Oscillating Engine Vibration

[fbien]

I have a strange engine vibration... 1986 c34 with original Universal diesel (400 hours, just tuned-up).  When underway, say 2400rpm, i get an oscillating vibration.   2-3 second cycle, the engine causes a vibration that starts and stops.   Rrrrrmmmmm... Rrrrrmmmmm... Rrrrrmmmmm... Rrrrrmmmmm...  Enough to rattle that cockpit locker a little bit, etc.  It is very steady 2-3 seconds on, 2-3 seconds off.  At lower RPM it goes away.  

I would think a shaft alignment problem would be a constant vibration.  Maybe a bearing somewhere?  Anybody had a similar problem?

-fb

[malgase]

Frank,

I have a similar vibration as well.  Between that and a stuffing box that didn't want to fully seal, I figured it was time to pull the shaft and inspect the fittings.

I found some normal wear at the cutless bearing, some noticeable wear at the stuffing box, and - heres the kicker - some wear in the region of the shaft log / exit tube.  I think (and I could easily be wrong) that there is a vibration that sets up a standing wave between the transmission and the cutless bearing.  That wave takes a couple of seconds to build in amplitude, and then contact with the interior surface of the shaft log knocks it back down again.  The bronze shaft is soft enough to actually bend under its own weight, so the force imparted by an eccentric / off balance prop should easily cause some deformation when larger loads are applied to the blades (elevated RPM's).

I am ordering a new shaft shortly, already have a new cutless bearing and new PSS seal, and then realized I may not have found the source of the standing wave.  Some other thoughts:

About 10 years ago I wrapped a line around the prop.  I have to wonder if the prop is just bent enough that it is a hair out of balance, and it takes a short while for the oscillation to translate into a standing wave in the shaft, then the phenomenon I describe above occurs.  So, I plan on getting the prop off the boat soon, and taking it in to check balance and pitch before I re-assemble the whole deal.  I'd hate to launch the boat only to have the vibration return, and be frustrated by it all season.

I won't be back in the water for about 7 weeks, so I won't be able to report back for a while on my success or failure.  Also need to realign the shaft as the engine being just a shade out of alignment with the shaft could have similar symptoms.

One other thought is to verify all motor mounts are tight, but I think that would be continuous vibration and not the cyclical stuff we are experiencing.

Any other comments on what this might be would be of interest to me as well as FrankB I am sure.

Frank B, feel free to contact me off-list to further discuss my long-winded theory. :confused:

Michael

[Roc]

I would suggest a stainless steel shaft.  Bronze does tend to 'whip', because it is softer.  While you're at it, look into a flexible coupling.  I have both the ss shaft and flex coupling and highly recommend them.

Roc-

[Gary Wilson]

Frank -

I have been through the same problem with our 1986 model.  When we first bought it, it had that vibration at all engine speeds over 1700 rpm.  It was especially irritating because there was a loud slapping sound as well.  It had a two blade prop and the previous owner had recently installed a stainless shaft.

I added a flexible shaft coupling, hoping it would solve the problem. I think it helped, but the noise was still there.  In reading through past issues of Mainsheet, I found that the dealers did not always align the shaft to the stern tube before aligning the engine to the shaft.  I performed this operation and had to add 1/2 oak blocks under the engine mounts to get the engine high enough to align properly.  This helped very much with the slapping sound, but vibration was still present.

I also found in the old Mainsheets that many people solved vibration problems by installing a 3 blade prop.  We did that, and now our vibrations are fairly minor.  Of course we sail a lot slower with a 3-blade prop, but we find in the Northwest, we are motoring quite a bit anyway.

We have developed a new sound now that I would like to eliminate.  It is a high pitched whine at rpm's between about 2200 and 2600.  Anybody know what this could be?

Gary Wilson

[jentine]

Just a few questions since with a meager 400 hours, most of the problems listed should not exist.  What have you done prior to the start of the vibration?  What new equipment have you added to the boat (any where on board)?  Have you changed the prop or added anything to the drive train?  What kind of "tune-up" was done to the engine?  
Generally any vibration from rotating parts is constant.  It may rise and fall in pitch, but it doesn't stop and restart.
Jim Kane :rolleyes:  :confused:

[jentine]

Just a few questions since with a meager 400 hours, most of the problems listed should not exist.  What have you done prior to the start of the vibration?  What new equipment have you added to the boat (any where on board)?  Have you changed the prop or added anything to the drive train?  What kind of "tune-up" was done to the engine?  
Generally any vibration from rotating parts is constant.  It may rise and fall in pitch, but it doesn't stop and restart.
Jim Kane :confused:

[Roc]

That noise you are hearing is because the blades are 'too sharp'.  Have a prop shop dull the ends of the blades.

Roc-

[Dionysus]

This "Harmonic Vibration" is normal!  I also have an 86 (#218) and have the same thing occurring at around 2000 Rpm (can't say for sure that my tach is calibrated perfectly).  I had a new shaft made and my 2 blade prop balanced, added a shaft seal, and replaced all the engine mounts (thanks Ron Hill).  This did reduce vibration, but not the RYTHMIC rrrm, rrrm that I think you're talking about.  I have spoken to other C34 80's model owners and they get the same thing.  I spend time on my freinds 46 Post Sport Fish with twin Detroit diesels...his boat does it at certain RPM's.  My father designed Submarines for 33 years...he says it's normal (subs have special homoginizing engineering to get rid of it).  So balance the prop, align the shaft, rplace the engine mounts, and do yourself a favor and install a shaft seal.  But don't sweat it as long as it goes away at most other RPM's.

chp

[andrewrbell]

I also have a vibration at around 1700rpm. it can be heard at the dock in forard but not reverse. Presently I have the prop shaft removed to replace the cutless bearing. I also found that the shaft log I.D. is little bigger than the 1" shaft. The shaft has been rubbing against the shaft log but not the tube. I am hoping that if I do a good alignment and ensure there is no contact between the shaft and the shaft log. I do however have another problem, the hose is 1-5/8" to fit the tube and shaft log which are also 1-5/8" does anyone know where I can get a repalcement hose since it appears it has been in plave for 16 years.

Thanks  Andre

[malgase]

Andre,

Although it is a mite more expensive than a piece of hose, I am replacing my conventional stuffing box with a PSS shaft seal.  I got mine from Boat/US for $149.99 less a 10% coupon discount.

From what I have read and heard, the 1.5 inch ID PSS seal can be stretched over the 1.63 OD shaft log.  I'm doing mine hopefully this weekend, so I will let you know how the installation goes.  I am hoping a little hot water will go a long way . . .

Regards,

Michael