[Public-List] stern tube question

David Fay davidfay999 at gmail.com
Sat Feb 2 18:34:06 PST 2019

[Clay, I hope you don't mind my including your email to me (below) on the Public-List. I'm sure it will be useful to others.]

You raise a couple of issues that have been bothering me also. See my comments below.


> On Feb 2, 2019, at 7:38 PM, Clay Pass <clay.q.pass at gmail.com> wrote:
> I'm going to throw this out there for discussion.  When I got #449 I had to replace the engine.  I rang my hands quite a bit when I read the alignment procedure calling for .004" (don't remember exactly what the spec was but it was in the thousandths) or tolerances on the engine shaft flange mounting surfaces.  I just didn't see how that was going to be possible to measure.  When the engine was removed the shaft dropped loosely to the bilge with movement in the order of inches (because of wear in the cutlass bearing).  So there is no way to hold that shaft by hand or assume it is in the correct position to consider positioning the engine such that a .004" tolerance is achieved at the flange.

I think that might be why aligning an engine is so tedious and time-consuming. You have to align it initially very roughly by eye, then bolt the two flanges together, then measure the gap between them all around, then shift the engine on its mounts to make the gap more uniform, then measure again, and so on. It could take hours to get it to the 0.004" or whatever tolerance.

> In manufacturing it is about impossible to repeatably measure .004" on anything other than a slab (rigid) table and carefully calibrated instruments.  Considering that the shaft is held in place with a rubber bushing on one end then there is no way to align the other end to achieve flange to flange tolerances that tight.  Also consider the engine itself is mounted on rubber bushings.

Yet people claim to be able to do it.

By the way, not only is the cutless bearing lined with rubber and the engine mounts riding on rubber, but the stern tube is also floating in a flexible caulk (thiokol). Nothing is fixed to thousandths of an inch.

> I think it would be better to recommend that any time you remove the engine, that you also remove the cutlass bearing and get a true alignment at the business end of the shaft to exit hole interface.

> Theoretically, your flex coupling is supposed to compensate for small amounts of misalignment, so you might expect to see some small amount of wobble as you rotate the shaft in neutral.  However, I would still try to minimize this since you obviously have some resulting damage to your cutlass bearing from too much side pressure.

You might be right about expecting a "small amount" of wobble. But how much is small? I wonder how much I can ignore.

Yes, I think I did have some damage to the cutless bearing from the sideways pressure (that plus it was old and worn anyway). That's why I could move the prop shaft back and forth by 1/4" or so. And it's why I replaced the cutless bearing. When I dry fit the new cutless bearing and housing onto the shaft, I couldn't wiggle the shaft at all. But that doesn't mean it isn't loading one side of the bearing more than the other. To eliminate that, I need to align the engine perfectly (assuming the shaft is not bent or the flexible coupling damaged).

> Ideally, the engine with the shaft mounted can be considered one rigid unit.  What you really want is that the shaft exit the stern tube and be perpendicular to the mounting interface of the cutlass bearing.  Such that when you slide the cutlass bearing onto the shaft there is no side pressure on the bearing surface and it perfectly aligns with the mounting interface and bolt holes so that when you tighten it down you are not introducing side pressure.  Does that make sense?

Absolutely. I agree completely. It's how to achieve it that's the issue.

> The way to do that is to adjust the engine mounts until the shaft exits the stern tube in the center and makes it possible to mount the cutlass bearing as I said before.  All assuming that the shaft is true, and doesn't wobble when turning.  You will be surprised to see how little movement on the engine mounting bolts will greatly impact the shaft exit location because of the offset length magnifies movement.  What I did was somewhat snuggly tighten the rear mounting bolts (my engine is mounted on rubber pads all around) and I then used a car scissor jack to move the front of the engine from side to side in controlled manner while someone was watching the rear, to get the correct side to side position.  Then I tightened or loosened the front bolts to control the up and down motion until I got the shaft perfectly centered at the exit.  Slipped the new cutlass bearing/housing on the shaft.  Bedded with 5000, screwed it on the stern tube and the mounting holes perfectly lined up without causing any pressure on the bearing surfaces.  That's what you want in the end.

So you didn't bother to measure alignment at the flanges? You just eyeballed the shaft to center it in the stern tube and called it a day?

If that works, it would be a huge help to me because, with my flexible coupling in place, there is no way to measure the gap between the flanges. It's hard to explain but my gearbox flange is shaped kind of like a four-leafed clover and the bolts on the flexible coupling line up with the gaps between the leaves. So there's really no way to reliably measure a gap between the bolt heads and the gearbox flange, which is what the flexible coupling alignment instructions require. Apparently the instructions assume that your gearbox flange is round, which mine isn't.

With your method I can forget all about the gap between the flanges.

> Hope this helps.  You might consider doing away with the flex coupling and going back to a rigid mount.

I've thought about that. It makes alignment much harder, and provides uncertain benefit, at least to me. However, Beta Marine must think it is worthwhile because they include it with all their engines. It's not even optional.

Thanks for your help.

> Clay
> On Thu, Jan 31, 2019 at 4:12 PM David Fay <davidfay999 at gmail.com <mailto:davidfay999 at gmail.com>> wrote:
> Clay,
> Interesting that you mention shaft alignment because, when I removed my bearing housing, my shaft was not centered in the stern tube. I'm about to tackle that problem. If it is off-center but doesn't wobble, I guess I have an alignment problem. If it does wobble, either the shaft is bent or the shaft is not aligned properly with the gearbox flange.
> I think the problem began when I had a bad rope wrap around the prop shaft last fall in a squall. It didn't stop the engine but the rope melted from the heat. Since the rope was attached to the bow of the boat (a long unstowed bow line), it could have exerted some sideways force on the prop shaft when the rope tightented. I doubt it would be enough to bend a 7/8" stainless steel shaft, but what do I know.
> Complicating the whole matter is that I have a flexible coupling between the shaft flange and the gearbox flange. It could be that the flexible coupling was damaged and needs to be replaced. Or perhaps just re-aligned. We'll see.
> David
> Blue Teal #284
> > On Jan 30, 2019, at 9:59 PM, Clay Pass via Public-List <public-list at lists.alberg30.org <mailto:public-list at lists.alberg30.org>> wrote:
> > 
> > I would check to make sure the shaft is coming directly out of the center
> > of the stern tube and rotate it by hand to make sure it doesn't wobble on
> > rotation. 

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