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Old 19-05-2020, 13:18   #1
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Moving motor sideways to correct alignment

Hey folks,

Iím hopefully nearing the end of a long sagaÖ failed stern gear, new stern gear, weird vibration under sail, now a new propeller. Iíll spare you most of those details but Iíve discovered what I think is an underlying problem possibly at the root of all of this.

I had to shorten the prop shaft for my new folding Variprop. On reassembly, itís clear that the prop shaft is slightly off center relative to the transmission coupling, despite the alignment previously being checked and rechecked with a feeler gauge and reported to be good within .003". I can see that the prop shaft itself is off by about 1/16Ē of an inch to port (see pic). When the couplings begin to engage, they look great all around with a nice even gap, but with a slight sideways flex on the shaft. If I bolt the couplings together first THEN try to insert the shaft into its coupling, it wonít go in because of the lateral friction. I also noticed the shaft is harder to spin by hand when the couplings are tightened than if theyíre loose. I never had vibration while motoring but this certainly doesnít seem right. Iíve read that you can still have an alignment issue even when the feeler gauge looks good, so Iím thinking thatís what's happening.

I think what I need is to shift my entire motor slightly to port. On the bottom part of my motor mounts, there are slider holes that allow for lateral adjustment, but the aft starboard mount and the forward port mount bolts are already maxed out to one side (see pics and drawings). If I shifted the front of the engine to starboard and let the back of the engine pivot, it would move the transmission coupling to port as desired, but I think it would make the actual alignment worse since it would twist the face of the transmission coupling away from the shaft coupling on its port side.

(Assuming you followed all that) What do you think my course of action should be? I could move the front of the motor as I described see if it helps. Or I could widen the slide-holes on the motor mounts slightly to allow the back of the motor to shift over a little further to port (not easy but possible), and maybe move the front of the motor to starboard as well if needed. Or I could fill and redrill the threaded bolt hole in the fiberglass mounting structure to give me a better range of motion (also not easy but possible).

Before I attempt any of these things I thought it wise to consult your collective knowledge. Is it indeed possible to have a coupling that appears to be aligned when the shaft itself being flexed? Do you have any experiences or advice you could share? Thanks a lot in advance.
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Old 19-05-2020, 13:29   #2
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Re: Moving motor sideways to correct alignment

What would happen if you rotated the maxed out mounts 180 degrees, would that give you room to move the engine over to align it? maybe in the past someone mixed up the orientation and misaligned the engine/shaft/mount arrangement.
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Old 19-05-2020, 14:02   #3
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Re: Moving motor sideways to correct alignment

Hmm thatís an interesting solution I hadnít thought of.
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Old 19-05-2020, 14:10   #4
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Re: Moving motor sideways to correct alignment

Maybe in all the other stuff you allude to at the beginning of the post the shaft moved sideways a bit?

I would just take the bolts out of the maxed out holes and try it out. If you shift the whole engine to port (and that sounds like it is needed from the description) and it all works then you can look and see how much meat would be left if you slightly enlarged the slots, or look and see where a new bolt hole would land, then decide on which approach to take based on how it all looks.
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Old 19-05-2020, 14:22   #5
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Re: Moving motor sideways to correct alignment

Also a great point. I might as well figure out the ideal position of the motor before committing to a plan.

As for the cause, my best guess is that the mechanic who installed my stern gear last year (shaft log, cutlass bearing, stuffing box and new shaft) may have put in the shaft log slightly askew, then moved the motor as much as he could to correct it and decided it was ďgood enoughĒ when we gave it a shakedown and there was no obvious vibration. Itís also possible it was like that since the motor was installed in 2013, before I owned the boat.
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Old 19-05-2020, 19:28   #6
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Re: Moving motor sideways to correct alignment

What you show and describe is VERY serious misalignment. It is certainly not "good enough." It is bad enough it WILL destroy your transmission seals and bearings, and other expensive bits of your drive system.

Is it possible that the shaft was bent?

Here is the best way to align a prop shaft. It is probably a lot more detailed than what you have seen before, but it is also the most comprehensive and accurate way of doing it. If you follow the procedure carefully, it will also tell you if your shaft is bent, or there are other problems that need to be corrected that don't involve moving the engine.

http://marinegears.com/pdf/GetInLine.pdf
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Old 19-05-2020, 20:16   #7
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Re: Moving motor sideways to correct alignment

Before you go crazy.....

Ensure that the shaft is centered in the shaft log...the packing gland could be clamped onto the shaft log off center, pushing the shaft off-center to the motor.
This will be more pronounced depending on how much free shaft you have.

A method I’ve used is to cut some thin plywood the right size to support the shaft, with a v-notch in it, and glue the ply to the hull with hot glue. Locate this ‘stand’ correctly by taking the stuffing box hose completely off the shaft to ensure the shaft is centered in the log. (Both horizontally and vertically)

This becomes more complicated if the boat is in the water, of course ;-). If so, butyl, modellers clay, silly putty, whatever, where the shaft enters the boat will minimize intrusion.

