Isuzu 38 and PRM150 - Knocking noise

[MikeH]

Hi all,

 

Our NB has a Isuzu 38 mated with a PRM150 and there is a dull knocking noise resonating through the hull (in neutral or in gear), best heard standing alongside the boat or in the cabin. It is much harder to make out in the engine bay or standing on the engine covers when in place. Even standing on the bow, you can hear the noise in the water tank below - it is loud.

 

There was a similar thread by @GrahamSop a few months ago. Graham uploaded a sound file and the noise sounded identical to the one we are hearing, sadly no conclusion on the thread.

 

We use the boat very little, perhaps 30 to 50 engine hours a year. We keep the fuel tank full during winter in an attempt to avoid condensation (which we have never had a problem with), the sound has been evident for some time. Last time we went out prior to this year was two years ago, but we recently started going out every weekend and the noise seems much louder than I recall from before. We don't know how many hours the engine has done as was previously a hire boat, the tacho has just shy on 13,000 hours on it, and while we suspect a reconditioned engine has been fitted at some point, we have no idea.

 

The engine runs very well and smoothly, a very small amount of smoke on start up, gone in a second or two, oil stays clean and doesn't seem to burn any.

 

Things I have checked / tried:

1. Checked and adjusted tappets.
2. Changed engine oil filter and oil.
3. Changed gearbox oil.
4. Replaced fuel filter.
5. Added injector cleaner.
6. Cracked injector pipes with engine running, one injector possibly had less effect on the engine than the other three but was marginal.
7. Checked water trap (clean), fuel looks clean.
8. Purchased 20 litres of fuel from a filling station, disconnected the fuel line and ran the engine from the can of fresh fuel for about 45 minutes, during which time the pump sucked about 5 litres from the can. Added the remaining fuel to the tank.
9. Probed the engine and gearbox for noise with a mooring pin. The noise is very clear and loud where the gearbox meets the adapter plate, the engine, injectors and mounts all sound quiet.
10. Disconnected the propshaft at the coupling at which point the noise can no longer be heard in the cabin etc., so at least I know how it is being transmitted to the hull. If I then probe the coupling it is very clear, it can be faintly felt through the probe.

 

Other then removing the gearbox and checking the driveplate etc., I am out of ideas. Several engineers I have sought advice from both say driveplates normally only make noise when a gear is engaged. A fellow boater a few boats up from us has the same engine / gearbox combination and they have the same sound, but it can barely be heard inside the cabin, certainly not to the point I would think it is a problem developing.

 

The noise is very regular so I don't believe it is an engine mount, as surely disconnecting the propshaft would not make any difference to it? Graham's post went as far as removing the gearbox and driveplate only to find the noise continued - last post he made said he was looking at possible fuel issues - but I have kind of ruled that out with the exception of a bad injector, although they all sound good when listening.

 

So before I get into removing the gearbox or talking myself into thinking it is an engine problem, any ideas / things to check would be welcome.

 

Thanks.

[Guest]

What kind of stern gland is it? As in a conventional greaser or a watercooled vetus or similar? If the latter, sometimes the engine mounts drop with age, and the propshaft bangs against the tube. This occurs due to the flexible nature of the rubber cutless bearing. Just a thought.

[MikeH]

1 minute ago, catweasel said:

What kind of stern gland is it? As in a conventional greaser or a watercooled vetus or similar? If the latter, sometimes the engine mounts drop with age, and the propshaft bangs against the tube. This occurs due to the flexible nature of the rubber cutless bearing. Just a thought.

Conventional. I did think this but dismissed it as when I recouple the propshaft it engages very easily. I also put feelers around the shaft where it enters the stuffing box (?) and it seemed pretty central. 

 

I guess that is my question, is the noise normal, just being transmitted for another reason such as alignment or is the noise louder than it should be?

[WotEver]

It’s not the engine or gearbox touching the engine bearers?

[Guest]

21 minutes ago, WotEver said:

It’s not the engine or gearbox touching the engine bearers?

Good point, our vetus did this after I dropped it 5mm for a different gearbox. The casting where the starter motor fits was just a couple of mm off the bearer. I had to cut a half round recess in the bearer.

[MikeH]

11 hours ago, WotEver said:

It’s not the engine or gearbox touching the engine bearers?

No, all noise disappears when I disconnect the propshaft. Noise is the same whether in neutral or in gear.

10 hours ago, catweasel said:

Good point, our vetus did this after I dropped it 5mm for a different gearbox. The casting where the starter motor fits was just a couple of mm off the bearer. I had to cut a half round recess in the bearer.

