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Old 05-20-2009
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Drivetrain Vibration Troubleshooting

I am trying to help another owner troubleshoot an engine vibration and would like the group's input.

The engine is a Volvo 2003 3 cyl 28hp. Prior to any repair there was peridoc vibration at certain engine speed ranges.

Over the winter, new motor mounts were installed, the cutlass bearing was replaced and the engine aligned once relaunced. An initial seatrial revealed tremendous vibrations above 1500 rpm, way worse than before any repair.

Subsequent diagnosis by the yard suggested the Martec folding prop pins were worn; the prop has been replaced with a Varifold 2 blade geared this week.

There is still vibration in the drivetrain. I have not confirmed, but I assume the vibration only occurs in gear. The alignment has been re-checked again and pronounced satisfactory.

We are trying to understand where to continue the diagnosis process? Where would you look next?

My theory at this point is that the shaft half coupling (not replaced or removed during repairs) could be out of tolerance....even if the engine is aligned...this might cause an issue?

Any thoughts or suggestions would be appreciated. The owner has over $600 in diagnosis charges and we're trying to educate ourselves to minimize the carnage.

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Old 05-20-2009
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Sounds like the driveshaft might not be turning true.
Did you check it when you had it out?
I had the exact same symptoms with my boat last season.
I replaced my martec due to it having excessive play causing a nasty vibration.
I had to have my driveshaft sent out(strightened/balanced),prop strut removed,sealed,torqued down and faired and cutlass bearing replaced due to consequential damage.
Also, did you measure the thickness of the shaft where it sits in the cutlass bearing and the neck of the shaft where it rides in the stuffing box(the shaft can wear down in these areas).I would also check the prop strut for play if the boat has one.
Im not familiar with the engine described but maybe check the manual to see if the driveshaft flange is supposed to be phased(there would be marks on the flanges if so).
c&c 33 tall rig

Last edited by cnc33voodoo; 05-20-2009 at 08:37 AM.
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