Getting the true center of the log is extremely important, but bear in mind that other factors could be throwing the shaft off. If you have an external strut/bearing that’s kicked to one side, you could be inducing a curve in the shaft that continues inside the boat.

Once you have that v notch holding the shaft correctly, rotating the shaft 180 degrees will tell you if the shaft is bent - the offset will move to the other side (this is called runout). Once you have the shaft properly aligned, you can reinstall the packing gland hose, and after aligning your engine, you can remove the ply with a bit of force.

Good luck,

Matt
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Old 19-05-2020, 20:28   #8
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Re: Moving motor sideways to correct alignment

Two options I can see. The most "correct" would be to drill and re-tap new holes in the bed as required. This is probably impractical for a number of reasons. The second idea is to cut the "bridge" out of the maxed out side of the affected slots. This will be easier than just trying to lengthen the slot if you don't have the correct tools. Cut a suitably sized piece of nice thick flat bar with a suitably drilled hole to act as a clamp down plate (and hide the slot!). Use this in place of a washer.
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Old 19-05-2020, 20:50   #9
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Re: Moving motor sideways to correct alignment

The shaft was new last year, I donít think it is bent but I guess itís possible. The shaft is out of the boat right now but Iím ready to put it back in when I get this figured out, so I could test its straightness first. The boat is out of the water too and the stuffing box is off, not the whole hose but the outer nut with the packing. There are no struts besides the bronze shaft log/stern tube (new last year), since itís a full keel with prop in aperture. The cutlass bearing is also a year old, holding the shaft snugly in place (albeit seemingly at the wrong angle). Since I had the work done last year I cruised from NY to Canada and back with no perceptible vibration under motor and only a slight flutter felt while sailing above 5 knots. At first I attributed it to turbulence off the deadwood onto my 3-blade prop but also suspected alignment so was paying close attention.

I havenít worked on these parts of the boat much before, but I think the angle of the cutlass bearing is the whatís wrong and moving the motor to match it is the solution.

I will read that alignment procedure carefully though, thanks.
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Old 19-05-2020, 21:03   #10
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Re: Moving motor sideways to correct alignment

Quote:
Originally Posted by Reefmagnet View Post
Two options I can see. The most "correct" would be to drill and re-tap new holes in the bed as required.

Both good ideas, but to be honest filling the old hole with marine-tex and drilling&tapping a new one an inch over sounds a bit easier to me than the plate option.
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Old 19-05-2020, 21:09   #11
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Re: Moving motor sideways to correct alignment

Correction: I had the work done two summers ago and did the Canada trip last year. Jeez time flies
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Old 19-05-2020, 22:20   #12
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Re: Moving motor sideways to correct alignment

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Originally Posted by OrangeCrush View Post
Both good ideas, but to be honest filling the old hole with marine-tex and drilling&tapping a new one an inch over sounds a bit easier to me than the plate option.

It would be my preference, for sure if you have the access. My line of thinking was that perhaps the engine would interfere with the drilling and tapping. whereas the slots can be cut out with a Dremel if need be if the any significant boat yoga was involved.
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Old 20-05-2020, 00:02   #13
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Re: Moving motor sideways to correct alignment

Have you checked the coupling faces?
They can bend if bolts are tightened before they are in alignment.
I’ve seen transmission side bent in shipment, and from rough handling on the shop floor.
If the faces are OK, then you gotta do what you gotta do to make correct alignment happen.
Side to side alignment is every bit as important as up and down, and I’m assuming that you don’t care to start over and reset the stern tube/bearing correctly.
Correct being a precise metal sleeve in place of the cut less bearing during installation.
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Old 20-05-2020, 05:50   #14
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Re: Moving motor sideways to correct alignment

The coupling faces look extremely flat and parallel as they come together. The misalignment is most obvious with the shaft coupling off, as in the pic.

I canít get back to the boat for a few days but Iíve got the shaft with me here. Does anyone know how I can reliably check its straightness without specialized tools? When my packing gland was first installed they packed it badly and it kept heating up which I suppose could have warped the shaft (although it looks perfect). Since I cut a half inch off the front, the keyway is shorter now so I was thinking of taking it to a shop anyway to have a new keyway cut.

And yes, removing and straightening the stern gear would be my last choice in terms if access and difficulty. The easiest (not cheapest) solution would be replacing the shaft if itís bent, followed by rotating the mounts 180 degrees (if possible), followed by a tie between widening the slots or retapping the mounting holes.
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Old 20-05-2020, 09:15   #15
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Re: Moving motor sideways to correct alignment

I would suspect the stern tube (“new last year”) as the culprit. Since boat is on the hard:
1) have the shaft & coupler checked/trued by a pro shop
2) use laser guide to ensure stern tube orientation hits center of transmission coupler within adjustment tolerance of slotted motor mounts
3) replace stern tube using laser guide if 2) not possible
Probably better to start with correct stern tube orientation rather than chasing the mounts excessively.
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