First partial engine alignment--some thoughts to share

We had some new vibration on our boat with the engine running, following installation of a new transmission by a diesel mechanic. I hoped it might improve on it's own once the transmission wears in a bit, but that didn't happen.

I read up on engine alignment, and debated whether to attempt it at the risk of breaking everything, as I hadn't done this before and am not mechanically inclined. I decided to proceed today, and achieved some success, so would encourage any anxious newbie to try this if they suspect their engine is not aligned correctly. The books said that if I could turn the drive shaft by hand, the alignment is probably close, so I didn't know if I would gain anything by trying to improve the alignment. As well, the diesel mechanic had told me that he got the alignment "very close, within about 4/1,000 of an inch". Also, David Pascoe, who writes alot about boats, says that trying to be too perfect about engine alignment is unrealistic as the cutless bearing is rubber and will allow movement, the engine will vibrate and shift a bit due to torque, throwing the alignment off, the hull flexing will affect alignment, etc.

I undid the bolts at the transmission/drive shaft coupling (4 bolts with nuts easily accessible). Though very tight, I was able to undo them with normal wrenches, without dropping them into the bilge. I moved the prop shaft back slightly by hand, then snugged it up again to the coupling, and then measured the space between the faces of the coupling at the top, bottom and both sides with an automobile feeler gauge in thousandths of an inch. The books say that differences of up to 0.001 per inch diameter of the coupling face are acceptable; my coupling is about 3" in diameter, so the tolerance on my boat shouldn't exceed 3/1,000s of an inch. The measurements were slightly different, but not as much as I had expected:
0, 5, 6, 4.

I mainly worked on the front motor mount on my universal 5416, as it is easily accessible, and the up/down adjustment is straight forward. It took many small adjustments of the nut to raise or lower the front of the engine just the right amount to achieve an equal gap at the top and bottom of the coupling. I found it important to raise or lower the bottom nut, and then also to tighten down the top locking nut--not doing so gave a false reading, as tightening the top nut slightly changed the position of the engine as well. Raising/lowering the front of the engine also slightly affected the measurements at the sides of the coupling, which I hadn't expected.

I was able to achieve measurements of 0, 2, 4, 3 thousandths. I was unable to shift the engine side to side as the side/aft motor mount nuts are almost impossible to reach with normal tools and the engine is too heavy to move by myself. The books say you can hit it with a hammer to move it, but there isn't any swing room, and I don't like the idea of hitting the engine with a hammer.

I decided that maybe I was close enough to be ok as the top/bottom were within 1/1000 and the side to side were within 4/1000, and replaced the nuts and bolts on the coupler, tightening them hard on opposite sides so they tightened evenly, and so they can't come undone.

I took the boat for a test run, and the vibration that had been quite pronounced at 5.5 to 6.2 knots was essentially gone. There is still a vibration at about 4.2 knots, but I'm told that all diesel engines have a speed/RPM where the engine harmonics make everything vibrate. But otherwise, the engine runs smoothly now with almost no engine vibration, and no vibration discernible when I put my hand on the transmission. I can turn the drive shaft by hand fairly easily, and it all feels smooth.

I would have liked to get the measurements perfectly even all round, but I didn't think that was possible; and I'm pleased that I got it closer than the professional diesel mechanic had done, and my annoying vibration is gone. I'm surprised that the small improvement in my measurements over how the diesel mechanic left it could make such a noticeable difference.--just a few thousandths of an inch!

So for those who haven't attempted this, and suspect their alignment may be a problem, I think this is one job that is relatively easy for an amateur to do.

Frank

Now you know why some of what Pascoe writes is bunk..

Seemingly little differences can mean big changes in vibration.

Some things the books won't tell you.

1- After making an adjustment re-tighten the motor mounts and wiggle the engine from the valve cover re-settling it in the mounts.

2- Please don't hit your engine with a hammer...

3- A 2X4 used as a lever can be used to move the engine side to side.

