Thumping sound when motoring ahead - worn cutlass bearing or engine mounts?

sebastiannr

sebastiannr

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Hi there all,

I've been asking quite a few questions on here recently and am eternally grateful for the advice that has been given. I'm a little a little out of my depth on this boat that I inherited last year.

Anyway, when putting the motor in gear, occasionally there is a terrifying thumping noise, as if the prop has lost a blade, it only appears to happen when going ahead. When it happens I take it out of gear then try again and usually it's OK. I worry that it's happening more frequently however.

Possible causes:

1) I dived down and and saw a hairline crack going around the p-bracket about half way down, but I couldn't feel any play in it, but that's not to say that there isn't play when there are greater forces acting on it. Does this crack mean that the boat should be taken out immediately and repaired? And would it just need sheathing in glass mat? I'm at a place where it's very cheap to get hauled out at the moment.

2)At certain engine revs, particularly in the lower range, the engine bobs (vibrates) around all over the place. Could the problem be with the rubber engine mounts not being tight enough?

3) I have no idea about the condition of the p-bracket cutlass bearing. I didn't notice any problems last time I was out the water, but I'm ashamed to say that I didn't properly inspect it. I have read that excessive wear can be the cause of similar thumping sounds coming from that area.

4) I have an autoprop. Although the thumping sounds like the blades have not correctly set themselves, I doubt this is the case as I recently gave it a full service and the blades spin freely.

If anyone has any thoughts as to the the cause of this issue I would really appreciate hearing them. I'm very concerned about doing further damage to the p-bracket and I kind of need to decide whether or not to get hauled out here (Kalibia, Tunisia) in the next day or so.
 
sebastiannr said:
Anyway, when putting the motor in gear, occasionally there is a terrifying thumping noise, as if the prop has lost a blade, it only appears to happen when going ahead. When it happens I take it out of gear then try again and usually it's OK. I worry that it's happening more frequently however.

Possible causes:

1) I dived down and and saw a hairline crack going around the p-bracket about half way down, but I couldn't feel any play in it, but that's not to say that there isn't play when there are greater forces acting on it. Does this crack mean that the boat should be taken out immediately and repaired? And would it just need sheathing in glass mat?

2)At certain engine revs, particularly in the lower range, the engine bobs (vibrates) around all over the place. Could the problem be with the rubber engine mounts not being tight enough?

3) I have no idea about the condition of the p-bracket cutlass bearing. I didn't notice any problems last time I was out the water, but I'm ashamed to say that I didn't properly inspect it. I have read that excessive wear can be the cause of similar thumping sounds coming from that area.
Click to expand...

Is your P-bracket cracked? The bronze, if cracked, could be what we in the trade call A Very Bad Thing. To the layman, that means "not terminal; can be fixed with an intensive course of careful remuneration". But if the metal has lost its bond with the glass, there are several ways to refix the joint, and sheathing is fine.
A whippy propshaft will soon show whether a cutless bearing is past its sell-by date. If you go overside and try and waggle the prop, if you feel any movement at the bracket that could possibly make a sound, therein lies your problem. But do check your engine mounts for premature ageing too.
If your engine can be moved by hand at all, especially, or if it feels like a restaurant table when you try and rock it, then turf all your mounts. You won't believe that Luxury Yachting used to be a silent pastime once you get your troubles sorted!
Good luck!
 
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Adamastor said:
But do check your engine mounts for premature ageing too.
If your engine can be moved by hand at all, especially, or if it feels like a restaurant table when you try and rock it, then turf all your mounts. You won't believe that Luxury Yachting used to be a silent pastime once you get your troubles sorted!
Good luck!
Click to expand...

Thanks for the response.

I just gave the engine a push and it can be rocked back and forth on the mounts. It's strange, as I remember my dad complaining about how much the genuine Yanmar mounts cost when he replaced them 5 or so years ago. When I got to the boat after he passed away, the engine compartment was full of water though, so maybe that degraded them prematurely.

Would tightening the bolts on top of the mounts help reduce the problem (if the mounts are the problem) or should they not be tampered with and replaced immediately?

It certainly would be a relief if that solves the problem!

