Rope around prop shaft - vibration

[mottseng]

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?

[Unity]

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 that’s what happened to us! (yep, both problems on two separate occasions - now have all new mounts and shaft!)

[mottseng]

Originally Posted by Unity

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 that’s 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.

[Suijin]

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

It’s worth checking the shaft for runout to confirm that it’s OK.

[mottseng]

Originally Posted by Suijin

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

It’s worth checking the shaft for runout to confirm that it’s 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

[mottseng]

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.

[Suijin]

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 it’s 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 it’s expensive to fix. It’s why the distance from bow to prop on my boat is 35’ and my bow lines are 30’ lol

[mottseng]

Originally Posted by Suijin

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 it’s 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 it’s expensive to fix. It’s why the distance from bow to prop on my boat is 35’ and my bow lines are 30’ 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.

[S/V Alchemy]

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

[Suijin]

Originally Posted by S/V Alchemy

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. I’ve always been militant about stowing lines properly and that experience put a fine point on it.

[SVTatia]

Originally Posted by Suijin

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. I’ve 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

[Suijin]

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.

[capt jgw]

Originally Posted by mottseng

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.

[Suijin]

Originally Posted by capt jgw

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 it’s REALLY bent. Even runout you can’t see with your eye can be a problem.

Personally I think it’s 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.

[GeneM]

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.