STARTING Engine Alignment- shaft centered in sterntube?

[senormechanico]

While it's all well and good to align the engine/trans to the shaft while hauled out, things usually change when the boat is back in the water. It's supported by water, not stands.
When it's been in the water for a day or so, recheck the alignment, and don't be surprised if it's not as you set it. After all, the alignment should be completed when it's in the water !

[Paradox111]

From: https://www.cruisersforum.com/forums...as-232953.html

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Re: SigmaDrive coupling on Catalina's
"I replaced motor mounts and very precisely aligned the engine, however realize that as long as your motor is sitting on rubber mounts, it’s only aligned when sitting there with the engine off, engine torque will cause a misalignment, as will a rolling sea way or heeling.
I installed the Sigma drive more because I was chasing an unsolvable prop vibe issue, and as a secondary consideration of never having to do another alignment, but even with everything perfectly aligned and new motor mounts etc when we would surf slightly I would pick up a vibe that would come and go with each surge from a following sea."

Now as a Retired aircraft mechanic and helicopter test pilot I’m overly sensitive to vibrations, I hear and feel them that most people can’t even when I try to point it out, and the Sigma Drive does help reduce vibrations, even in a perfectly aligned system with new mounts.

So, is a Sigma Drive “needed”? Of course not, boats have done without them for longer than I have been alive, but I bet you could have said the same thing back in the day when soft motor mounts were introduced too, or fiberglass for that matter.

The real question in my opinion is it worth the expense to you for a nice to have, but not necessary to have item to reduce vibrations and noise.

ALSO:
https://www.cruisersforum.com/forums...ns-179781.html

[skipperpete]

Originally Posted by Bowdrie

That is only true if the Cutless is exactly centered and in plane with the stern tube.
You can center the shaft in the tube all you want, but if the Cutless does not have the same center and plane you end-up with what the OP spoke of, siezed up bearings and a wallowed bearing support.
The shaft must run "true" in the Cutless, what's between the Cutless and the coupling flange has to be adapted to that.
You can't use a stuffing box as a fulcrum to hold a shaft where you want it.

Under what circumstance would an outboard shaft bearing NOT be concentric within the stern tube, the OP made no suggestion that he had an outboard P bracket. If the shaft isn’t in the centre of the stern tube, its simply WRONG, the tube is the datum plane to which the engine must align to, (where a P bracket is fitted, its subject to the same alignment rule and needs to be adjusted accordingly….. not regarded as the datum point. For clarity, I do a lot of engine alignments on all sizes and rotational speeds of propeller shafts including Chockfast mounting where very high accuracy is required.

[sanibel sailor]

Originally Posted by skipperpete

Under what circumstance would an outboard shaft bearing NOT be concentric within the stern tube, the OP made no suggestion that he had an outboard P bracket.

If the fiberglass tube was not manufactured to close tolerances, if the hull was pulled out of the mold "green" and bent the tube during curing, if the hull has changed shape over the last 45 years are a few reasons that shaft would not be concentric that come to mind.

In my case, I have repaired the outer section of the stern tube by drilling it out and sleeving it. When I epoxied the new sleeve in place, I had the cutlass bearing installed in it and used the prop shaft inserted into the coupling which was mounted to the output flange to align the new sleeve and bearing. It was the best I could think of at the time, but I am certain the accuracy achieved would not impress a precise machinist. I think it is close; I cannot tell any difference moving the shift around in the tube, but it may not be quite perfectly concentric.

[skipperpete]

Originally Posted by sanibel sailor

If the fiberglass tube was not manufactured to close tolerances, if the hull was pulled out of the mold "green" and bent the tube during curing, if the hull has changed shape over the last 45 years are a few reasons that shaft would not be concentric that come to mind.

In my case, I have repaired the outer section of the stern tube by drilling it out and sleeving it. When I epoxied the new sleeve in place, I had the cutlass bearing installed in it and used the prop shaft inserted into the coupling which was mounted to the output flange to align the new sleeve and bearing. It was the best I could think of at the time, but I am certain the accuracy achieved would not impress a precise machinist. I think it is close; I cannot tell any difference moving the shift around in the tube, but it may not be quite perfectly concentric.

So is it safe to assume that when you installed the new shaft bearing (and bushing), the shaft was already centered in the sterntube at the inboard end……. By “attached to the output “, how could the engine alignment have been precise with no stern bearing.
Or is it the case where you installed the bushing/bearing assembly with no accurate reference to the propeller shaft until after the bushing was locked in place. Since the shaft had some sort of trauma involving the flogged out rear bearing and its only 1” diameter did it get checked for straightness during the procedure?

