New vibration in drive shaft

annageek

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We were heading into Newton Creek a couple of weekends ago. With full revs near the entrance (it was a spring tide and we were heading in on the ebb) I noticed the engine tone drop barely perceptably and a short while after a horrible noise. It was a sort of periodic squealing / screaching noise that corresponded with the engine revs. I have no idea what it was but you could feel it through your feet. However, I dropped the revs to the minimum to just make progress against the flow and managed to get moored. My partner dove on the prop and she found a ton of weed mangled around it - the sort of light green, wet-cotton-wool type stuff.

Anyway, since then, there's been a slight, but very noticable 'wobble' in the prop shaft where it enters through the stern gland. Video with slo-mo here:
. The gland is a touch more drippy than it was before, thought no deluges yet. Before all of this, I'm pretty sure it was running almost perfectly true with no discenrnable wobble. Weirdly, at full revs, the wobble all but disappears, but it's definitely there at everything from tickover to 95%.

Hand rotating the shaft shows the run out is between about 1mm and 2mm (measured by looking at the shadow of a torch on a ruler - not a DTI!). Not a huge amount, but despite the flexible shaft coupler, this vibration looks to be coupled into the engine and the whole thing's moving a bit more on the engine mounts.

Questions are:

1. Any idea what could have happened?

2. Is 1-2mm runout too much / potentially damaging for the gearbox / stern gland etc.

The boat seems to run OK. I can't repeat the problem with the screaching noise, even after running at full revs for several minutes to test. My preference is to see out the rest of the season and take a closer look when she's out of the water over winter.

Just for context, it's a 26' Westerly Griffon with a Volvo Penta MD7A 13hp engine.
 
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Graham376

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The squealing could be the shaft not centralised in the stern tube or, if you have a Stripper or Spurs rope cutter, it may be too far into the V block and binding as the shaft moves forward under load. Checking engine alignment with a feeler gauge in the R & D coupling is easy enough but you first need to make sure the shaft is centred in the tube.
 

annageek

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Funnily enough, we do have a spurs rope cutter, but it's not currently fitted, so can't be that.

I'm guessing the only wa tto check the shaft is centred in the tube is to remove the stern gland = boat out the water.
 

laika

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I'm guessing the only wa tto check the shaft is centred in the tube is to remove the stern gland = boat out the water.

Having recently been chatting to an engineer about shaft alignment...quite the opposite (if I understand correctly, which I may not so people who know rather more than me, please correct me...). According to my local marine engineers shaft alignment should be checked with the boat in the water because out of it the cradle will flex the boat. I believe the process involves undoing the coupling (with the stern gland still doing its job of keeping the water out) and then taking a micrometer to it
 
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Moodysailor

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If something has changed, it's a case of working through to find out what it could be.

If you think you have shaft run out, then the shaft should really be removed and checked for bow in some v blocks.

A few options (some already mentioned)
  • Engine/shaft alignment out
  • Cutlass bearing worn/damaged
  • Prop damaged/bent
  • Shaft bowed
  • Shaft bent after P bracket
  • Engine mount collapsed

Final note - be careful of confirmation bias - i.e you think some damage has happened, so you are looking for it. You mention that you can't be sure this wasn't there beforehand. If the boat makes the same speed at the same revs as before, if there is no additional vibration and no new/different noises then it may just be your mind looking for something. In that case, I would suggest you are OK to keep an eye on it and have the shaft pulled during your next scheduled ashore time.
 

parbuckle

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Before attempting any in depth stripping down you can try listening via a stethoscope or metal rod to identify if any strange noise is coming from the gearbox or stuffing gland area , run the engiine at various revs also you could unbolt the coupling and listen to gearbox at various revs all you can do is look for clues such as noise or vibration sometimes it is easier to diagnose with moving parts .
 

Tranona

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Having recently been chatting to an engineer about shaft alignment...quite the opposite (if I understand correctly, which I may not so people who know rather more than me, please correct me...). According to my local marine engineers shaft alignment should be checked with the boat in the water because out of it the cradle will flex the boat. I believe the process involves undoing the coupling (with the stern gland still doing its job of keeping the water out) and then taking a micrometer to it
This is a hangover from old wooden boat days and is less likely with GRP boats, although not impossible, just highly unlikely that a well built GRP hull would be that flexible! Relatively easy to check as you say by undoing the coupling bolts and sliding the shaft back so that you can measure the gap around the circumference, but a PITA if you have a rope cutter as it can slip out of the block.
 

