Vibrating shaft at slightly high revs.

[ramblinrod]

Originally Posted by Cheechako

I have a feeling 24" unsupported prop shaft would end up being a right angle taking the hull bottom with it! :>)

See post above about exaggeration, but lets not throw the baby out with the bath water.

The important point is that an inch or even two, (as long as minimum clearances are maintained) won't make a significant (vibration at some freq vs none at any) difference if the prop shaft and alignment is true and the bearings are good.

[MYTraveler]

My experience probably won't explain your problem, but for what it is worth, immediately after my most recent haul our (for bottom paint), I had a noticeable vibration about 1100 rpm that got progressively worse at higher rpms (so not a resonant frequency thing). The vibration was slight but noticeable, and since I have owned the boat since new I know that something had changed.

I first suspected that I had wrapped up some sea weed, so I put the boat hard in in reverse to clear it. None came up and the vibration didn't go away, so I was concerned that my shafts had been bent by improper sling placement. I had my diver look for any other cause. Other than a little sea weed, he could see nothing that could create a problem. But, low and behold, that little bit of sea weed was all it took to create that vibration.

[ramblinrod]

Originally Posted by Cheechako

I see nothing unusual in your video.

Way too much shaft vibration at the stuffing box.

https://youtu.be/fUkl3v5gD4k

Looks like at least +/- 0.020".

That is not normal and should be fixed to prevent further damage.

[a64pilot]

Originally Posted by ramblinrod

See post above about exaggeration, but lets not throw the baby out with the bath water.



The important point is that an inch or even two, (as long as minimum clearances are maintained) won't make a significant (vibration at some freq vs none at any) difference if the prop shaft and alignment is true and the bearings are good.

I'll find out later for sure, but I have 2.5" of exposed shaft with a shaft diameter of 1.25", and am awfully sure that is the source of my vibration.

[ramblinrod]

Originally Posted by a64pilot

I'll find out later for sure, but I have 2.5" of exposed shaft with a shaft diameter of 1.25", and am awfully sure that is the source of my vibration.

Question: If everything is jake (alignment, play, balance), how can 1" of additional shaft extension "cause" a non-vibrating shaft to suddenly vibrate?

Answer: It can't, the shaft will still rotate happily about its centre.

This is why I can confidently state that if you adjusted the shaft extension and the vibration went away, it was not the change in shaft extension that fixed it. (Most likely the inadvertent change in alignment, since there was not likely anything done to affect balance or play in the assembly.)

Edit: If the assembly has a very high Q on its resonant frequency, theoretically a minor change in length could hit that specific frequency or harmonic which suddenly causes a significant change in vibration. Highly unlikely, and certainly not the first thing to consider. In most cases if one extends the END of the shaft from the cutlass bearing by 10%, the increase in vibration will be about 10% more, at a slightly different frequency (RPM).

[Hoohaa]

It shouldn't be the cutlass bearing as it was replaced along with the shaft and coupling.
I don't think its a harmonic resonance or cavitation off the hull as it occurs even at very low rpm. I think its always there.
I am also now thinking that the prop is not the cause as the wobble occurs at very low revs.
So that leaves the alignment at the coupling.
I will be back on my mooring in a few hours and i will unbolt it and take a look.


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[a64pilot]

A shaft will flex. Look up harmonics, you hit a harmonic resonance freq and that will generate a lot more force than you would think.
I have I think about seven feet of shaft completely unsupported, except for the ends. Now if I had a pillow block bearing half way, then I feel sure that would solve my problem, but before I go to such extreme lengths, I think it prudent to shorten the shaft to the recommended length.
Has to be a reason for the recommendation don't you think?

[GILow]

I suspect in the case of our boat we have a slight imbalance in the prop. Highly likely given the prop is 30 years old. Moving the shaft aft probably allowed this imbalance to have more leverage on the shaft as a whole (albeit tiny leverage) and introduce the vibration. Moving the shaft forwards eliminated the leverage and allowed the cutlass to better control the imbalance. The prop is to be replaced so the cost of balancing is money poorly spent, provided I don't chew out the cutless, which has not happened so far.

But for the op it really feels like the flange connection at the gearbox end to me from that video. There appears to be a slight alignment issue in the video at the gearbox end amplifying to a full on skipping rope whipping movement by the shaft seal end.

Finally, I take the point about the types of marine professionals needing to be appropriate to the job, but when they have gone to the trouble of putting their skill set on their business card and they still get it wrong I reckon I have some cause to be discontent. I know there are good ones out there, keep your ear to the ground and you learn who to trust, but some of them, frankly, have a nerve offer the services they offer. I am sure Ramblinrod does not fall into this latter category. and if exaggeration were a crime I'd be serving a life term by now.

Matt

[ramblinrod]

Originally Posted by a64pilot

A shaft will flex. Look up harmonics, you hit a harmonic resonance freq and that will generate a lot more force than you would think.
I have I think about seven feet of shaft completely unsupported, except for the ends. Now if I had a pillow block bearing half way, then I feel sure that would solve my problem, but before I go to such extreme lengths, I think it prudent to shorten the shaft to the recommended length.
Has to be a reason for the recommendation don't you think?

See post #35. That one would hit a high Q resonant freq, by adjusting a shaft in or out 1" is theoretically possible but highly improbable.

The reason for the minimum clearance dimension is to provide sufficient room for water to get through the bearing unimpeded (note that most deadwood installed cutlass bearings get no water flow), so it isn't that a big of a deal.

Some gap is important to allow for take-up in the assembly to avoid contact between stationary and rotating parts, especially if one has a resilient coupler that is compressible under full load.

There is really no maximum extension specification other than what is practical 1.25 or 2.5 x diameter should make no difference under all but extremely rare circumstances.

Again, the most likely "cause" of the change was the change in alignment, not the (very minor) change in extension length.

PS, Seven feet is quite a span for a small diameter shaft, and it is possible that it is sagging enough in the unsupported section to cause an issue.

[Hoohaa]

Ok. Take a close look at this and you can see that there is some misalignment horizontally.
I will buy som feeler gauges tomorrow and get some accurate measurements before I make any adjustments.
Its very annoying having paid someone to align the motor and shaft.
I even suspect that the wobble in the shaft had led them to believe that the shaft was bent.
I am starting to get a bit angry when i think about what all this cost me...
Oh well. I estimate the misalignment is about 3mm. Do you think thats enough to cause a wobble?


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[Hoohaa]

Pics

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[Three Sisters]

Earlier mention was made on the importance of the motor mounts.

Hopefully you now have a spare shaft after the previous repairs.

[Hoohaa]

I pretty shore the mounts are good. The engine has never been properly aligned on the mounts.
And no i never did get the old shaft back.
They will be getting a call from me on Monday. Once I tally up what was spent on the repairs. I don't really think i will get anything back but it's going to be interesting to know what evidence they based the bent shaft on.
But i won't dwell on it. I still need to fix the problem.

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[Three Sisters]

Cannot tell from the photographs but the flange bolts need installed.

Just slightly loosened, otherwise, if removed, the weight of the shaft and coupler prevent any semblence of a successful alignment.

[Hoohaa]

Sounds like a catch 22. If i install the bolts then it will pull it into alignment. I can definitely make a horizontal adjustment but vertically will be out. I'll be getting some measuring instruments on Sunday. Then i should be able to get some accurate measurements between the flange faces. Obviously I haven't done this before so any advice is appreciated.
Maybe a YouTube video on the subject?



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