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

[Albert Ross]

Your problem does sound very similar to mine, I don't have an auto prop just a standard 4 blade, the boat was hauled out yesterday cutlass bearing is fine, 2mm of play at most, no wear on the rubber inside the cutless bearing so I'm assuming that would rule out engine alignment aswell, does your noise happen frequently or just every now and then? I have a Borg Warner v drive box with a ford mermaid 180hp

It happens every time when engaging forward gear, when we have been sailing but not after being moored overnight (possibly something to do with the temperature of the gearbox?), and never when engaging reverse. If you explore the web, there are lots of people with the identical problem. It is important to note that this is a really loud clonking, banging noise, not a little rattle. It lasts for about 10 seconds after engaging gear.

 

[sebastiannr]

OK an update,

After removing the rope cutter and moving the shaft anode back by 3 about 30mm so that there is nothing impeding back and forward movement of the prop shaft...

The problem persists, so it wasn't the rope cutter.

I have noted that the banging generally happens if I am going ahead without drive from the prop (either sailing or after going into neutral), and then engage in forward again, that's when it happens. So I think the only answer is that in these situations, the prop isn't feathering correctly at first, causing massive imbalance.

Any further comments welcome,

Seb.

 

[vyv_cox]

OK an update,

After removing the rope cutter and moving the shaft anode back by 3 about 30mm so that there is nothing impeding back and forward movement of the prop shaft...

The problem persists, so it wasn't the rope cutter.

I have noted that the banging generally happens if I am going ahead without drive from the prop (either sailing or after going into neutral), and then engage in forward again, that's when it happens. So I think the only answer is that in these situations, the prop isn't feathering correctly at first, causing massive imbalance.

Any further comments welcome,

Seb.

See post #18. Your symptoms sound very similar to mine. Even with an Aquadrive my shaft still manages to contact the stern tube on occasion. It sounds bad but up to now I have seen no evidence of damage.

 

[boatmike]

OK having eliminated the rope cutter and anode as a source lets talk about autoprops. It is inherent in their design that when first engaging gear (forward or reverse) the blades take a few seconds to find their nominal thrust position. Before they do they can indeed cause the shaft to be unbalanced and if there is any clearance at all in the bearings they may rattle a bit or indeed if there is very little clearance in the stern tube itself this out of balance will cause the shaft to hit the inside of the tube. This will not be helped if you have very soft engine mounts or they are in need of replacement. HOWEVER when the blades have settled down and you are motoring forward (or aft) this should stop whatever the condition of your mounts. The question is "Is this a momentary clunking that goes away under thrust or does it persist?"

 

[sebastiannr]

OK having eliminated the rope cutter and anode as a source lets talk about autoprops. It is inherent in their design that when first engaging gear (forward or reverse) the blades take a few seconds to find their nominal thrust position. Before they do they can indeed cause the shaft to be unbalanced and if there is any clearance at all in the bearings they may rattle a bit or indeed if there is very little clearance in the stern tube itself this out of balance will cause the shaft to hit the inside of the tube. This will not be helped if you have very soft engine mounts or they are in need of replacement. HOWEVER when the blades have settled down and you are motoring forward (or aft) this should stop whatever the condition of your mounts. The question is "Is this a momentary clunking that goes away under thrust or does it persist?"

When it happens, if I increase the revs it only becomes more violent and faster, I haven't persisted with that as it sounds like damage must be being done. So I reduce the revs again and let it settle for maybe 5 seconds. I then slowly increase the revs again until I'm sure that it's sorted itself out, sometimes I may have to go back to tickover again and give it a while longer.

It's got to be something to do with a force of water passing the prop as I'm going ahead which make it more difficult for the prop to set itself properly. That's all I can think of.

 

[vyv_cox]

It's got to be something to do with a force of water passing the prop as I'm going ahead which make it more difficult for the prop to set itself properly. That's all I can think of.

That's what I think mine is doing. For 99% of the time it's perfectly OK. If I rev hard in either forward or reverse, e.g. turning using prop walk in a confined area, it will knock. The other particular time is if we are motoring with a swell from astern, when bigger waves will cause the knocking as the stern lifts.

