strange engine vibration

[tgalea]

Hi All,

Just recently a new engine vibration developed at a particular rpm ; below that it's ok and all the way up to full throttle is ok.

Any clues ?

Regards

Tyrone Galea


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[PuffTheMagicDragon]

Hi Tyrone,
I believe that all engines have a "critical speed" where vibration is felt in a particular RPM range, e.g 1500-1700. This is normal and it varies from one maker to another. Nonetheless I would still check (1) each mounting - assuming that they are rubber, (2) barnacles on the prop - more on one particular blade, (3) chipped prop.

<hr width=100% size=1>Wally

 

[AlanPound]

We have a 'knocking' type of vibration that seems to be absent below 1500RPM, appears above 1500 without really increasing much with revs. It is absent at, say, 1800RPM as the boat initially accelerates from stationary, but sets in as it reaches cruising speed (ie. unloads the prop/engine a bit). The problem has been getting more noticable with use...

First thought was the spurs (dived and removed them), sterngear, then gearbox, but each of those seem to have been eliminated for one reason or another. Nor does it seem to be the engine (6 cyl 120HP Perkins).

Current theory is the (3-blade) MaxProp, which seems to exhibit some play at the blade tips (MaxProp say there should be approx zero play). Propbably (sic) a wear propblem...

So now the prop will have to come off, and go to a service agent in Barcelona (boat is in Gib).... they say around 2 weeks to either rebuild the prop (if feasible), or get a new one.

8(


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[alan]

With my Autoprop, when it is dirty (i.e. covered with growth), there is a vibration which is noticible at a particular rev range and also the engine will not produce max revs. Similar symptoms exhibit when I have picked up a fishing line/plastic bag !!!!! I would check carefully for a fouled or damaged prop, particularly if you have a "feathering" type.

Good luck.

Alan.
Nettuno, Italy.

<hr width=100% size=1>Alan Cloke

 

[tgalea]

Thanks for your time.

Yep. Spoke to some other guys locally and they seem to either suggest a prop balancing problem or maybe shaft bush worn.

Its unbelievable. Last winter i stripped the whole boat down and re-built everything from scratch, pulled the engine out, etc... apart from the shaft assembly... and guess what.. here we are discussing vibrations, props!

Cheers
Tyrone.


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[tgalea]

Hi Wally,

Hope all is well.

Actually my concern is that this boat never had any vibration in any RPM range in the past. Spoke to some people who suggested the bush in the shaft assembly must be slightly worn.

I must admit it is the only thing i did not renovate or inspect so there must be a good chance it is the problem.

Regards,
Tyrone.


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[longmoor]

You didn't say if the vibration occurs when the drive is engaged or when the engine is run in neutral. If, for example, you get it when in neutral then don't worry about props, shafts and things. If however, it's there when engaged ahead or astern you need to look at the areas mentioned by others.

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