Vibrating shaft at slightly high revs.

[Cheechako]

While threaded rod shown in the pic with rod is obviously just that, it appears (?) to fit the prop centered. It still appears to be a small shaft compared with the prop.
If the shaft is showing runout turned slowly inside the boat... hmmm... that's a different thing, maybe the prop shaft is bent a bit.
With a long skinny shaft, I would probably not put that shaft zinc on the center of the span, just on general principles... It's an unbalanced heavy mass, better off on one end or the other. Just a small thing...
A double cutlass bearing makes sense to me with that long and small dia shaft.

[Hoohaa]

Here are the results of a fresh dip in Sydney Harbour today. A little rough in places but it should give everyone a better idea of what we are dealing with.
I can pretty much confirm that the stern tube has no bearings in it. There does appear to be one or two bearings in the strut. I can't be 100% shore but although not identical in appearance there was something in both ends of the strut.


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[Hoohaa]

Sorry. Try this one.

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[jimbunyard]

[QUOTE=Hoohaa;2245564]So normally stern tubes don't have a bearing?
Again, there really isn't a 'normal', probably a more typical small boat setup with an average length shaft of appropriate diameter will have a single bearing in the stern tube or the strut and the other end will be anchored by the companion flanges at the gear. But for other applications, such as yours, it is 'normal' to have a 'mid-ship' bearing to keep the shaft under control.

Do you put a bearing back and front of the strut normally?

No, that was a somewhat clumsy attempt to say that there are two cutlass bearings in your application, and to also remind that sometimes 'professionals' do some things that are not strictly standard practice, for a variety of reasons ranging from good, such as trying to work around a bind or get someone home, to bad, such as deliberately cutting corners to make more money from the unsuspecting or the ignorant...


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Originally Posted by Hoohaa

So if stern tubes normally do not have a bearing. Then there should be even clearance around the tube.
If the tube was missaligned then it could be deflecting the shaft and making it appear to be bent. Does that sound feasible?

Yes, it is feasible, but it's also feasible that the shaft can be off center in the stern tube at the stuffing box end with the axes of the strut and stern tube bearings in perfect alignment.

Originally Posted by jimbunyard

Anyway, just for grins, next time you have the flanges disconnected and the drivesaver out, try pulling the flanges close together, but not touching, and rotate the shaft and see if there is runout in the prop flange as compared to the gear box flange. Obviously, if there is something's not kosher with the running gear from the prop flange back...

While it may appear so, I didn't make this suggestion idly. Since the shaft is held essentially rigidly (there is always a little play designed into the system) by the two brgs, any shaft or coupling issues should make their self (or themselves) apparent by doing this simple test. If there are prop or shaft whip issues everything could still appear perfectly aligned...

I realize that you're getting a lot of what might seem conflicting ideas, but this is a complex, dynamic system, and most of the suggestions are reasonable and good. Just that you're getting such a variety illustrates how complex it is...

For whatever reason, it appears nowadays that short, direct answers are the only ones that have any value. I'm going to break with that and give a fairly long, and, (I hope) a bit more comprehensive answer...

First, I would do the simple test described directly above. You don't have to go in the water, or even move the boat, so (a cardinal rule of boating) simplest things first. If there is runout of any kind, that must be sorted before going any further. A prime suspect in this area would be the clamp-on prop flange; the face of the flange must be absolutely perpendicular to the shaft (and the shaft must be straight).

If everything is good there, I would next move the shaft anode to the strut end of the shaft (it will keep the shaft in the boat that way if the shaft were to break [or temporarily remove it altogether]), and also temporarily remove the flex coupling (to reduce the overhang on the prop end of the shaft and to eliminate any runout issues it may have or induce) and see if conditions improve in gear at idle. If they do, then increase speed to see if everything remains good.