Thank you once again.IMGP0448.jpg
 
Couple of observations. The mounts are critical, particularly on a Yanmar. The two main problems are the locking nuts on the top coming loose so the engine is floating rather than loading the mounts. The second is that the mounts can delaminate - that is the rubber becomes detached from the metal - difficult to check without taking them off. Poor mounts can also lead to misalignment of the engine and prop shaft which will lead to excess wear in the cutless bearing. There are alternative suppliers of mounts but the really good ones are even more expensive than Yanmar originals.

Moving to the other end. Not entirely clear from your photo, but there seems to be a quite a big gap between the P bracket and the front of the prop, which looks to be a folding type, not an Autoprop. This should be typically 20mm on that size shaft although it is usually not a big issue until it gets more than 50mm. It can cause excessive flexing, again causing wear in the bearing. That is a very substantial P bracket, so doubt it is really cracked. It also looks to be longer than normal - which is 4* shaft diameter or 4" for a 1" diameter shaft.

To summarise, the noise is almost certainly a worn cutless bearing, although it could be the folding prop having worn teeth or pins. Bearing wear is a consequence of misalignment and excessive shaft movement.

Time to haul out and inspect.
 
Thanks Tranona, I will haul out. A question though:

Tranona said:
Couple of observations. The mounts are critical, particularly on a Yanmar. The two main problems are the locking nuts on the top coming loose so the engine is floating rather than loading the mounts.

Moving to the other end. Not entirely clear from your photo, but there seems to be a quite a big gap between the P bracket and the front of the prop, which looks to be a folding type, not an Autoprop.

To summarise, the noise is almost certainly a worn cutless bearing, although it could be the folding prop having worn teeth or pins. Bearing wear is a consequence of misalignment and excessive shaft movement.

Time to haul out and inspect.
Click to expand...

Does this mean that I can attempt to re-tighten the locking nuts if they are loose? Would I likely be mis-aligning the prop shaft if I did this?

The prop isn't actually on the shaft in this photo, I think it's another boats boom that gives the illusion that there is one, and it looks like the shaft hasn't been fully inserted into the hull as the prop is much closer to the p-bracket than this photo would suggest.
 
I can't find info on how you can check the condition of the mounts.

I can rock the engine back and forth with relative ease, but I believe that Yanmar mounts are known to be very soft and flexible. Is there a way that I can find out whether or not the mounts need replacing before I spend £600 on new ones (I would prefer to stick with genuine Yanmar mounts)?

Thanks!
 
Checking the security if your hold down bolts and engine mounts should be part of your periodic routines. Find out if there is a specific torque for them and re-tighten them. If no specific torque make them as tight as you can. If they have come loose you better check your alignment to the prop shaft. Do this by removing the coupling and marking each flange at 12 o'clock and measuring the distance between them. Then move the gearbox flange to 3 o'clock and measure again then move it to 6 o'clock and 9 o'clock and repeat then do the same with the prop shaft flange. If all is true straight and in line the distance between the flanges will be the same. Then fit the flex coupling back.
 
sebastiannr said:
Thanks Tranona, I will haul out. A question though:



Does this mean that I can attempt to re-tighten the locking nuts if they are loose? Would I likely be mis-aligning the prop shaft if I did this?

The prop isn't actually on the shaft in this photo, I think it's another boats boom that gives the illusion that there is one, and it looks like the shaft hasn't been fully inserted into the hull as the prop is much closer to the p-bracket than this photo would suggest.
Click to expand...

As Cryan says the first thing is to check is that the lock nuts on the top are tight. You can do this in the water. The back left hand one is usually a bit of a pig to get at, but put a spanner on all of them and see whether they move. You will find they twist a bit on the rubber. Also worth checking alignment before you take the boat out as it is best done in the water - although if you do end up changing mounts you will need to re-align. Not sure where you got the price from, but they are nowhere near as expensive as that - although the genuine Yanmar ones are more expensive than some pattern types.

Thanks for the clarification on the prop and shaft. If you do have a folding or feathering prop then worn bearings or pins could be the source of the noise, but the favourite is still worn cutless.
 
Tranona said:
Thanks for the clarification on the prop and shaft. If you do have a folding or feathering prop then worn bearings or pins could be the source of the noise, but the favourite is still worn cutless.
Click to expand...

Would a worn cutlass bearing explain the fact that the terrific thumping only happens about 10% of the time the engine is engaged? I checked the engine mounts and the lock nuts are tight, and a Swiss guy on the boat next to me had a look and thought that there was the right amount of movement of the engine (of course the 'thumping' didn't happen when I tried to demonstrate it to him).

