Vibrating shaft at slightly high revs.

[Three Sisters]

Do you have one of these ?

[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.
Photo 26-02-2015 9 47 29 am.jpg Views: 214 Size: 413.2 KB ID: 134044" style="margin: 2px" />
Photo 2-03-2015 3 15 28 pm.jpg Views: 202 Size: 410.6 KB ID: 134045" style="margin: 2px" />

[Hoohaa]

Originally Posted by Three Sisters

Do you have one of these ?

If its a feeler gouge or strait edge. Then I have both.

[GILow]

Originally Posted by Hoohaa

I am in the process of finding out from the people that did the work, if and or what bearings where replaced and why they put the flexible coupling into the system.

While you are at it you can ask them what the hell chewed the coupling edges up.



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[jimbunyard]

Originally Posted by jimbunyard

You are of course correct, it appears that it is manufactured by Polyflex, though I can't be sure either way without looking through the entire line of drivesavers and polyflex couplings.

However, the point is the same, these type couplings are not meant to allow for and absorb misalignment, as traditional flexible couplings are. For drivesavers, it is explicit in their literature that the primary function is impact absorbtion, for the polyflex, who knows, I couldn't get any technical information to load, but based on what they look like I will assume the function is identical.

Originally Posted by jimbunyard

...is primarily intended to avoid damage to the marine gear in case of impacts... It is not what is traditionally known as a 'flexible coupling', which are designed to absorb slight misalignments and vibration.




...you will not be able to align the engine without the fine adjustment provided by the adjusting nuts.

...the axis of the crankshaft and the prop shaft have to match, and since the shaft is held at the cutlass end, the engine has to be moved to accommodate it.

First off the, let's get these two oversimplifications out of the way. You can align the engine without the 'fine adjustment' provided by the adjusting nuts, it's just even more tedious and inefficient than with them. Indeed, I remember trying to align a pair of 8v53s with shim stock and giving up in frustration...

The axis of the propshaft and the crankshaft have to match only if the gear is straight through (which appears to be so in your case). More comprehensively, it should be said that the axis of the propshaft and the
output shaft of the gear should match.

Originally Posted by GILow

Well Polyflex call them simply "flexible disc couplings" on their web site and there's no literature on them that I could find either. In talking to them recently when replacing my coupling, they were at pains to point out that alignment still needed to be spot on so I assume they are not intended to manage alignment issues, just vibration and shock loads as you say.

Originally Posted by OceanSeaSpray

Look at the shadow of the shaft/steel coupling against the white background paint after 0:36 and you will see that the engine is shuddering just as well.

If you shake a spinning shaft that is not being held perfectly straight and true at its end - courtesy of the wonderful flex coupling - you can easily cause it to start whirling out of control from a certain speed up.
The seal is at the end of a rubber hose, so it offers no support at all and I would bet anything that the shaft is also floating in the tube at the inboard end.

So back to where I had started, this installation should NOT be using a flex coupling, because the engine is rubber mounted and the shaft is floating, only supported by the cutless bearing at the far end.
Make a hard spacer with a precise spigot, get rid of the junk coupling and it will all start running smoothly.

Alternatively, because there is such a long length of exposed shaft behind the engine, add a second cutless bearing at the inboard end of the tube, before the seal, and then you should leave the flex coupling in to absorb the motion of the engine.
If the unsupported shaft length was excessive (which we still don't know), then this would be the pathway to follow.

Either way, the installation is incorrect as it is, so there is no point trying to tune it. The better you align, the worse it will get actually.

Your eyes are better than mine. What I see is at .36 is a very slight intermittent engine vibration, that is not in concert with the much more pronounced, regular vibration at the seal end, and possibly a very slight regular vibration at the engine end that may be in concert with the seal end...which leads me to ask, 'which is causing which?'

Sorry for the redundancy in reference to the 'flexible' coupling, but I am trying to strongly illustrate that this is not a flexible coupling in the classic sense. It is not flexible in the ordinary sense of the word, any more than (well, slightly more than) a block of aluminum is flexible.

I agree with most of the rest that you say (minus the hyperbole and unfounded judgements about things we've not yet been informed of), but I can't agree with your assertion that, 'The better you align, the worse it will get actually.'

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

Oooh, what a long shaft you have...

Now we have some information to ask questions about. Is that a cutlass bearing that is glassed into the hull? If so, you most likely need a coupling that is more flexible. I'd also check with the yard to see how they aligned the two. If not, and the only cutlass bearing is in the strut, the hard, sacrificial drive saver type coupling, with proper alignment, should be OK. Or you can take it out and see if things improve without it.

I'd still be checking that prop for balance and trueness...

[Three Sisters]

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

Magnified them as much as the computer would go, and just hope it's the screen or optical illusion.

Looking at the angles of the stern tube and strut cutlass bearing, the two do not appear to be on the same axis.

Drawing lines at the stern tube exit and strut opening.
And referencing the photo of the stern tube construction.

Can someone magnify the photo and verify that that the two are indeed in alignment ????

[Cheechako]

One thing I notice from those pics is that is a big diameter prop for the shaft size. At least it looks that way. The forces on a prop are not equal from all sides as the hull interferes with water flow. Just scaling from the pic, if that's a one inch shaft then the prop is in excess of 20" diameter?

[DeepFrz]

Originally Posted by Three Sisters

Magnified them as much as the computer would go, and just hope it's the screen or optical illusion.

Looking at the angles of the stern tube and strut cutlass bearing, the two do not appear to be on the same axis.

Drawing lines at the stern tube exit and strut opening.
And referencing the photo of the stern tube construction.

Can someone magnify the photo and verify that that the two are indeed in alignment ????

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

[Cheechako]

Whoa... two cutlass bearings? Hmmm.... not sure I've seen that...

[DeepFrz]

I hope minaret checks out this thread.

[Hoohaa]

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.

[jimbunyard]

Originally Posted by Cheechako

One thing I notice from those pics is that is a big diameter prop for the shaft size. At least it looks that way. The forces on a prop are not equal from all sides as the hull interferes with water flow. Just scaling from the pic, if that's a one inch shaft then the prop is in excess of 20" diameter?

You ain't kidding. I believe it was you who brought up the 15% clearance between prop and hull (in this case skeg); here it seems more would probably be even better...

And I don't think I've ever seen a place where a 'V' type strut could have a better application, even if everything else is in perfect balance and alignment.

[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.


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[dfelsent]

If it is shaft whip move that zinc from the middle of the shaft (worst place) to right in front of the strut. Then try again.

[Sailmonkey]

Originally Posted by dfelsent

If it is shaft whip move that zinc from the middle of the shaft (worst place) to right in front of the strut. Then try again.

Good idea, or do away with the egg and put a donut just aft of the strut


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