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Mystery Vibration

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CaboJohn

Regular Contributor
Boat is a 2002 Pursuit 3000, twin Captain's Choice 5.7 engines, Hurth ZF's, dripless seals, 500 hours, located in San Jose del Cabo, Mexico. Last time down I noticed the starboard prop was slightly bent backwards at tips. It looked like the boat backed into a mud bank or something while running. All three tips, no evident dings. Since we are all rocks here (and I have never run aground) it had to happen several years ago in Florida before the boat came down. I had the marina send the props out to be worked on. An American has a prop shop here and supposedly does really good work. They also recommended changing the cutlass bearings as the starboard was a little loose. Their homemade push out device for the bearings did not work and eventually they pulled the shafts to get at and cut the old bearings out. They put everything back together and on the test ride I noted the boat was now silky smooth at low rpm but at about 3000 a vibration develops. It is quite severe up until about 3400 rpm when it greatly moderates or goes away altogether. I also noted when idling in that there is a high pitched squeal on the starboard side. It starts at about 1000 rpm and increases with engine speed until at 2000 rpm it is too high for my ears (they give out at about 7-8000 cycles) to hear. I have heard that new bearings sometimes do that until they "break in", but I don't have any experience in that department. They pulled the boat to make sure everything looked clean then put it back in the water and aligned the shafts to the transmissions. The port was a little out, starboard looked good. Took it out for a test ride again and found that the vibration was still there as before. Shutting down alternately showed it definitely was coming from the starboard side. They are unsure as to what to do next. They are particulary mystified by the fact that all seems smooth at lower rpms and then it comes on suddenly as engine speed increases. They feel that balance or bent shaft problems will be noticable over a much wider range. I know dynamic problems can sometimes have strange interactions but don't know if this sort of vibration is common or unusual.

I haven't scanned the engine (yet) but it seems very smooth and will run to 4950 at WOT so it is still strong. In the absence of better ideas we will likely send that prop back to the shop to be checked, then pull and inspect the shaft and seals on that side, and then ??. Doesn't seem like much of a plan. Any suggestions or ideas are welcome. Thanks.

CaboJohn
 
Is it possible that they did something to the strut when they did the work or installed the cutlass bearing wrong? I would also check the shaft for being true just on principal.
 
If the props were done using the old blocks method (not digitally scanned) one or the other is suspect.

the shaft should have been checked while it was out...since they had an issue with their push out tool, damage could have been done during the futile part of that exercise...

Engine mounts could also be contributors, especially if other items have tightened up some..

Bearings usually are water lubricated so the "break in concept" is an old wive's tale...the high pitched squeal, to me, suggests a misalignment between the bearing and the shaft...

Misalignments and other issues can be present over very small RPM or speed ranges or be quite broad...in my limited experience
 
I've done a dial indicator on the shaft end and also mid shaft while installed. If those are true, that should eliminate the shaft itself. These can be tough to debug. No spare props?
 
Thank you for the responses. I talked again to the yard and they said they removed the starboard prop to have it checked. Then they put the boat back in the water and ran the engine up with no prop attached. They report the same vibration at the same rpm is present. I am a little surprised that would be the case with no load pushing on the shaft but since the yard really wanted it to be the prop (other guy's problem) I believe them. So for now we have ruled out the prop. I did ask them to look at the motor mounts and specifically to see if the motor is jumping around when the vibration starts up. I concur that a mount could have been bad for a while and it only shows up now and makes the vibration worse (the cause of which still must be found). Yard has suggested balancing the shaft and flanges. I have suggested taking them apart and seeing what they did that caused the issue...... I don't know how these things are keyed. Is it possible to put together 90 or 180 out and create a balance issue on something that was previously good??

And what about "balancing". I don't see how that is necessary or possible in a shaft. Are the flanges really ever balanced? How is that done? And a shaft of that length is going to look like a piece of spaghetti to a dial indicator. Do you use lots of V blocks and try a look for a bend? Lots I don't know.

The homemade bearing remover was a pretty good version of the Strut Pro and I doubt it did any damage as it simply pushed on the bearing and the housing with no load on the shaft or strut itself. And the mechanic pointed this out and said it was critical that he didn't bang it with a hammer as that is how struts are bent or loosened. I was pleased to hear that. So I suspect it was just a case of the bearing being tight, and there are instances on the net of people commenting the "real" Strut Pro would not get the bearing out. But I would be interested in any contrary opinions to what I just said.