Nothing is touching, the noise is only being transmitted through the propshaft.

[MikeH]

Went and listened to another identical engine/gearbox combination yesterday, couldn't hear the noise through the hull, but could hear it when probing the adapter plate.

 

I think the noise is a characteristic of the engine / gearbox, possibly getting a little louder as the engine accrues more hours. I am now satisfied the noise is not a problem, the transmission of it through the propshaft is.

 

When I disconnect the propshaft from the coupling, it re-engages with no effort so seems to be aligned. The engine mounts looked pretty old although they are still doing their job it seems. If they have dropped a little over the years, causing the engine to go out of alignment, I wonder if the propshaft is touching the side of the stern tube, causing the noise to be amplified?

[Tony Brooks]

So what type of "flexible" coupling do you have and how old and/or oily are the engine mounts? How often does the  stern gland need repacking or adjusting?

 

Engines will move on flexible mounts and that would tend to wave the prop shaft about. Unless you have a Cutless rubber shaft berating, which is unlikely but not impossible if you have a conventional gland, the engine movement transmitted to the shaft may well produce noise as the shaft bangs about in the stern gear unless the flexible coupling will allow some radial misalignment. Many do not.

 

If the rubber in the mounts is old or perished by oil the engine will move more than with new mounts. Can you get your boot against the engine when its knocking and try pushing it sideways to see if it makes a difference?

 

 

Posted just as you posted. I think you may well be correct. Mike.

Edited July 21, 2019 by Tony Brooks

[MikeH]

Hi Tony,

 

The coupling is a R&D disc, engine mounts look old but no oil or diesel on them. The problem is only the transmission of the noise through the propshaft, I don't believe it is being caused by movement of the propshaft in the sterntube.

 

Not sure what cutlass bearing I have, although a comment was passed last time the boat was out of the water a couple of years ago that it had recently been replaced. I have only adjusted the stern gland once, and it only needed a 1/4 turn. 

[Tony Brooks]

12 minutes ago, MikeH said:

Hi Tony,

 

The coupling is a R&D disc, engine mounts look old but no oil or diesel on them. The problem is only the transmission of the noise through the propshaft, I don't believe it is being caused by movement of the propshaft in the sterntube.

 

Not sure what cutlass bearing I have, although a comment was passed last time the boat was out of the water a couple of years ago that it had recently been replaced. I have only adjusted the stern gland once, and it only needed a 1/4 turn. 

 

The R&D coupling in my view is not really flexible enough to allow any radial misalignment and all but zero axial so I would say there is a good chance the engien is waving the shaft about.

 

A Cutless bearing is a fluted rubber "sleeve" in which the shaft runs contained in a rigid tube. These are not normally fitted to boats with a conventional stern gland and a remote greaser but are fitted to boats that use packless (non-adjustable) stern glands. The rubber sleeve will allow the shaft to wave about so the stern glands used are usually on the end of a rubber section that allows the shaft to move without hitting anything. Usually the hole the shaft passes down is far larger than the shaft so unless the engine drops a long way on its mounts there is no chance of such noise.  None of this is likely to apply with an ordinary greased stern gland & metal bearing.

 

Lets have a photo showing the gland, shaft etc. Some things you say do not add up to me so we need to know exactly what you have.

 

If its a conventional gland the chances of a Cutless (trade name) shaft bearing is unlikely.

If you have a Cutless bearing the chances are your stern gland is not adjustable.

Conventional stern gland designs are legion but with a typical canal style gland an adjustment of just 1.5 flats (1/4 turn) seems a very unlikely small adjustment unless you are confusing turning the greaser with adjusting the gland and if so then there is no way its enough.

[blackrose]

Are there any widebeams or people who use fenders in the area? If so it's probably their fault. ?

[Mike Adams]

You could wedge the propeller using a piece of wood through the weed hatch it would tell you if is the shaft vibrating - in neutral of course. A waterlubricated bearing can easily be destroyed by something wound around the shaft.

[MikeH]

10 hours ago, Tony Brooks said:

 

The R&D coupling in my view is not really flexible enough to allow any radial misalignment and all but zero axial so I would say there is a good chance the engien is waving the shaft about.

 

A Cutless bearing is a fluted rubber "sleeve" in which the shaft runs contained in a rigid tube. These are not normally fitted to boats with a conventional stern gland and a remote greaser but are fitted to boats that use packless (non-adjustable) stern glands. The rubber sleeve will allow the shaft to wave about so the stern glands used are usually on the end of a rubber section that allows the shaft to move without hitting anything. Usually the hole the shaft passes down is far larger than the shaft so unless the engine drops a long way on its mounts there is no chance of such noise.  None of this is likely to apply with an ordinary greased stern gland & metal bearing.