4- Dish soap can be used under motor mounts to make them slide on the beds easier for side to side. It will dry and become non slippery by the time you are done.

5- If a gap rotates around the coupling and does not always stay at say, the bottom, or top, it is a good sign the shaft is bent.

6- Every time you put the feeler gauges in between lightly push the couplings together.

7- Leave the coupling bolts in but loose.

8- Always do a final adjustment with the rig tuned and the boat having been in the water for a few days. You should always do a rough alignment on shore but will usually need to tweak it with the boat in the water.

9- When aligning out of the water be sure the shaft is passing though the cutlass in the dead center. This will insure that the shaft is aligned to the cutlass. The shaft when passing through the cutlass bearing must have "constant clearance" through the center over it's entire bearing length. After the shaft is centered in the cutlass check to make sure the shaft is passing through the shaft log in the center. I have seen many DIY's "go to town" on a re-alignment and not pay attention to the shaft centering and only to have the shaft slapping the shaft log under power later. If you get to 98% on land, and centered, you'll only need light tweaking once in the water.

Remember this entire procedure is predicated on the premise that the factory built your boat so the shaft log center lines up with the cutlass bearing center. If you are skeptical of this you'll need to confirm these two line up before doing anything. The shaft log may not line up with the cutlass and in this case you always want to align the shaft to the cutlass so it rotates perfectly centered in the cutlass, preferably with a new cutlass bearing, or only slightly worn. On many boats this means the shaft does not or can not pass through the center of the shaft log without major surgery. This can lead to other issues with stuffing boxes etc.. The engine should be aligned to the cutlass bearing firstly then the shaft log secondly but both are important as you don't want an off center stuffing box wearing the shaft either. On our boat the shaft log and cutlass bearing line up almost perfectly but this is not all that common.


10- If you have an exhaust hose or engine driven compressor belt connected to the engine that can "hold it" disconnect it while aligning so you don't have any competition fighting you. Once aligned re-connect these items in the aligned position.

11- A lot of shaft vibration on small dia shafts comes from shaft whip and not the actual small alignment issue. There is a rather long, bearing free, distance between the gear box and the cutlass and if the couplings are out, even a little, it will induce shaft whip or what I refer to as the "jump rope effect". If you really want to get anal about alignment do the following. Once the couplings have been aligned and the bolts are tight locate a dial indicator, set it up, and turn the shaft to by hand to be sure you have a true concentrically rotating shaft. If the couplings are not aligned the shaft won't rotate concentrically.

12- It should be noted that an improperly centered packing box, after alignment, can induce side loading of the shaft and can also cause shaft whip. Once aligned simply loosen the hose clamps and adjust the stuffing box to a "relaxed" and centered position around the shaft so it does not impart any side loads..

13- To help minimize shaft whip a balanced prop that has been lap fit to the shaft, a proper alignment and a coupling that has been fit and faced to the shaft are all necessary components of the mix. It is also important that the exposed space between the prop and strut does not exceed the diameter of the shaft it's self.

Thanks, Maine Sail, for this extensive additional information. The next sailor doing an engine alignment will have much more knowledge with which to approach this job.

I'm so happy to have gotten rid of the vibration that I had, that I'm hesitant to do anything further in relation to alignment on my boat, for fear that the vibration might return if I screw it up. But I'll think about what you wrote, and probably gradually build up the courage to "dabble" some more.

Thanks again!
Frank

Hi Frank,

Think of it this way. If you mess with it some more and go backwards, it's just "time" to get it back to where you started. You'll get pretty good at it after about the 5th go 'round

As True as it gets

Alignment done. Now : within a half a thousandth.
A mechanic friend did the main work with me acting as the uneducated helper. He was able to do a partial alignment last spring before we went on vacation but found that the two front mounts were just a little bit shy of enough room to move, within the cast ovals in the slots on them.
Last week he had time to return to this project. We raised the engine in the front with a short piece of wood for a pry and put some small blocks under each mount bracket. We only needed to move it vertically about a half inch so that the long SS lags could be freed and the mounts removed.
He did a bit of milling on each one of the 2 holes (enlarged toward one side) on each mount.