If it does, you're stuck with a dilemma. With a rigidly held shaft you must have a flexible coupling when you have flexible engine mounts. If you put the flex coupling back in, you change the geometry and potentially reintroduce the problem. If you don't put the flex coupling back you risk breaking the shaft. I would lean toward getting a more flexible coupling and shortening the shaft to get the prop as close to the strut as possible.

If conditions haven't improved, then it's pretty much down to shaft whip or prop balance/trueness/configuration issues. Given from what we see in the pictures, to me it seems that shaft whip should be pretty much in control; the mid-ship bearing appears to be fairly close to centrally located, probably the best place to control whip.

And that leaves prop issues. For starters since it is the only thing that hasn't been changed, and the symptoms have remained with other changes being made, it at least makes sense to question that there might be something amiss there. It is simplest to check for balance and trueness, but the relatively large size, in comparison with the shaft size, and the proximity of the blade tips to the skeg (which, by the way, if you shorten the shaft the tip clearance will be reduced) are both issues of concern. It is possible that a less high aspect blade configuration will work better in this application.

Of course, there is also the possibility that you'll have to go up to a larger diameter shaft, but this is likely to be the most expensive solution. Better, I think, to go with simple system tweaks at first to try and get a handle on what's happening, and make changes as you learn.

I hope I haven't made things even more murky, but there just really isn't a single, simply correct answer on this one...

[Snowpetrel]

Originally Posted by Hoohaa

Sorry. Try this one.

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Wow, 2 or so meters of unsupported shaft, 25mm. Thats seems too long to me, For my engine I think 1.35m or so was the limit. Have a look at the specs on this sheet. http://www.nannidiesel.com.au/upload...heet/N3.30.pdf.

Maybe somewhere along the way a bearing inside the sterntube got removed, or a more rigidly mounted stuffing box that acted as a partial bearing got replaced by the floating stuffing box?

Possibly something like an aquadrive system would be another solution if nothing else works.

The real solution is probably a bigger shaft, but that is likely much more expensive than even the aquadrive.

I am interested in this as I have a slight vibration at revs. In my case I think my prop is slightly unbalanced, one of these days I am going to have it dynamically balanced... Its good to hear about your experiences and learn from them.

[jimbunyard]

[QUOTE=jimbunyard;2245227]









QUOTE]

Originally Posted by Snowpetrel

Wow, 2 or so meters of unsupported shaft, 25mm. Thats seems too long to me, For my engine I think 1.35m or so was the limit. Have a look at the specs on this sheet. http://www.nannidiesel.com.au/upload...heet/N3.30.pdf.

Maybe somewhere along the way a bearing inside the sterntube got removed, or a more rigidly mounted stuffing box that acted as a partial bearing got replaced by the floating stuffing box?

Possibly something like an aquadrive system would be another solution if nothing else works.

The real solution is probably a bigger shaft, but that is likely much more expensive than even the aquadrive.

I am interested in this as I have a slight vibration at revs. In my case I think my prop is slightly unbalanced, one of these days I am going to have it dynamically balanced... Its good to hear about your experiences and learn from them.

If you look closely at the second and third picture of the op's boat, it seems pretty clear that there actually is a middle bearing...

[Snowpetrel]

Nothings real clear about those images, to me it looks like the whole sterntube has been replaced, not the bearings, there are signs of black contaminated cracks on the hull near the base of the sterntube? I can't decide if the new tube is stainless with a fibreglass tube inside or just all Fibreglass tube? I can't see any evidence of a separate bearing in there, but maybe my interpretation of the images is wrong and there is.

[Snowpetrel]

I think I get what you are saying now..

That is the sterntube, on lots of modern boats the whole shaft 'floats' with only one true bearing inside the P bracket and nothing inside the oversize sterntube. The sterntube has clearance for the shaft to move. This is what I was thinking might have been the case. If so that system is very long for that shaft diameter.