Having said all that, there is another troubling observation. When they went to pull the shafts, they noted that the port shaft just cleared the rudder post so that the rudder did not have to be removed. The starboard shaft would not quite clear so they dropped the rudder. Pursuits are pretty well made so unless one believes the struts and rudders were misaligned at the factory (could be but unlikely), one could suspect that the strut on that side has been bent or shifted somehow. If so, it has worked great for two years until this work was done. Why would it be an issue now? What do you think??

I will let you know what happens next. I plan to go down tomorrow morning and take a look. And I will pass on any more pearls of wisdom from the forum. Weather is good but I would like to be fishing.

CaboJohn
 
Duh... You are right as always, Chris. If the load (prop in the water) can be taken off and the problem still shows itself, then isolating it should be a snap. We should find it tomorrow. Thanks.

CaboJohn

PS How do you check strut alignment? Just kind of bore sight it up to the hole?
 
I suspect there is an alignment problem. Was the propshaft alignment done after the boat sat in the water for a few days (or on land)?

Jeff
 
Might also consider the rotor on the dripless shaft seal. Make sure it was properly reinstalled. Also a seal without cooling water will squeal.
 
CaboJohn said:
Duh... You are right as always, Chris. If the load (prop in the water) can be taken off and the problem still shows itself, then isolating it should be a snap. We should find it tomorrow. Thanks.

CaboJohn

PS How do you check strut alignment? Just kind of bore sight it up to the hole?
Click to expand...

Well, when doing a strut replacement I am lucky enough to have access to a lathe and a Bridgeport. I will turn a solid bushing to push up into he stern tube and slide the shaft into place then adjust the strut accordingly. If you have an piece of cutlass that is the same size as your shaft and stern tube you can use that. Cut it in half length wise and slip both pieces in on the top and bottom.
 
Just visited the boat. The yard has pulled it out of the water and removed the prop, shaft and flange. All have been sent to the prop shop for inspection and fix if needed. They say he is equipped to do this. Their current theory is that the shaft is slightly bent. I have some difficulty with the translations, but think they believe they can see some wobble at the seal when hand rotating the prop. They think it has always been there but the tight cutlass bearing will no longer allow wobble at the prop end so it tries to wobble the other end. They agree it would have been good to run an engine test after unbolting the flange as Chris suggested, but cannot do it right now as the boat is on the hard with a hole in the bottom.

I cannot say for sure that there was no vibration at the 3000 to 3400 range before this all started as there was some at all rpm because of the bad prop. And we usually ran below that rpm to the fishing grounds and fish at 1700, which even today runs smooth. I really wish my lame surveyor in Tampa would have caught the prop. Anyway, nothing to do now except wait.

I did look at the flange on the other prop shaft. I have not taken one apart so I am not sure how it and the shaft go together. There seems to be several square headed bolts going into the body of the flange to the shaft. Is there a key way in there somewhere? Is it possible to assemble it somehow off center?

As for the bent strut theory, the mechanic says the shaft slides in easily though the cutlass bearing and into the prop tube and seal, and seems exactly centered. Not sure how that squares with the bent shaft theory. But I will come back to that if we continue to strike out elsewhere. I did try and move the strut and it seems solid to the boat.

Thanks for any help.

CaboJohn
 
Turns out the prop shop is located just off of the parking lot of a great bar (Latitude 22) so it was a nice visit yesterday. They checked out the prop, shaft, and flange and decided the flange was bent out of square by .010". Since we are trying to get the flanges lined up to less than .004" this is significant. They turned the face of the flange to make it square.

Back at the yard (before I got there from the bar) they mounted the whole thing, launched, fired it up, set sail and pronounced the vibration still there but greatly reduced. When I arrived they had decided to try and align the whole thing one more time. First I had them run the engine without any flange/shaft connected but with the transmission in gear. It ran up with no vibration, so all is OK from the tranny forward. We then started the alignment and found the flanges were off set vertically and horizontally by about an 1/8 of an inch. I could lift the prop shaft flange and push it sideways to make it line up but I don't think that is the whole idea. So they started in on the motor mounts and found one of them frozen (right rear). So obviously when they said they had "aligned" it several days ago it was BS. Finally took out the motor mount to see if it could be freed but it is trash. A new one will take more than a week to get down (we just missed today's once a week truck).

I am reasonably sure now the vibration is just an alignment issue (as predicted by Mark and Jeff) but it will take a while (and many missed fishing days) to get it resolved. The flange was likely bent when they tightened it up without really aligning it (because the mount was frozen). For all I know, the straightened flange is bent for the same reason. Ah, Mexico! On the other hand, I got in some golf today and we are having fresh shrimp and red snapper tonight with friends at home (locally caught but store bought) so there are compensations.

And this will give me time to start on the usual list of other small boat things that must eventually be done.

Thanks for the help, I will let you know when it is finally resolved.

CaboJohn
 
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