Vibrating shaft at slightly high revs.

[Three Sisters]

Originally Posted by DeepFrz

I see what you mean but its just too hard to tell from here. Same as the misalignment I thought I saw of the shaft and the stern tube.

Two cutlass bearings on the same shaft is just asking for trouble IMHO. It seems like the shaft is very long with a long unsupported section. I really think you need something like the "aquadrive" system.

Welcome to Aquadrive

And the bottom photo from Hoohaa in post 131 appears to show alignment.

[Three Sisters]

Originally Posted by Hoohaa

I will be heading to the boat shortly to have another crack at it.
I am thinking of sorting out the existing engine mounts so that they can be adjusted properly and it will give me an opportunity to check there condition.
Attachment 134075

Attachment 134076

Attachment 134077

Attachment 134078

I have to applaud your ongoing application to resolve the coupling alignment. And if memory serves, this is your first time trying the coupling alignment.

Not being able to properly adjust the engine mounts is making the total alignment difficult. And preventing other considerations into the causes of the shaft wobble. If the mounts can be cleaned up and made adjustable, as you stated, you can complete the alignment and move to other possible causes should the wobble continue.

At this point everything can go back together as found.

[jimbunyard]

[QUOTE=Hoohaa;2244972]The shaft is for all intensive purposes 1 inch. But the 3rd picture shows the prop sitting on a screwed rod, I don't know what its diameter is. so it wouldn't be the best pic for scaling.
Looking at the invoice. They did replace 2 bearings. I would have thought a bearing on the strut and one on the stern tube would be normal? or are you referring to 2 bearings on the stern tube?
I have been talking to the company that did the work this morning and they are looking at the job to re familiarize themselves with it. So far they have been friendly and helpful.
I will hopefully here back from them soon.
Here are a few more pics I managed to find.
I will be heading to the boat shortly to have another crack at it.
I am thinking of sorting out the existing engine mounts so that they can be adjusted properly and it will give me an opportunity to check there condition.
[QUOTE]

There really isn't a 'normal'; it is more common, probably, around smaller
boats to see single cutlass bearings. It all depends on the requirements dictated by the boats' design.

With a relatively long shaft like yours (it appears on the order of 6-7 feet),
a midshaft bearing could be specified to keep shaft whip under control. This complicates things a little but is not too uncommon. Alignment of the two bearings is very important, and since the shaft is held rigidly, alignment of the engine to the prop shaft coupling is also. For the same reason, if you have flexible engine mounts a flexible element between the output flange and shaft coupling is also necessary (as stated by OceanSeaSpray previously).

[senormechanico]

Originally Posted by Hoohaa

Here some picks I found of the repairs showing a new strut and new tube. I would assume that all the bearings were replaced but will confirm tomorrow.
Attachment 134044
Attachment 134045


Attachment 134046

Attachment 134047

Maybe it's just my old eyes or the camera lens, but that last photo when blown up on my computer screen looks like the cutless bearing hull exit, the strut axis and shaft are not in perfect alignment with each other.

Edit: I just read post #136. Alignment or misalignment??

[Hoohaa]

Originally Posted by Sailmonkey

Holy schnikies that's a long skinny shaft!!! I'd bet what you're fighting is shaft whip!

Only known cures are an increase in shaft size or adding an intermediate bearing.


Sent from my iPhone using Cruisers Sailing Forum

Ok i rechecked the alignment , adjusted the height a smidgen and got a pretty good result. Reassembled everything and went for a test run.
No change. Its still the same.
But. The wobble occurred even as the shaft rotated very slowly while the motor was idling in neutral. So that's got to rule out the prop.
If you grab the shaft just in front of the stuffing box, you can move it a bit. Its not loose but it definitely has enough play in it to produce the wobble.
So I am starting to think that there can't be any bearing in the stern tube, otherwise it wouldn't be possible to move it. So maybe it is shaft whip.
I am taking her out on the harbour tomorrow. If the sun comes out I'm going over the side for a closer look..


Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[a64pilot]

I apologize if I insult, but don't go near the prop if its turning, even slowly

[jimbunyard]

It's not entirely clear to me how the occurrence at low speed rules out a prop problem but not shaft whip, but...


Though I've never seen one like it or this type of installation, from this picture



and this picture



it appears that the metal tube glassed into the hull is some kind of cutlass bearing.

Also, if the yard replaced two bearings, unless they somehow put two little short ones back to back in that strut, one of them is in the shaft log, or as above, and in the picture, is the shaft log.

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...

[fryewe]

Didn't read all the entries on this thread or watch the videos, but noted in a recent post that you said your shaft turns with the transmission in neutral.

Damper plate worn?

Could this contribute to your vibration?

[Sailmonkey]

Originally Posted by fryewe

Didn't read all the entries on this thread or watch the videos, but noted in a recent post that you said your shaft turns with the transmission in neutral.

Damper plate worn?

Could this contribute to your vibration?

The damper plate has nothing to do with shaft wobble, unless it's so bad it's throwing the rear of the engine around. A bad damper plate will be usually accompanied by a bad noise.


Sent from my iPhone using Cruisers Sailing Forum

[Hoohaa]

Originally Posted by a64pilot

I apologize if I insult, but don't go near the prop if its turning, even slowly

She will be anchored with the engine off. But thanks for your concern. More worried about the cold water.

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[Hoohaa]

[QUOTE=jimbunyard;2245227]It's not entirely clear to me how the occurrence at low speed rules out a prop problem but not shaft whip, but...

You are correct. Rules out the prop and I am clutching at straws trying to come up with an alternative.



Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[Hoohaa]

Originally Posted by jimbunyard

It's not entirely clear to me how the occurrence at low speed rules out a prop problem but not shaft whip, but...


Though I've never seen one like it or this type of installation, from this picture



and this picture



it appears that the metal tube glassed into the hull is some kind of cutlass bearing.

Also, if the yard replaced two bearings, unless they somehow put two little short ones back to back in that strut, one of them is in the shaft log, or as above, and in the picture, is the shaft log.

So normally stern tubes don't have a bearing?
Do you put a bearing back and front of the strut normally?


Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[Hoohaa]

Originally Posted by fryewe

Didn't read all the entries on this thread or watch the videos, but noted in a recent post that you said your shaft turns with the transmission in neutral.

Damper plate worn?

Could this contribute to your vibration?

She has a Hydraulic gearbox so i think its just fluid turning it very slowly. You can stop it with your finger.

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[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?

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[OceanSeaSpray]

Whether there is a bearing in the stern tube depends on the shaft length, diameter and RPMs... You clearly have what looks like a Tufnol bush at the outer end.

But how long is that tube in the hull and how long is the unsupported length from that bush to the coupling? It might be hard to tell as you are in the water and you didn't see the job getting done, but these things are important.

Stainless shafts are quite easy to bend if you get them whirling (and not that difficult to straighten either). Your shaft is clearly very thin and not supported well enough for its length and it could well be bent since you can see a wobble turning it by hand slowly.

In the end, you might just have to pull it out completely and then make a decision between supporting it at each end of the stern tube, moving the existing bush further up the tube, going to 1 1/4'' shaft etc.

My experience with these things is that you have to gather all the data and then check that you actually have an acceptable design/configuration before doing things, or you can be chasing your tail forever. You could discover a bent shaft now, straighten it and fix the problem just to end up in the same place again in a year's time.