 

[boatmike]

I think you both have either too little clearance in the propshaft tunnel or more likely knackered engine mounts. I had a catamaran with saildrives and autoprops but even then there was vibration when first putting it into gear. The blades are feathered if sailing forward and you need to ensure your throttle opening is sufficient to maintain drive if motor sailing otherwise they wont "set" to a pitch. When stationary (on a mooring or pontoon for instance) you might well find you need to give a burst of throttle to "set" the blades, especially going from forward to reverse (or vice versa) and there is a delay sometimes when using reverse to "brake" when coming in to a pontoon. This is all normal for an autoprop. You should be able to set the blades with a short burst of throttle. If your engine is solid this happens very quickly but I guess if the shaft is not rigid because of oversoft mounts it will oscillate due to the out of balance and this may in turn prevent the blades setting so easily. My guess, especially as its a Yanmar, is that if you jack up your engine and remove each bearer in turn, you will find that the rubber has parted company from the steel it is supposed to be bonded to. This is even more likely if as you originally said, they have been swimming about in a wet bilge and have rusted. If so chuck the crappy things and buy some better ones from ASAP supplies.

 

[sebastiannr]

Thanks boatmike,

I suspect you're right, I did check all the mounts and the rubber hadn't parted from steel, but they are very soft and there's a lot of movement of the engine when starting up, for example, so that may be a cause for the blades not being forced to immediately set correctly in some situations.

Put the rope cutter back on?...

 

[micheal.rock@gmail.com]

Just came across this post. If you hear a bad thumping noise when going ahead at certain speeds and no thumping noise when going astern you almost certainly have a gearbox problem. Haven't time to read all posts but if the noise stops when you increase speed this indicates the gears engage fully because of the increased forward pressure .

Mike

Thanks boatmike,

I suspect you're right, I did check all the mounts and the rubber hadn't parted from steel, but they are very soft and there's a lot of movement of the engine when starting up, for example, so that may be a cause for the blades not being forced to immediately set correctly in some situations.

Put the rope cutter back on?...

 

[colind3782]

I had a similar problem and, after investigating all of the above, the problem turned out to be undersized bolts on the shaft coupling, allowing just enough movement to cause the knocking. Replaced the bolts and the problem went away.

 

[boatmike]

The rope cutter has not been helping either way. Leave it off, solve the problem before putting it back.

Ignore distractions, I doubt very much there is anything wrong with your gearbox. The noise you are hearing is the shaft hitting the outside of the tunnel. If you watch the rear of the shaft as this is happening I think you will see it is moving about radially while all this is going on. The bearers must be solid enough to keep the shaft on centreline before and during the time they are adjusting pitch. If they are not, replace engine bearers, realign the engine, make sure your flange bolts are tight and then you will find a quick burst of throttle will set the blades without the noise.

As an afterthought I am also worried about the "hairline crack" in your P bracket. I think you need to ensure that this is just a surface defect and not structural. Were the P bracket to break you could have serious damage and if indeed it is moving it could be the source of, or contributing to, your problem. If it is a structural crack you need to clean it up and grind out the crack and have it welded. Overlaying with glass matt won't suffice!
Let us know how you get on.

 

[sebastiannr]

Update:

Turns out it's the gear box slipping. I finally got into a situation where I had time to check out the engine room whilst it was happening and there is no doubt. Hopefully its just the cable needs adjusting, otherwise a pretty major job.

Thanks for all the input

 

[charles_reed]

If you do need mounts, these from ASAP are much cheaper than the Yanmar originals

http://www.asap-supplies.com/search/yanmar+mounts

Yanmar prices (in UK) are prodigious - alternates, try ASAP, Halyard Engineering or purchase in the US of A via a travelling friend.

 

[charles_reed]

Update:

Turns out it's the gear box slipping. I finally got into a situation where I had time to check out the engine room whilst it was happening and there is no doubt. Hopefully its just the cable needs adjusting, otherwise a pretty major job.

Thanks for all the input

Yanmar boxes - Deserved ill-reputation for wearing/glazing the cone clutch. I finally lost forward completely, took box out between tides on the sand @ Casteltownberehaven, and found that the selector fork had worn on the one side. Replaced the other way round and fiddled adjustment and had a perfect box for another 4 years until I replaced the engine with a 3YM.

 

[sebastiannr]

Yanmar boxes - Deserved ill-reputation for wearing/glazing the cone clutch. I finally lost forward completely, took box out between tides on the sand @ Casteltownberehaven, and found that the selector fork had worn on the one side. Replaced the other way round and fiddled adjustment and had a perfect box for another 4 years until I replaced the engine with a 3YM.

That sounds promising, thanks I shall take the gearbox off tomorrow and check it. Do you think the fact that there is no issue with reverse implies that it's not the cone that's the issue, but the selector fork as it was with you? I certainly hope so!