Prop shaft rattle/knocking sound

[Carduelis]

I have an in-gear knock from the prop shaft of my Bene 36. This occurs only at around 1400-1700 rpm and then goes away at faster engine speeds. It also diminishes after the engine has been running for a while.

The boat has just been taken out of the water and I'd expected to find a worn cutlass bearing, but there is no discernible play. The boat has a Stripper rope cutter but this seems secure.

Question: could a large build up of fouling, either on the prop itself, or around the shaft water intake have caused this effect?

The cutlass bearing is the all rubber type in a keel log, with no P bracket.

 

[colind3782]

I had something similar and it turned out to be play in the coupling where someone had used the wrong fixings, machine screws instead of bolts.

 

[Tranona]

First thing is to check the engine mounts for both security and wear. Then the coupling for alignment. For the shaft to touch the side of the tube there must be significant movement or misalignment to deviate that much from centre.

 

[MoodySabre]

Ony boat it was the bearings in the Stripper.

 

[Spyro]

Sounds like the shaft hitting the stern tube Try listening to or feeling the inboard end of the tube. Try motor sailing see if it's worse on one tack or the other. Do you ever get one loud knock when the engine starts up. All Could be caused by the shaft not being central in the tube. It's difficult to line up without being able to see the shaft in the tube If you are sure it is that then you really need to slide the shaft seal forward to get the shaft centralised by moving moving the engine on it's mounts.

 

[Carduelis]

Thanks for that. Real food for thought. If it is a shaft alignment problem, why would it disappear at higher rpm? And why would it reduce after running for a while?

I had wondered if it was a lack of water flow due to the fouled up intake, but I guess that if this was the case, the cutlass would have worn...?

 

[Gymnopedie]

Worth checking play in the (wheel) steering linkage which can cause similar symptoms

 

[Spyro]

Thanks for that. Real food for thought. If it is a shaft alignment problem, why would it disappear at higher rpm? And why would it reduce after running for a while?

I had wondered if it was a lack of water flow due to the fouled up intake, but I guess that if this was the case, the cutlass would have worn...?

Possibly at higher rev the engine runs smoother and doesn't move as much. I had the problem of a knocking shaft when I first got my boat. It was due to an off centre shaft. It would only do it on tick over or just above and when motor sailing on one tack.
It's just one possible option to consider

 

[Robin]

Ony boat it was the bearings in the Stripper.

We had that on our Jeanneau Sun Legende that we had before we left for the 3rd world too, cured with new nylon bearings in the Stripper, the originals were to be fair many years old. Another possibility is that the Standstill part has come out of the 'V' block that holds it from rotating, had that once on earlier W33, cured by a swim and a little underwater brute force with the coupling inside separated, pull shaft back relocate to get the standstill bit into the 'V' part and reassemble coupling , don't test until swimmer out of the water!

 

[Carduelis]

Possibly at higher rev the engine runs smoother and doesn't move as much. I had the problem of a knocking shaft when I first got my boat. It was due to an off centre shaft. It would only do it on tick over or just above and when motor sailing on one tack.
It's just one possible option to consider

Thanks. That sounds quite plausible. Is shaft alignment realistically a Diy job, and if so, how do you go about it?

 

[Adamastor]

Check the inboard end of the propshaft too. If you have an inboard bearing, that might be gaffed, and the engine movement (you did check the engine mounts for slackness, didn't you?) may be driving the near end of the shaft out of alignment.

 

[Spyro]

Thanks. That sounds quite plausible. Is shaft alignment realistically a Diy job, and if so, how do you go about it?