I wonder, after servicing the autoprop, there was a little more resistance on the blades spinning on their axis (not much, just resistance of the grease). Is there anyway that occasionally one of the blades doesn't set itself correctly, causing imbalance in the rotation of the shaft and hence the extreme thumping?
 
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Noise associated with a worn cutless bearing tends to be constant. The play is there all the time, so one would not expect any noise and vibration to go away. If the P bracket has a crack in it, then yes this does need to be dealt with. Vibration may have caused it or it could be causing the vibration. If you pick up a rope on the prop, expect the bracket to break under the shock if it is already cracked. My P bracket cracked in this situation, and broke about two hours motoring later. Fortunately we had beached to check the sterngear after the fouling - and it just "came orff in me 'and, guvner" That has put paid to sailing this season while we get it sorted. Glass in P brackets from Lancing Marine in various sizes from about £250.

Otherwise as above, engine mounts and engine alignment are all suspects as causes of vibration. Also, the shaft itself may be bent. Again, have you had fouled sterngear recently? As I found, it may do nothing, or it can do a lot of damage - including overstressing flexible engine mounts, causing them to fail too. A really bad high engine speed foul can even tear the engine right off its mounts, and bend the crankshaft. All that power has to go somewhere if the engine is prevented from turning suddenly!
 
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My feeling is that the occasional thumping is not from something that is fixed i.e. not the prop shaft, engine mounts or cutlass bearing. The vibrations from these would there all the time. Vibrations, not thumping. So that would lead me to something that moves i.e. the prop not properly opening. That's where I'd start. Out you come.

Other things that can come loose and make a din are a shaft anode and the rope cutter. Once again it would be surprising if they were only a problem 10% of the time.

Does a quick burst of reverse stop the thumping? If so then prop opening problem is likely.
 
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The last guy I talked to with such symptoms found that it was actually the exhaust pipe that had fractured one of its brackets and was banging on the hull at certain speeds.
 
sebastiannr said:
Hi there all,

...hen putting the motor in gear, occasionally there is a terrifying thumping noise, as if the prop has lost a blade, it only appears to happen when going ahead. When it happens I take it out of gear then try again and usually it's OK. I worry that it's happening more frequently however.
Click to expand...

My Yanmar 2GM did this when the cone clutch in the gearbox needed to be refaced. It's not a big job once the gearbox is out and there are plenty of threads on here about it. In my case it was worse when the gearbox oil was warm after motoring for a few hours, and not so bad when the engine was cold.

As others have said, you also need to check the p-bracket and engine mounts/alignment.
 
I experience deep knocking noises occasionally, almost certainly due to the shaft contacting the stern tube. I also have an Autoprop, which I believe under certain conditions, e.g. turning sharply, following seas and the like, is able to cause slight bending of the shaft when the blades are not all at identical angles. Allied to the fact that on my boat there is very little clearance between the shaft and tube this can lead to contact, even with my Aquadrive.

Before splashing out on new engine mountings I would check the alignment of your shaft coupling to gearbox, but pay particular attention to getting the shaft central in the tube inside the boat. Also, try to ensure that all mountings carry a similar load, not easily done but it should be possible to estimate using a strip of steel.
 
Thank you for all the responses, it's amazing how much I learn through these forums.

As a follow up, I was able to dive down and take some pictures and check for play in the cutlass bearing yesterday. I couldn't feel any play in the bearing, but visually inspecting it was difficult.

I've noticed that the thumping occurs mostly after I've been in reverse, then go forward. So perhaps it's the prop blades not setting properly?

In the pictures you can see where paint has come off at the point of the crack on the p-bracket. There's no play in it so hopefully it's just the glass fibre sheathing that's cracked. Am I being reckless if I leave it until the next haul out (in October before we cross the Atlantic)?

http://i1347.photobucket.com/albums/p705/sebastiannr/IMGP0455_zpsd984bebd.jpg

http://i1347.photobucket.com/albums/p705/sebastiannr/IMGP0457_zpsf8bbf245.jpg

Anyway, thanks once again for all the input, invaluable as always.

N.B. Those shaft anodes are only 1 month old, is that normal corrosion or is there serious stray current somewhere?
 
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blimey that's a big prop which could cause twist on the shaft when you first put it in gear. How ever I would suggest you get the whole lot looked at by a marine engineer along with what every one else has said, I also suspect the gear box

cheers
mick
 
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