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Old 08-08-2017, 11:07   #1
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Bent Propeller Shaft

Hi,

Any advice would be greatly appreciated.

We had our propeller shaft bearing (through hull) and shaft support bearing (between motor and propeller shaft bearing) replaced in our 1989 Morgan Classic 41 sailboat. During the sea trail we noticed a big vibration coming from the motor.

The mechanic who did the work told us that the shaft was bent between the shaft support bearing and motor and that it was like that before the repairs. He said that the bearing was probably damaged and allowed for bend in the shaft so that there was no vibration coming from the motor.

Is it possible the bearing was probably damaged and allowed for the bend in the shaft so that there was no vibration coming from the motor?

Is it possible that the shaft wasn't aligned with the motor properly and that is why the shaft is bent now?

Thanks in advance
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Old 08-08-2017, 11:24   #2
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Re: Bent Propeller Shaft

I'm not sure what your set-up is like near the prop but if the shaft is bent, the propeller will wobble and will cause vibration whether or not the strut and cutlass bearing (If you have them) are also bent in the same direction. Your only ray of hope might be that if there is a new cutlass bearing installed, sometimes it takes a few hours for the shaft to wear the rubber and settle in. Before removing the shaft, you will also want to check that it was properly aligned to the gear box. If it were bolted without being aligned within the correct tolerances, it would also cause a vibration. (My experience is based on a couple of similar occurrences. Hopefully you will get a more expert opinion).
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Old 08-08-2017, 11:45   #3
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Re: Bent Propeller Shaft

If it's bent, it will wobble, you can see this wobble and take a pencil and just touch the shaft with it turning and see how much it's bent by holding the pencil against something that keep it from moving.
Now a gross misalignment will look bent, but it may not be, it may just be in a bind and flexing
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Old 08-08-2017, 12:09   #4
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Re: Bent Propeller Shaft

Forgive me !,-- but you didn't actually PAY this person after receiving that explanation.
I have replaced these systems a number of times, and would not think of not checking the alignment of the engine, pilot bearing, and cutless bearing as a matter of course.
Maybe I'm reading something wrong here ??
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Old 08-08-2017, 12:14   #5
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Re: Bent Propeller Shaft

Put a mechanics lien on the guy's house until he fixes what he screwed up.
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Old 08-08-2017, 12:27   #6
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Re: Bent Propeller Shaft

Something doesn't smell right.

The mechanic put a known-bent shaft BACK into service without even notifying you first?

Was there the vibration before? If not, I'd bet the shaft wasn't bent before the last service.

Perhaps the shaft got bent when the boat was hauled out? (In which case, the mech still should have caught it) Maybe it got bent refloating the boat? ( In which case, the mechanic shouldn't have known it was bent)
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Old 08-08-2017, 12:32   #7
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Re: Bent Propeller Shaft

How does one bend a shaft?
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Old 08-08-2017, 13:47   #8
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Re: Bent Propeller Shaft

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Originally Posted by Flyingriki View Post
How does one bend a shaft?
Catching the prop or prop shaft with a lifting sling while hauling out or refloating would prolly accomplish that.

Maybe manhandling the engine while the coupling was attached would do it too.

Cranking down on coupling bolts while the pilot wasn't properly seated?
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Old 08-08-2017, 14:34   #9
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Re: Bent Propeller Shaft

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Is it possible the bearing was probably damaged and allowed for the bend in the shaft so that there was no vibration coming from the motor?
It would have been very noisy and still had a vibration. If it's bent your mechanic is a dumb ass for putting it back together like that. It's not that hard to straighten a shaft. He could have even made more money on the job by aligning it better or taking the shaft to a machine shop. banghead:
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Old 08-08-2017, 14:35   #10
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Re: Bent Propeller Shaft

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Originally Posted by Flyingriki View Post
How does one bend a shaft?
The easiest way to bend a shaft is to drop a dock line in the water where the other end is firmly attached to a cleat then go full throttle. You will bend the shaft, the strut, pop up an engine mount may be. Do not ask how I know.
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Old 08-08-2017, 14:39   #11
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Re: Bent Propeller Shaft

I think you might try to loosen the 4 mounting bolts for the bearing (blue flange)just enough to allow the flange to move slightly. Run the engine in gear at the dock, and see if the vibration goes away. If it does, the bearing was improperly installed. If no vibration, re-tighten the bolts (evenly).
Hope this helps.
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Old 08-08-2017, 15:40   #12
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Re: Bent Propeller Shaft

If the shaft was bent previous, you would have felt it. The bearings don't wear in such a way that they would mask that problem. Him offering that for an explanation means he knows exactly how the shaft did get bent or he shouldn't be doing bearing repairs.

Sounds like you need to get hauled and get the shaft straightened by a machine shop and then aligned by a competent mechanic. I'd also run a dial indicator on the shaft after it was assembled in both the area between the strut and hull and also just after strut if there's room to verify it's running true before getting splashed.
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Old 08-08-2017, 16:07   #13
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Re: Bent Propeller Shaft

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Do not ask how I know.
Really sorry to hear that!
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Old 08-08-2017, 23:25   #14
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Re: Bent Propeller Shaft

Check the alignment before you do any thing else,

1) just undo the bolts holding the shaft coupling to the gear box coupling, slide the shaft a few millimeters (there may be a grub screw holding the shaft on the new plumber block bearing which needs undoing)

2) Then slide it forward again -it should match up with the two adjoining spigots (flanges) with no hesitation (if the alignment is good)

3) if not re alignment will need to be done with a few thousands of an inch by using feeler gauges, and adjusting the engine mounts

Marine Engines : Drive System Alignment by David Pascoe, Marine Surveyor
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