Yes it's possible but in my caes the boat was out of the water. I don't know what type of shaft seal you have.
There's a description here of how to align the shaft with the engine http://www.tb-training.co.uk/10sgear.htm#bmn33 but you also have to get it central in the tube, thats the part where you have to be able to see the shaft at the inboard end. For me with a conventional stuffing box that meant loosening the stuffing box hose off the shaft log, sliding the stuffing box and hose forward and measuring the gap all around between the shaft and the inside of the log. The shaft also needs to be supported so that there is no droop or bend with the weight of the coupling. I made up some spacers (wooden wedges) to hold the shaft central while I aligned the engine. Once I was happy the shaft was central and straight. I moved the enging on it's mounts to meet the shaft and aligned it as shown in the link. It's also suggested enging alignment is done with the boat in the water because the hull might flex different from the way it might sit in a cradle.
I would want to be sure it was shaft knock first before going down that route. There are other things it could be. You could disconnect the engine coupling and move it up, down left to right and get an idea if it was off centre before doing anything drastic but to do it right you need to be able to see where the shaft enters the log.

 

[Tranona]

Thanks. That sounds quite plausible. Is shaft alignment realistically a Diy job, and if so, how do you go about it?

You have (I think) a Volvo seal and there is no inboard bearing. Normally the seal, which has an integral bearing automatically self centres the shaft in the tube. So just undo the coupling bolts and push the shaft back. You will soon see whether the alignment is out because the half of the coupling on the shaft will not align with the engine. You will need to do this when the boat is out as pushing the shaft back may disengage your cutter from the striker plate and you cannot be certain it will re-engage when you pull the coupling back after you have adjusted the engine mounts.

As has already been suggested, it is worth checking the cutter for worn bearings and the damper springs on the striker plate are still there as either of these faults could cause random rattles.

 

[Carduelis]

Thanks for all that excellent advice. It is a Volvo seal BTW. I'm at the boat this week and will check alignment as advised. I suspect I'll change the cutlass bearing anyway as a precaution.

 

[Spi D]

Not to give any false hope, but I had similar just before lifting out last fall. Appeared to be an anode on the shaft gone loose, eventually sitting against the P-bracket at speed.

 

[bluerm166]

I had a noisy knocking at certain revs which was put down to loose spur cutter components.It turned out to be the greaser tube to the stuffing box which laid lightly against the hull and resonated at certain revs.and slapped the hull producing a surprisingly stoccato note.
If not a flexible greaser tube then perhaps a teleflex or similar cable ?

 

[DipperToo]

Possibly at higher rev the engine runs smoother and doesn't move as much. I had the problem of a knocking shaft when I first got my boat. It was due to an off centre shaft. It would only do it on tick over or just above and when motor sailing on one tack.
It's just one possible option to consider

I had the same - where the propshaft passed through a cut-out space between the engine and stern gland, and when the engine was ticking over (and vibrating more), the excess movement made the propshaft aft of the gearbox lightly touch one side of the hole it passed through before the stern gland.

 

[Adamastor]

Now Spi and Bluerm have both summed up the chronic skills deficit in the marine industry. Seems to me that in ye olde dayes there would be a kindly pipe-smoking gent with a scrunched face sitting whittling a telegraph pole into a toothpick at the head of the slipway who would caution you "ter check that owle stern assembly fer tings banging up on t'hull 'fore yer spend good money haulin' the boat out." And often they'd be right, too!

 

[charles_reed]

Kittens learn by failing at play, so do yotties.

There are so many possible causes of "rattles" that I'd first try and locate the nearest point from which the noise is coming - that means being down below, with a funnel (stethoscope even better) and looking and listening.
If it is the shaft knocking on the stern tube it will be very apparent by the amount of movement of the engine - that would lead to concern about engine mounts (unbolted, delaminated etc).
If the shaft runs true it's probably in prop or ropecutter.
However, in deference to all the suggesters, first comes observation, then diagnosis. With the dearth of knowledge about the OP's engine/boat we're all whistling in the dark.

 

[Spyro]

Now Spi and Bluerm have both summed up the chronic skills deficit in the marine industry. Seems to me that in ye olde dayes there would be a kindly pipe-smoking gent with a scrunched face sitting whittling a telegraph pole into a toothpick at the head of the slipway who would caution you "ter check that owle stern assembly fer tings banging up on t'hull 'fore yer spend good money haulin' the boat out." And often they'd be right, too!

Hence why I said twice there other things it could be.