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PeterCr

Diagnosing a bent propeller shaft

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2 or 3 weeks ago I started hearing a reverberating noise when motoring. It seemed to me to be about the same frequency as I would expect the prop to be turning, and increased with revs. Nothing in neutral. Not loud but definitely there.

 

I'm wondering if I have a bent propeller shaft.  About 3 or 4 weeks before that I hit a nasty piece of wood, though the noise seems to be more recent.

 

I don't feel anything through my feet and not really through the tiller, though on rare occasions I do think there's something through the tiller, but may be imagining it.

 

I have also had another strange noise, though rarely. Every now and then I get a clattering noise in reverse, nothing in forward. Most of the time it's perfectly normal in reverse.

 

I have a vetus water lubricated stern gland/propeller shaft. If it's bent then it's certainly only minor.

 

Any ideas how I could determine if it's the propeller shaft? Or what else it could be? Cheers

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4 minutes ago, PeterCr said:

2 or 3 weeks ago I started hearing a reverberating noise when motoring. It seemed to me to be about the same frequency as I would expect the prop to be turning, and increased with revs. Nothing in neutral. Not loud but definitely there.

 

I'm wondering if I have a bent propeller shaft.  About 3 or 4 weeks before that I hit a nasty piece of wood, though the noise seems to be more recent.

 

I don't feel anything through my feet and not really through the tiller, though on rare occasions I do think there's something through the tiller, but may be imagining it.

 

I have also had another strange noise, though rarely. Every now and then I get a clattering noise in reverse, nothing in forward. Most of the time it's perfectly normal in reverse.

 

I have a vetus water lubricated stern gland/propeller shaft. If it's bent then it's certainly only minor.

 

Any ideas how I could determine if it's the propeller shaft? Or what else it could be? Cheers

 

Fix a block of wood or something to the hull in some way so one end is very close to either the shaft where it enters the gland or as its a flexible gland the gland itself. Then rotate the shaft by hand using the coupling. If it is bent you will see the gap between shaft and wood alter as you revolve it.

 

Have you checked all the engine mounts are tight, both the the bed and to the engine foot? If any of the later are loose tighten the bottom nut up against the top one. never turn the top nut.

 

 

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9 minutes ago, PeterCr said:

Or what else it could be? Cheers

I guess you have checked the obvious. Is the prop clear from debris. Feel the prop blades, are they in good condition without any bends/nicks.

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14 minutes ago, Tony Brooks said:

 

Fix a block of wood or something to the hull in some way so one end is very close to either the shaft where it enters the gland or as its a flexible gland the gland itself. Then rotate the shaft by hand using the coupling. If it is bent you will see the gap between shaft and wood alter as you revolve it.

 

Have you checked all the engine mounts are tight, both the the bed and to the engine foot? If any of the later are loose tighten the bottom nut up against the top one. never turn the top nut.

 

 

Good idea Tony, thanks.  Yes the engine mounts are ok. Also the prop is fine thanks Rusty, I've looked at that.  I did expect a bend in the prop when I hit the wood but it's fine.

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We used to get an occasional clattering blouse in reverse. Turned out to be the bottom part of weed hatch plate knocking against the side wall of the hatch. As long as I ensure it is pushed as far back as possible then no more noise.

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6 minutes ago, pearley said:

......… clattering blouse in reverse.

At school , the 8 year olds were set some homework "I want you to give me a sentence with the word 'fascinate' in it"

 

Next day little Johnny gives his answer - "my sisters' blouse has 12 buttons on it but she can only fasten eight"

 

I wonder if she has the same problem if she puts in on in reverse ?

Edited by Alan de Enfield
  • Haha 1

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I put the lump hammer on the bottom with the handle to within a mm or so of the shaft and did a full rotation. I don't see any movement. I tried it in a couple of different places. So that's though I'm still mystified about the reverberating noise.

13 minutes ago, pearley said:

We used to get an occasional clattering blouse in reverse. Turned out to be the bottom part of weed hatch plate knocking against the side wall of the hatch. As long as I ensure it is pushed as far back as possible then no more noise.

I had wondered about that exact same thing and moved the plate around a bit for that reason. Glad you found that as I suspect now that's what it is.

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1 hour ago, PeterCr said:

2 or 3 weeks ago I started hearing a reverberating noise when motoring. It seemed to me to be about the same frequency as I would expect the prop to be turning, and increased with revs. Nothing in neutral. Not loud but definitely there.

 

I'm wondering if I have a bent propeller shaft.  About 3 or 4 weeks before that I hit a nasty piece of wood, though the noise seems to be more recent.

 

I don't feel anything through my feet and not really through the tiller, though on rare occasions I do think there's something through the tiller, but may be imagining it.

 

I have also had another strange noise, though rarely. Every now and then I get a clattering noise in reverse, nothing in forward. Most of the time it's perfectly normal in reverse.

 

I have a vetus water lubricated stern gland/propeller shaft. If it's bent then it's certainly only minor.

 

Any ideas how I could determine if it's the propeller shaft? Or what else it could be? Cheers

More than likely it will be a bent propeller.  Prop shafts are much tougher than propellers.

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Just now, Flyboy said:

More than likely it will be a bent propeller.  Prop shafts are much tougher than propellers.

A bent propeller shouldn't be as much of an issue as a bent prop shaft I would think. We've been motoring along as normal so there's no problem I can see other than the noise, which is only annoying in so far as I don't know if it's serious or not. So if there's no obvious problem then a small bend in the propeller, which I haven't noticed when feeling around, should be ok hopefully.

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1 hour ago, PeterCr said:

A bent propeller shouldn't be as much of an issue as a bent prop shaft I would think. We've been motoring along as normal so there's no problem I can see other than the noise, which is only annoying in so far as I don't know if it's serious or not. So if there's no obvious problem then a small bend in the propeller, which I haven't noticed when feeling around, should be ok hopefully.

I recently  hit an underwater object so hard that it bent one of my propellor blades. This caused a lot of vibration in the tiller at anything above medium engine speeds. I had to have the boat dry docked and removed the prop to have it straightened. While it was off we tested the prop shaft with a dial test indicator, but it was OK. This would be what you would need to do if you thinks your shaft is bent. You wouldn't be able to tell if its a few thou out by any other way.

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Thinking about a bent propeller. I had thought that if I had a very slightly bent prop shaft it might take some weeks for it to wear out something in the stern gland, and therefore a while before any noise began. But if the prop was bent then I'd assume the noise would have started straight away, not some weeks later.

5 hours ago, monkeyhanger said:

I recently  hit an underwater object so hard that it bent one of my propellor blades. This caused a lot of vibration in the tiller at anything above medium engine speeds. I had to have the boat dry docked and removed the prop to have it straightened. While it was off we tested the prop shaft with a dial test indicator, but it was OK. This would be what you would need to do if you thinks your shaft is bent. You wouldn't be able to tell if its a few thou out by any other way.

 I might see about that when I'm back to my marina. I'd still like to find out what this reverberating noise is. 

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It is possibly propeller 'singing', which is caused by deformation of the propeller, but normally, this tends to occur at a certain speed and disappear below and above that speed.

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