 

Lets have a photo showing the gland, shaft etc. Some things you say do not add up to me so we need to know exactly what you have.

 

If its a conventional gland the chances of a Cutless (trade name) shaft bearing is unlikely.

If you have a Cutless bearing the chances are your stern gland is not adjustable.

Conventional stern gland designs are legion but with a typical canal style gland an adjustment of just 1.5 flats (1/4 turn) seems a very unlikely small adjustment unless you are confusing turning the greaser with adjusting the gland and if so then there is no way its enough.

Quite agree re the R&D but that is what it currently has.

 

It is not the engine waving the shaft about in the the stern tube. As I said, the noise is evident from the engine / gearbox with a probe - while the shaft is disconnected. The shaft is simply transmitting the noise to the hull.

 

The stern gland (stuffing box) entry is a precise fit with the greaser attached. The adjusters for the stuffing box only needed a 1/4 turn, any more made it difficult to turn the shaft, not confusing it with the greaser which takes a regular 1 to 1 1/2 turns after cruising.

 

I rechecked the shaft to coupling alignment today and the coupling hole was 1mm low compared to the shaft, so I adjusted the mounts to compensate and the the noise has all but gone. I am going to replace the mounts as they look old and not in the best shape.

 

Thanks again for the help, looks like I have a solution. ?

[GrahamSop]

On 20/07/2019 at 14:30, MikeH said:

There was a similar thread by @GrahamSop a few months ago. Graham uploaded a sound file and the noise sounded identical to the one we are hearing, sadly no conclusion on the thread.

 

Things I have checked / tried:

1. Checked and adjusted tappets.

Other then removing the gearbox and checking the driveplate etc., I am out of ideas. Several engineers I have sought advice from both say driveplates normally only make noise when a gear is engaged. A fellow boater a few boats up from us has the same engine / gearbox combination and they have the same sound, but it can barely be heard inside the cabin, certainly not to the point I would think it is a problem developing.

 

The noise is very regular so I don't believe it is an engine mount, as surely disconnecting the propshaft would not make any difference to it? Graham's post went as far as removing the gearbox and driveplate only to find the noise continued - last post he made said he was looking at possible fuel issues - but I have kind of ruled that out with the exception of a bad injector, although they all sound good when listening.

 

So before I get into removing the gearbox or talking myself into thinking it is an engine problem, any ideas / things to check would be welcome.

 

Thanks.

Hi Mike,

Apologies for the lack of conclusion to my thread! Nice to hear (or not nice actually) somebody with very similar issues.

 

I removed my gearbox back then for a separate reason (I was replacing my driveplate), it just happened to coincide with me investigating the knocking noise, so it was handy being able to rule out the gearbox/driveplate from the knocking problem.

 

I know the first thing you did was adjust the valve clearances, but I checked mine as thoroughly as possible last night and I can definitely say it reduced my engine knocking noise a lot. It is still there slightly, but for sure it has made a big difference.

 

Graham

[MikeH]

20 minutes ago, GrahamSop said:

Hi Mike,

Apologies for the lack of conclusion to my thread! Nice to hear (or not nice actually) somebody with very similar issues.

 

I removed my gearbox back then for a separate reason (I was replacing my driveplate), it just happened to coincide with me investigating the knocking noise, so it was handy being able to rule out the gearbox/driveplate from the knocking problem.

 

I know the first thing you did was adjust the valve clearances, but I checked mine as thoroughly as possible last night and I can definitely say it reduced my engine knocking noise a lot. It is still there slightly, but for sure it has made a big difference.

 

Graham

Hi Graham,

 

Thanks for the response. I will check the valve clearances again but I believe the sound is a characteristic of the engine and can be heard to varying degrees dependant on a number of factors, such as fuel quality, valve clearances engine mount condition, # hours on engine. I listened to a friends engine and probing above the flywheel on the adapter plate, the same sound can be heard in the 4 Isuzu / PRM combinations I have listened to. It is just a question of how loudly that noise gets transmitted to a place where it can be heard. In my case I found the engine mounts had compressed a bit over the years, causing the weight of the engine/gearbox to put weight on the shaft. I adjusted the mounts to lift everything about 1mm and the noise has all but gone again. I have ordered new mounts as they have been on there many years by the looks of things.

 

Mike