If it has a bearing inside the sterntube then it's a rigid shaft, not a floating shaft and totally different from an engineering perspective. Edit.. Looking closely that may be a tufnol bearing (as oceanseaspray spotted) with a single flute? And the tolerances in the second picture do look real close. No sign of a gap.. Hard to be sure from the pics but I think you may be right and it is a bearing in there.

I may have missed something inportant in the previous posts, or this may have been discussed, I have only skimmed over the 150 odd posts. If so appologies.

[OceanSeaSpray]

The pictures clearly show a bearing, yet the OP says there isn't one after diving and the footage of the shaft running seems to show it shaking far too much near the tube to be supported there.

Has the bearing gone missing? In any case, that shaft is way too long and thin to run without intermediate support. Most small boat shafts can reach at least 1000RPM, and very often more. A missing bearing would essentially explain everything.

[Cheechako]

[QUOTE=jimbunyard;2246019]

Originally Posted by jimbunyard

QUOTE]



If you look closely at the second and third picture of the op's boat, it seems pretty clear that there actually is a middle bearing...

I don't clearly see a Cutlass bearing in the stern tube...? A cutlass bearing should have visible evenly spaced grooves. I don't see them. Although if there isn't one that's a thick tube. Is that a metal tube? Why do that?

[jimbunyard]

[QUOTE=Cheechako;2246322]

Originally Posted by jimbunyard

I don't clearly see a Cutlass bearing in the stern tube...? A cutlass bearing should have visible evenly spaced grooves. I don't see them. Although if there isn't one that's a thick tube. Is that a metal tube? Why do that?

I don't know that it is a what I'd normally call a 'cutlass bearing' in that
there doesn't appear to be staves or channels directing water in for lubrication. But if you go back to HooHaa's original post with this picture



and blow it up to 400% you can clearly see some kind of composite in the I.D. of the tube. Also, if you blow up the picture that has the entire outside view of the whole shebang, it appears that the clearance at the strut and at the stern tube/bearing are the same. It's a little hard to tell because at high magnification the picture is a little blurry. I may be wrong, but it certainly looks like a bearing of some kind to me.


I only question Hoohaa's statement about it because he seems a little unsure, and the idea of a qualified yard installing a 7 foot, 25mm shaft supported only at each end is almost unimaginable...if they did they certainly aren't qualified.

[Sailmonkey]

[QUOTE=Cheechako;2246322]

Originally Posted by jimbunyard

I don't clearly see a Cutlass bearing in the stern tube...? A cutlass bearing should have visible evenly spaced grooves. I don't see them. Although if there isn't one that's a thick tube. Is that a metal tube? Why do that?

I see what looks like a phenolic bearing in the stern tube


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[Hoohaa]

[QUOTE=Sailmonkey;2246433]

Originally Posted by Cheechako

I see what looks like a phenolic bearing in the stern tube


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If its A phenolic bearing, should there be any movement or play in the shaft because when I grab it and move it there is at least 1 or 2mm of play. I was even contemplating poking a wire up between the shaft and stern tube to see if it bumped up against anything as there was enough room to do it..

Jim. thanks for you thoughts. I am still getting my head around what you have posted for me. Will require a couple more reads for me..

Just another thought. The stuffing box has never been touched in this process . It barely drips at all so could do with some loosening and it was mentioned earlier that the heavy rubber hose clamped to it could be influencing the alignment?

[DeepFrz]

I looked at your stuffing box drip and I think it's just fine. The best way to tell though is to take its temperature.

I really am on the side of those who feel that you are going to have to bite the bullet and either go with a larger diameter shaft or something like an aquadrive or both.

[Hoohaa]

Originally Posted by DeepFrz

I looked at your stuffing box drip and I think it's just fine. The best way to tell though is to take its temperature.

I really am on the side of those who feel that you are going to have to bite the bullet and either go with a larger diameter shaft or something like an aquadrive or both.

Won't an aquadrive just deal with an alignment issue?
Would installing a cutlass bearing at the back of the stern pipe, support the shaft and prevent or reduce the wobble?