[sanibel sailor]

Originally Posted by skipperpete

So is it safe to assume that when you installed the new shaft bearing (and bushing), the shaft was already centered in the sterntube at the inboard end……. Or was the shaft flange attached to the gearbox flange?
Or is it the case where you installed the bushing/bearing assembly with no reference to the propeller shaft until after the bushing was locked in place. Since the shaft had some sort of trauma involving the flogged out rear bearing and its only 1” diameter did it get checked for straightness during the procedure?

The shaft flange was attached to gearbox flange. I used the shaft inserted into the flange to align the new sleeve/bushing. I did not pay attention at the time where the shaft was in relation to the inner end of the tube. Since the engine had not moved since I have owned the boat, I assumed the flange was a good reference point. Given that the bearing is only 4" long and has some flexibility, I wouldn't be surprised if the bearing did not center the sleeve perfectly in the tube.

And I did get the shaft checked. It is within .002", much less than the allowable .018" runout for a 3' shaft.

[skipperpete]

Originally Posted by sanibel sailor

The shaft flange was attached to gearbox flange. I used the shaft inserted into the flange to align the new sleeve/bushing. I did not pay attention at the time where the shaft was in relation to the inner end of the tube. Since the engine had not moved since I have owned the boat, I assumed the flange was a good reference point. Given that the bearing is only 4" long and has some flexibility, I wouldn't be surprised if the bearing did not center the sleeve perfectly in the tube.

And I did get the shaft checked. It is within .002", much less than the allowable .018" runout for a 3' shaft.

No worries, for future reference the propshaft centering is done with no connection to the engine. The engine isn’t that smart about its position, the propshaft knows exactly where it needs to be. If I saw .018” runout on my dial indicator between the coupling and the shaft seal I’d be unhappy but on vee blocks over the entire shaft length, less unhappy unless you happen to be using a Borg warner or other reversing gear without a reduction, high shaft speeds really show up minor deflections. .002” is fine!

[sanibel sailor]

To close the loop:
Engine alignment completed. Shaft centered in tube within 1/16”, flange aligned within .004”. This required considerable change in mount adjustment suggesting that alignment previously was poor perhaps contributing to the cutless/stern tube failure.
I think these are close enough as I am installing a Globe DriveSaver that is supposed to compensate for minor discrepancies.

[Wotname]

Originally Posted by sanibel sailor

..... I replaced one engine mount that was bad.
.....

To re-open the loop now your original question has been satisfied - what caused only one mount to fail?

What was the difference between the one bad mount and the other three(?) good mounts?

[sanibel sailor]

The nuts on that mount were seized.

[nmccubbin]

IF, and only if, the cutlass is perfectly aligned with the stern tube.
The shaft and the engine have no idea where the stern tube is, and don’t care.
What counts is the alignment of the cutlass with the shaft, and the shaft with engine. Of course, if the cutlets is installed perfectly, they will all align, but this author has some doubts due to prior damage to his cutlets mount.
The OP who suggested feeling for the sweet spot with the cutlets dry seem on the ball to me

[sinnerman]

BTW, you are checking the alignment of the shaft to the engine with the boat in the water -correct?

You can do a rough alignment in the blocks, but the final (true) alignment is performed in the water.

My two cents

cheers

[sanibel sailor]

Thanks for the replies.

Boat is ashore for a prolonged refit. Alignment will be rechecked after launch.

Tried to find the “sweet spot” but could not detect any difference. Of course, we are talking about 1/4” of movement with about a 2’ distance. That alone helped convince me that the minor off-center isn’t likely to cause trouble.

Incidentally, I just sent my MaxProp off to PYI to check dynamic balance. That’s the last remaining potential culprit that might have caused my cutless/stern tube woes.

[Frank60]

If using PSS, I'd recommend their pro shaft seal. More heavy duty and will hold up better in your situation.

[sanibel sailor]

Originally Posted by Frank60

If using PSS, I'd recommend their pro shaft seal. More heavy duty and will hold up better in your situation.

Not quite sure what is unique in my situation requiring the silicone bellows. The current one has held up 13 years without problems.
I already have purchased the refurb kit with standard bellows. The only other thing the pro offers is a shaft collar which is easily sourced and not really needed if the set screws are properly installed