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Discussion Starter #1
This weekend I noticed a different sound when I motored above 2,000 RPM. It's a sound that wasn't there before. It sounds like a slight knocking or gurgling noise (no perceptible vibration though) and it only appears above 2,000 RPM. Everything in the engine and drive system seems normal: no visible vibration or movement of the shaft while spinning. I dove under the boat and found the bottom and prop clean and without any dings. I did, however, find that I could move the prop shaft slightly from side-to-side in the bushing (cutlass bearing). It's only a slight play of maybe a millimeter or two. I don't recall there being any play before. Is a little play normal? Could this play be causing the noise?

Additional information in case it helps:

My engine is a Yanmar 3GM30F coupled with an Aquadrive thrust bearing which allows the engine to work out of alignment (so engine alignment can't be the problem). The prop shaft enters the hull behind the full keel (prop in aperture) and buts up against a cross beam (plate) glassed in the hull which bears the thrust of the drive shaft. The system has always worked flawlessly and silently (zero vibration and very quiet due to soft engine mounts). It seems the sound appeared from one day to the next (I'm very sensitive to any changes in sounds on the boat and would have noticed it if it had started gradually). The problem is the summer cruising season is upon us and I wasn't planning on hauling the boat until next winter. I would like to eliminate all other possible causes before changing the cutlass bearing (bushing). Help! :confused:
 

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Get down in the water and and have a pull on your prop and shaft, if you get some movement then probably time to replace.
 

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Shouldn't be any play..but vibration could be a symptom of other possible causes..like growth on the prop and shaft or maybe a loss of zinc if one is mounted on prop or shaft..
 

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Discussion Starter #4
Just to clarify- I did dive under the boat- that's when I found a slight play in the shaft and bearing. The prop is clean and has no dings. The zinc is almost 50% worn (and perhaps unbalancing the prop?). I'll try changing the zinc and see if that as any effect.
 

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his weekend I noticed a different sound when I motored above 2,000 RPM. It's a sound that wasn't there before. It sounds like a slight knocking or gurgling noise (no perceptible vibration though) and it only appears above 2,000 RPM. Everything in the engine and drive system seems normal: no visible vibration or movement of the shaft while spinning. I dove under the boat and found the bottom and prop clean and without any dings. I did, however, find that I could move the prop shaft slightly from side-to-side in the bushing (cutlass bearing). It's only a slight play of maybe a millimeter or two. I don't recall there being any play before. Is a little play normal? Could this play be causing the noise?
 

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2 mm is HUGE

If my Cutlass Bearing had 2 mm of play, that would be way past time for it to be changed. 2 mm is 0.078 inches, 1 mm is 0.039 inches, but close to the limit. 1/16 (0.0625 in) is the most I would ever do, but if you're at 1/16th, you should replace it.

In terms of mils, there are 1000 mil/in, so ~4 mils/1 mm. If you get a feeler gauge, and can slide a 5 mil feeler gauge or thicker shim between the bearing and the shaft, your at the end of the bearing and it should be replaced. 4 mil or less is best.

But, as others have said though, ideally no play is desired.

DrB
 

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Discussion Starter #10
I'm just wondering if I have to haul the boat now to fix this or can I continue with my cruising plans for December and January, which will inevitably involve about 40 hours of motoring. I don't want to damage anything, but if I can get away with another 2 months like this, so much the better. I had planned a haul out anyway for after my December/January cruising.
 

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I'm just wondering if I have to haul the boat now to fix this or can I continue with my cruising plans for December and January, which will inevitably involve about 40 hours of motoring. I don't want to damage anything, but if I can get away with another 2 months like this, so much the better. I had planned a haul out anyway for after my December/January cruising.
What is shaft diameter ?
What is distance between strut and propeller hub ?
What kind (size) of propeller ?
 

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Discussion Starter #12
Boatpoker, to answer your questions:

1 inch shaft diameter.
The shaft exits the aft part of the keel and has barely 1 inch of room for the zinc between the keel and the prop.
The prop is a 3-blade one, but I don't know the size offhand. It's coupled with a 27hp Yanmar 3GM30F.
 

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Boatpoker, to answer your questions:

1 inch shaft diameter.
The shaft exits the aft part of the keel and has barely 1 inch of room for the zinc between the keel and the prop.
The prop is a 3-blade one, but I don't know the size offhand. It's coupled with a 27hp Yanmar 3GM30F.
It's definetly not ideal but should be ok for your proposed cruise.

Reason I asked those questions .....
I looked at one this morning that had 6" between the bearing and the prop hub on a 1" shaft plus a very heavy 3-blade feathering propeller. In this instance I recommended immediate attention as the heavy prop on 6 inches of unsupported shaft is like a whip that would wear through the bearing in very short order.
 
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As boatpoker has said there are other factors that can effect bearing wear and also vibration/noise, prop over hang and large heavy props are not good, gap should ideally be 12.00-15.00mm with soft drive but as you have a rigid drive with thrust bearings it could be reduced to 5.00mm. We work on an absolute min gap of 3.00mm to allow water to exit bearing.

For a new shaft bearing on a 25.4mm (1") shaft the clearance will be 0.13mm, our advice is to consider changing when clearance approaches 0.60mm.

Despite the fact that you have an aqua drive it is quite common to find that the shaft bearing in a P bracket/strut or less common but possible a stern tube, is not in alignment. So when it's dismantled check the bearing carrier alignment. You can do this with a new bearing made as a clearance fit in the carrier. Chock shaft and new bearing with a clearance of 0.05-0.15 (so quite close) should slide up shaft into carrier and still easily turn.
 

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Discussion Starter #15
Thanks Boatpoker and Kutter. I'm not sure if I mentioned this, but there is no vibration in the drive train, just a different noise. It's almost a gurgling noise like cativation and it only starts above 2,000 rpm. I can't see any vibration of the shaft at the stuffing box or anywhere else. I don't think anyone else would notice the slight difference in the noise, but as the owner of the boat I'm aware of any change in noise, however slight. I don't want to damage anything by running the engine with this play, but if another 40 hours won't hurt anything (except me- I HATE to have ANYTHING not 100% on the boat!) I'd really like to haul out after January when I have the time to take care of things properly.
 

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Thats a tough call. when I pulled my boat I had a little vibration and checked the cutlass for wear. When I went to replace the cutlass I also noticed my shaft was worn and needed to replace both. I didn't know if the shaft was original or not, might have been so I don't think I had done any more damage until I was able to get back to the boatyard I worked at to pull my boat but this is something you want to keep in mind.
 
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