Couple days later we reversed the process and the alignment started over. Note: the forward spring tension on the bellows on the PSS seal means you have to put one feeler in just to keep the faces apart... and then work around with a second feeler gauge. Shaft was also rotated, lastly, to check for trueness. It is straight.

After getting it down to a thousandth, we tightened every thing down, double checked all, shook it around... and rechecked the flange... and found that he was within half a thou. He was very pleased and said that the last half thou was good luck.

When we ran it under load there was noticeably less vibration and noise. (We started out with it at five thou out to port, and double checked that measurement from last spring's recollection.)

I am really impressed with how much subtlety and experience is involved in this. Three years ago a yard mechanic claimed to have done a great alignment as part of the replacement of the old flattened/shot mounts with the new R&D mounts. The boat is now quieter than it was when it left the yard.

So, no news here except to say that really great work can be done with patience and experience.... about 40 years worth of experience...


Cheers,
Loren

Hi Loren,

Thanks for your interesting post. I decided to try to improve on my earlier alignment, as I was still a few thousands of an inch out (4, 3, 2, 0, if I remember correctly). The vibration on my boat had improved substantially but there was still some at certain RPMs. So I thought it might only take an hour or two to try to improve on it.

Well, it went from bad to worse (about 0, 3, 11, 6). I spent the past two days working on it, eventually moving the motor mounts laterally a bit. After much measurement, readjusting, re-measuring, I finally got it to better than before at 2,2,0,0 and decided to leave it at that, as I wasn't able to develop a strategy to get it perfect and was concerned about losing the gains I had made. I haven't been able to test it by running the boat, so don't know yet what effect the improvement will have on vibration. I hope the improvement is noticeable.

I learned a few things about alignment in this go-round. Firstly, it is important to take the coupler bolts out completely, as without that it is not possible to move the prop side flange back far enough to ensure that it is still aligned. Secondly, improvement sometimes requires adjusting two of the mounting bolts at a time--ie port side and front or starboard and port side, etc. to get the right change; just adjusting one doesn't always lead to improvement.

It took alot longer than I had expected, certainly much longer than last time. It is one of the tougher jobs on a boat, and one I hope not to do for a long time. But I am pleased that I was able to do it, and to get a better result than last time.

Do you have any tips from your experience that may be helpful if I do have to do it again?

Thanks,
Frank

Frank Langer said:

It took alot longer than I had expected, certainly much longer than last time. It is one of the tougher jobs on a boat, and one I hope not to do for a long time. But I am pleased that I was able to do it, and to get a better result than last time.

Do you have any tips from your experience that may be helpful if I do have to do it again?

Thanks,
Frank

Click to expand...

I cannot add much more, but perhaps you might try sacrificing a chicken... invoking the "spiritual" part of this tedious process!


Loren

Hi Loren,

Earlier today, when the process wasn't going well, I would have been prepared to sacrifice almost anything to make some reasonable progress. At that point, I undid what I had done and started again (after quite a few hours of work). After that, it seemed to go surprisingly well, until I got to where the measurements were 0,0,2,2; but despite some attempts, I couldn't make any more progress and was concerned I might lose the progress I had made. Especially as the "experts" seem to suggest that 1 degree per thousand per inch of diameter of the coupler is ok. I think my coupler is either 3 or 4 inches, so my 2 degrees should be ok.

I'm thinking though that the professionals must have a formula of sorts--ie. if the top and port side are 4 degrees different from the bottom and starboard side of the flange, do what? At times I found that a couple of adjustments were needed to achieve progress--ie. lower both the front and the port side mounts by one turn of the motor mount screw; doing just one didn't show the progress I needed. But this was very much a trial and error process on my part, undoing what didn't work and trying again. I can't imagine that the professionals don't have a formula.

If anyone can shed more light on this, I would be very pleased.

Frank