ianat182

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Watching that video I did notice that the gland housing itself was moving,.
As you mentioned the cutless bearing has been renewed. I wonder if the stern bearing itself has been fully bolted in to the gland housing when it was replaced. I also looked for the stern gland greaser/pump and connection to the gland ,what looks like a plug of some kind seems to be where the greaser would have its fitting. The bearing housing on my old Tiger had two bolt s bove and below the bearing housing and were tightened by two nuts inboard above and below the shaft; this would be an easy fix to tighten them using a tubular type plug spanner or a ring spanner if there is room; something like a shaped"C" ring spanner would do it. I don't think the Griffon has a "P" bracket has it?(edit) I've just looked at the drawings
and it looks like it may have a P bracket afte all although none of the photos actually show one(or the drawings but as it is an Ed Dubois design it probably does. My thoughts as above would not apply I guess.

ianat182
 
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Bran

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It may be that the shaft is no longer central to the gearbox output flange. If access is ok I would disconnect the coupling bolts, then rotate the shaft by hand, try not to move it side to side but just rotate it and see if you get the same amount of runout that you saw previously. If there is still a lot of runout then it may be a bent shaft, however I think maybe it was just jarred slightly so that it is no longer central to the gearbox flange. Maybe just slackening and retightening the coupling bolts will improve it. To check roughly how central the alignment is to the p bracket, with the coupling disconnected apply an equal force up, down, left and right the the end of the shaft, the approximate centre of all the movements should be close to the centre of the output flange. This is very approximate but may show if an engine mount has been damaged or something else has shifted. Hopefully this would not take too long, the movement you show in the video is a bit on the excessive side and may cause more wear on the stuffingbox and cutlass bearing.
 

V1701

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It may be that the shaft is no longer central to the gearbox output flange. If access is ok I would disconnect the coupling bolts, then rotate the shaft by hand, try not to move it side to side but just rotate it and see if you get the same amount of runout that you saw previously. If there is still a lot of runout then it may be a bent shaft, however I think maybe it was just jarred slightly so that it is no longer central to the gearbox flange. Maybe just slackening and retightening the coupling bolts will improve it. To check roughly how central the alignment is to the p bracket, with the coupling disconnected apply an equal force up, down, left and right the the end of the shaft, the approximate centre of all the movements should be close to the centre of the output flange. This is very approximate but may show if an engine mount has been damaged or something else has shifted. Hopefully this would not take too long, the movement you show in the video is a bit on the excessive side and may cause more wear on the stuffingbox and cutlass bearing.

Yep that was my train of thought as well after checking the obvious...
 

annageek

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Thanks all for the input so far. It's all really helpful

It may be that the shaft is no longer central to the gearbox output flange. If access is ok I would disconnect the coupling bolts, then rotate the shaft by hand, try not to move it side to side but just rotate it and see if you get the same amount of runout that you saw previously. If there is still a lot of runout then it may be a bent shaft, however I think maybe it was just jarred slightly so that it is no longer central to the gearbox flange. Maybe just slackening and retightening the coupling bolts will improve it. To check roughly how central the alignment is to the p bracket, with the coupling disconnected apply an equal force up, down, left and right the the end of the shaft, the approximate centre of all the movements should be close to the centre of the output flange. This is very approximate but may show if an engine mount has been damaged or something else has shifted. Hopefully this would not take too long, the movement you show in the video is a bit on the excessive side and may cause more wear on the stuffingbox and cutlass bearing.
I had thought about the fact that the centre of the output flange may no longer be central to the shadt coupler, but was operating on a 'no problem is so bad you can't make it worse' principle, so didn't try this. It seems like a fairly easy / quick / low risk thing to try though.

I also considered the bent shaft idea. Whilst this would explain it, I can't believe that a bit of weed fouling would cause a 25mm stainless shaft to bend! But you never know!

Watching that video I did notice that the gland housing itself was moving,.
As you mentioned the cutless bearing has been renewed. I wonder if the stern bearing itself has been fully bolted in to the gland housing when it was replaced. I also looked for the stern gland greaser/pump and connection to the gland ,what looks like a plug of some kind seems to be where the greaser would have its fitting. The bearing housing on my old Tiger had two bolt s bove and below the bearing housing and were tightened by two nuts inboard above and below the shaft; this would be an easy fix to tighten them using a tubular type plug spanner or a ring spanner if there is room; something like a shaped"C" ring spanner would do it. I don't think the Griffon has a "P" bracket has it?(edit) I've just looked at the drawings
and it looks like it may have a P bracket afte all although none of the photos actually show one(or the drawings but as it is an Ed Dubois design it probably does. My thoughts as above would not apply I guess.

ianat182
Yep, the griffon does have a P bracket... well at least it did the last time I saw it!!! Diving on it showed it was still there though and firmly attached.
 
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