Shaft Vibration?

[gtmoore]

I've had a good look through all the engine vibration posts made here but I would like to run my problem by you all just to see if any of the myriad things it could be is more likely (sorry it's a bit long).

My Moody 29 was re-engined by the previous owner a few years ago with a 3 cylinder Beta 20hp which is super smooth and barely moves on it's mounts at any revs. I bought the boat last year and before I used it in anger my yard advised me to have larger engine bearers fitted (now 3"x2" steel boxes not 2"x2") which I did and obviously had the engine aligned. I also have the Volvo seal that doesn't need a greaser and certainly doesn't drip at all which is great.

My problem is that after about 1700 revs I get a sort of clatter that slowly comes and goes in about 3 second intervals. At higher revs the noise stays the same but the intervals get shorter. It truly sounds as if something is loose in a locker and is rattling but believe me I've emptied them twice but the noise remains. It's more of an annoyance as it's not extremely loud. My boat has a long stern tube built in i.e. no P bracket. It sounds as if the shaft may be knocking in the tube but oddly this doesn't seem to be audiable when tied to the mooring (maybe noise from the pop wash is hiding it).

Two more things, a previous post said to check that the shaft isn't bent by seeing if it moves the seal when in gear. It does move a little at idle but when increased it doesn't. Also, I'm not sure if this is related but after a trip round from Dover to Southampton we needed to remove a mass of fishing net around the prop.

Any ideas if this sounds more like alignment, bent shaft, prop problem etc...? I'm a bit worried this could be wearing something out?

Thanks


<hr width=100% size=1>Gavin

 

[tcm]

Are you sure the prop is clear now? Bits of rope still around the prop would make a noticeable but bearable and variable vibration.

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[colvic]

As TCM says, check that everything is off the prop and the shaft as even a small amount of debris will put it out of balance and cause a knocking sound.

Fishing net weighs quite a lot and if it was round the prop for some length of time it could well have set up a vibration in the shaft that could have loosend the stern tube out fitting, which is what happend to us after a 10 ft. length of 2" rope was round the prop for no more than one minute.

We have just re-engined and had to replace the existing shaft as it wasn't long enough. The whole assembly is now much smoother and quieter and I think our original shaft was everso slightly bent.

Don't mean to sound so gloomy


Phil

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[Anchorite]

I'd check for a prop problem: your 'catch' may have unbalanced it a little,
rather like a carwheel which is okay at low speeds but becomes obnoxious
at higher revs. Needs specialist gear to check out (and the prop off,of course).

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[mtb]

you don't say when it started, if after the net fine then it's prop damage but if it's always been evident then it could be cavitation re the noise of the water imploding on the prop surface or beating against the deadwood area due to an imbalance re the prop.
Shaft alignment !! dought it any more than a few thou will cause problems.
Is you outer cutlass bearing ok .
Were the new mountings of good quality if cheap they might be to soft allowing things to move .
Now get a screw driver and keeping well away from moving parts place the end of the screw driver at various points listening with your ear to the end just like the doc does with a stethoscope !! you can normally hear roughly where the noise is coming from
Cheers
Mick

<hr width=100% size=1>smile you'l be on your boat soon ;-)
<A target="_blank" HREF=http://homepage.ntlworld.com/boats/>http://homepage.ntlworld.com/boats/</A>

 

[VicS]

I was going to sugest the use of a listening stick, but you'll pobably need something longer than a screw driver to listen to the sterngear; the handle off the deck scrubber perhaps. It is a very useful trick which can often pinpoint the source of odd noises

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[freeman]

We had the same problem and were getting through cutlass bearings every year,
with noise at low revs.

We now have, I hope, discovered the problem.
Insufficient water passing through the cutlass bearing and
The shaft was bent. However it had to professionally tested to show that it was bent.

Good luck

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[gtmoore]

Lots of great advice here - thanks to everyone. Firstly, I think I'll check that all the netting is off the prop and shaft as per tcm's post cos thats the easiest to do. We sort of pulled the bundle of net off using a boat hook during a springs low water but without swilling around in the mud it's difficult to see properly on the mooring. I didn't realise the importance of this being completely clear. Then if still no joy I guess you're all telling me that the prop and shaft will need to be checked. What a pain! - I've only just got the boat in the water after an extended period in the yard after purchase and I'm really enjoying it!

The joys of boat ownership eh?

Thanks again.

<hr width=100% size=1>Gavin

 

[vyv_cox]

There are two aspects to shaft alignment. Firstly the shaft has to be central in the tube and then the shaft has to be aligned with the gearbox output shaft. Sometimes the first of these is difficult to achieve, especially because it changes with engine speed as torque reaction compresses the rubber mountings on one side. Dependent upon the seal type fitted you may be able to tell whether the shaft is in the centre of the tube quite easily. If not, then you may have the answer immediately.

Observe what happens to the engine when revs are increased under load. It may be necessary to realign the engine with a slight offset to compensate for the reaction. If you cannot do this yourself, discuss with a competent yard. It should not be necessary to haul the boat out to do it.

One other thing. A condition called "soft foot" is often the cause of myserious vibrations at specific revs. Caused because one mounting is not supporting equally with the others. Try to assess this by slackening off each mounting nut and checking whether it is possible to get a feeler gauge underneath the mounting plate on the engine.

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[longjohnsilver]

Yes just a bit of twine around the prop really will make a difference, put on your wet suit and mask and have a look. Have a good sharp serated knife to hand just in case.

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[mtb]

You should see the length of my screw driver .....

<hr width=100% size=1>smile you'l be on your boat soon ;-)
<A target="_blank" HREF=http://homepage.ntlworld.com/boats/>http://homepage.ntlworld.com/boats/</A>

 

[Sinbad1]

Most likely a combination of 3-4 problems. Its possible that the fishing net bent the shaft which will have damaged the cutless bearing as its no longer aligned.

There is no easy way of correcting this. You have to draw the shaft. Send shaft to local engineers and have it checked for straightness and trued if necessary. While shaft is out check cutless. You will find that it has worn. Replace it. Check also for lubrication. Make sure that you have waterflow through your shaft. This will either be pumped via the engine or scooped up via the bearing housing for the cutless.

Last, get the prop balanced.

You most certainly should not have metallic clanking and using it will invariably cause further damage to other components.

Its not a big deal. Just start at the gearbox after removing prop and start taking things off. You will learn lots!

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[Guest]

Picked up net ....

Boy I thought the boat was coming apart !!!! Cleared as you did with boat hook. After when motoring had a vibration and shudders .... turned out to be a small piece and I mean small still lodged .........

Luckily no damage to shaft or fittings !!



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !