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Old 20-08-2018, 06:27   #1
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Rope around prop shaft - vibration

This past weekend I unfortunately got a dock line wrapped around the prop shaft near the prop. I went down and managed to get it cut off. The engine starts and runs well except while in idle. In idle there is a thumping and vibration. Once I apply some power is smooths out. Any thoughts?
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Old 20-08-2018, 08:08   #2
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Re: Rope around prop shaft - vibration

Maybe check your engine mounts - the shock load to the shaft may have caused an already degrading mount to give up the ghost? Often vibration at idle that smooths out at speed indicates engine mount problems, where as vibration getting worse at speed can indicate shaft alignment, shaft bent, cutlass whacked, etc.

Or at least thatís what happened to us! (yep, both problems on two separate occasions - now have all new mounts and shaft!)
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Old 20-08-2018, 09:21   #3
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Re: Rope around prop shaft - vibration

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Originally Posted by Unity View Post
Maybe check your engine mounts - the shock load to the shaft may have caused an already degrading mount to give up the ghost? Often vibration at idle that smooths out at speed indicates engine mount problems, where as vibration getting worse at speed can indicate shaft alignment, shaft bent, cutlass whacked, etc.

Or at least thatís what happened to us! (yep, both problems on two separate occasions - now have all new mounts and shaft!)
Thanks, when I go up this weekend that will be the first thing I check.
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Old 20-08-2018, 09:49   #4
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Rope around prop shaft - vibration

And just because you donít have noticeable vibration at higher revs does not mean you didnít bend the shaft or skeg enough to cause problems down the road, like premature cutless bearing wear etc.

Itís worth checking the shaft for runout to confirm that itís OK.
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Old 20-08-2018, 10:31   #5
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Re: Rope around prop shaft - vibration

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Originally Posted by Suijin View Post
And just because you donít have noticeable vibration at higher revs does not mean you didnít bend the shaft or skeg enough to cause problems down the road, like premature cutless bearing wear etc.

Itís worth checking the shaft for runout to confirm that itís OK.
How do you check the shaft for run out? Can this be done in the water? The area where the rope was around is only about 6" between the prop and the bearing.

Thanks
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Old 20-08-2018, 10:40   #6
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Re: Rope around prop shaft - vibration

Ok just did some googling and this looks like a job when the boat is out of the water. She comes out in a month and that would be a good time to replace cutlass bearings both shaft and rudder, check run out and motor mounts.

Thanks to all.
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Old 21-08-2018, 07:11   #7
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Re: Rope around prop shaft - vibration

You check runout with a micrometer on an arm and a clamp. You check it at various points along the shaft to diagnose if the shaft is bent and where (which really does not matter...if itís bent you need a new one). A mechanic can do it in a matter of minutes.

It does not take much to bend a shaft just enough to cause all sorts of problems. And itís expensive to fix. Itís why the distance from bow to prop on my boat is 35í and my bow lines are 30í lol
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Old 21-08-2018, 07:16   #8
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Re: Rope around prop shaft - vibration

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Originally Posted by Suijin View Post
You check runout with a micrometer on an arm and a clamp. You check it at various points along the shaft to diagnose if the shaft is bent and where (which really does not matter...if itís bent you need a new one). A mechanic can do it in a matter of minutes.

It does not take much to bend a shaft just enough to cause all sorts of problems. And itís expensive to fix. Itís why the distance from bow to prop on my boat is 35í and my bow lines are 30í lol
Great, note to self.....don't use dock lines longer than distance to prop!!!

I am hoping to get a few more weekends in before hauling out for the end of season. During this time I hope I don't do further damage.
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Old 21-08-2018, 09:41   #9
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Re: Rope around prop shaft - vibration

You may wish to install one of these to avoid the problem in the future. https://www.ab-marine.com/shaft-shark/
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Old 21-08-2018, 09:51   #10
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Re: Rope around prop shaft - vibration

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You may wish to install one of these to avoid the problem in the future. https://www.ab-marine.com/shaft-shark/

They are cheap insurance when a new shaft and labor can set you back $5k.

During my last refit I saw about a half dozen boats come into the yard needing new shafts from incidents with their own lines. Iíve always been militant about stowing lines properly and that experience put a fine point on it.
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Old 21-08-2018, 10:12   #11
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Re: Rope around prop shaft - vibration

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Originally Posted by Suijin View Post
They are cheap insurance when a new shaft and labor can set you back $5k.

During my last refit I saw about a half dozen boats come into the yard needing new shafts from incidents with their own lines. Iíve always been militant about stowing lines properly and that experience put a fine point on it.
Yes, and this is test of some of them, I am still selecting the most efficient...

https://www.youtube.com/watch?time_c...&v=gU9bwlSTg-s
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Old 21-08-2018, 10:25   #12
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Re: Rope around prop shaft - vibration

I had a Spurs on my shaft but it broke. Decided something without moving parts made more sense. After watching that video, and some chin scratching I bought a Prop Protector. Very well made and stupid, stupid sharp.
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Old 21-08-2018, 12:31   #13
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Re: Rope around prop shaft - vibration

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Originally Posted by mottseng View Post
How do you check the shaft for run out? Can this be done in the water? The area where the rope was around is only about 6" between the prop and the bearing.

Thanks
The one and only time I wrapped a line it did bend the shaft. No good way to check it afloat, it would have to be really bent to see it in the water. If you pull the shaft and roll it across a smooth table top, the slightest bend will be obvious.
They really can't be straightened, so no real need to get technical. If you see it wobble on the tabletop it's toast. When mine bent I got a new one and kept the old one as backup in case I bent the new one even worse. It was just a tiny bit bent but that was enough to make some weird noises.
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Old 21-08-2018, 13:08   #14
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Rope around prop shaft - vibration

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Originally Posted by capt jgw View Post
The one and only time I wrapped a line it did bend the shaft. No good way to check it afloat, it would have to be really bent to see it in the water. If you pull the shaft and roll it across a smooth table top, the slightest bend will be obvious.
They really can't be straightened, so no real need to get technical. If you see it wobble on the tabletop it's toast. When mine bent I got a new one and kept the old one as backup in case I bent the new one even worse. It was just a tiny bit bent but that was enough to make some weird noises.

If you see it wobble on a tabletop then itís REALLY bent. Even runout you canít see with your eye can be a problem.

Personally I think itís easier to have a mechanic take 15 minutes to check it with a micrometer than to go through the trouble of pulling the whole shaft just to roll it on a table lol.
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Old 21-08-2018, 18:45   #15
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Re: Rope around prop shaft - vibration

It's good to be cautious, but also think about not being too concerned. I've had incidents that caused shaft vibration, but I believe my strut bearings just got a little sideways and worked out on their own. I have a fast cruiser with big shafts and struts.
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