle, but gets more smooth as you increase the speed.
> > > >
> > > > So, if the tach reads 1000 and you feel the boat shuddering, the idle
> > is
> > > too
> > > > low and your tachometer is probably reading a little high.  Increase
> > the
> > > > idle by adjusting the governor (on the cable right below the engine
> > > mounted
> > > > fuel filter).  If the tach is correct and it only smooths out (stops
> > > > shuddering) at higher RPM, your engine mounts may be shot.  Another
> > > thing to
> > > > notice is if you rapidly back the throttle down to idle position and
> > the
> > > > engine shudders or nearly stalls then again, your idle setting is too
> > > low.
> > > >
> > > > Cheers!
> > > > -Jeff Hare
> > > > #809
> > >
> > >
> >
>