Drive train vibration Cat 3208T with Twin Disc MG 506-1

I have owned a Jefferson 46 with twin 260hp 3208T for the past few years and have always felt some vibration and of course most of the vibration is at the best cruising RPM (1500-1800). I measured both shafts and couplings with a dial gauge the port drive train was out 5 thousandths and the Starboard was out 20 thousandths. From what I have read this reading should not be more than about 8 thousandths. So at this last haul out for bottom paint I had the yard do the following on the starboard drive train.
Removed prop for balancing.
(It had no dings and was in very good shape with very little balancing needed).
Removed shaft for straightening.
(The prop end was 7 one thousandths out and the coupling end was 15 one thousandths out. It was straightened to 3 and 5).
Removed and refit shaft coupling.
(The coupling was found to be wobbled out and would not face up to the shaft. It was also found that the keyway slot on the shaft was not straight. So they cut a new keyway slot 180 degrees on the shaft, replaced and refaced a new coupling).
Replaced motor mounts.
(2 motor mounts were bad but all 4 were replaced).
Replaced cutlass bearings.
(Cutlass bearings were worn but not bad).
Removed struts and realigned.
(The struts were out of alignment by 1/8th inch they were lowered and realigned).
The boat was launched and 48 hours later the engine was realigned on its new mounts and connected to the new coupling.
After running the boat for the first time the vibration is still there and the shaft and coupling is still out 20 thousandths of a inch. Does anyone know what the problem may be or if there is anything else I can do? Or did I just through 4 grand out the window?

Well, it should be fairly straightforward to re align the engine to the prop shaft and reduce the misalignment.

Also is it possible that you have a deteriorating torsional damper? Usually these cause vibrations near idle. You can do a quick check to see if the vibration occurs in neutral. That is an indication of a damper problem.

David

djmarchand said:

Well, it should be fairly straightforward to re align the engine to the prop shaft and reduce the misalignment.

Also is it possible that you have a deteriorating torsional damper? Usually these cause vibrations near idle. You can do a quick check to see if the vibration occurs in neutral. That is an indication of a damper problem.

David

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What I see and feel is 90% of the vibration is at 1500-1800RPM. Do you mean if the engine vibrates in neutral I could have a damper problem? and would the damper make the shaft and coupling out 20 thousandths? Can you check the damper with out removing the trans?

A few times in the last 20yrs I have found the output flange on gears were actually not true. With shaft pulled back, put a dial indicator on gear flange and rotated. Sure enough, there was runout. Should be near zero or maybe 0.001" max total runout.

Runout on gear flange tends to magnify down the shaft line and will set up an annoying wiggle in everything.

On several others, it was as simple as some corrosion on the gear and shaft coupling flange. Cleaned up and problem gone. And faces need to be clean clean clean. Does not take much rust to kick things out 0.002" on flanges, and that can magnify two feet down the shaft to 0.020".

You should have done a vibration analysis before doing all that work.

You should get one now before throwing more money at it.

which engine causes the vibration? Were the props scanned?

Could it just be phasing of the engines slightly out of sync?

We did all that on our boat last year and there is still a some vibrations under power, however, I am told that engine vibrations will always cause a little of this. There is no way to have that much spinning mass not cause at least SOME vibration. I do sleep better knowing that it is FAR better than when the previous owner had it.

If I were you, I would get them to look at it again (or get a second opinion). I suspect Ski's assessment is the likely culprit, but there are plenty of others too. Our shop sorta specialized in this sort of thing and the prop and shaft shops we used all work together and a partnership to do these things for lots of the big sportfish boats here in NC.

Ksoulant said:

What I see and feel is 90% of the vibration is at 1500-1800RPM. Do you mean if the engine vibrates in neutral I could have a damper problem? and would the damper make the shaft and coupling out 20 thousandths? Can you check the damper with out removing the trans?

Click to expand...

I really don't think that your problem is the torsional damper. It has nothing to do with alignment. It is splined to the engine's output shaft and turns all of the time, neutral or in gear. So if it is going bad, it will vibrate in both modes. Rev the engine to where you normally feel the vibration but in neutral. If you feel the vibration then it may be the damper.

Then look underneath the bell housing. Often you can see bits of rubber that have fallen from a deteriorating damper. You do need to pull back the transmission to look at the damper.

I suspect that your problem is alignment, either coupler faces out of true as Ski suggests, or simply a bad alignment job after spending $4K. You have eliminated everything else from the coupler aft. Go back and fix the alignment before doing anything else, probably with a different mechanic.

David

With all that work of course the engine/shaft would have had to be aligned.

Did they let the new mounts settle at least a couple of days?
Did the allow the boat to set in the water overnight before alignment?

Ksoulant said:

Replaced motor mounts.
(2 motor mounts were bad but all 4 were replaced).

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Hi Ksoulant,

Could I suggest a further simple test which won't cost you anything. Take the boat for a spin somewhere calm and wide open, run up to the rpm where you get the vibration, and then while it is vibrating turn the rudder slowly to port and then to starboard. If turning the rudders stops the vibration, then you may want to look at the rudders and have them dropped out of their stocks.

I offer this suggestion solely based on my own recent experience. I had a whole lot of work done, very similar to you, following an - ahem - incident where I may have found the bottom of Moreton Bay... Anyway, after all the work I still had vibration, and then one calm I turned the rudders about 10 degrees to port, and it all went smooth.

Good luck!

H.

Yes, the above suggestions, and you should have them adjust everything again now you are back in the water, as everything settles once in the water.

Unless you hit something with your prop that should have been obvious, your shaft and alignment was bad from the start. Like most new builds I have seen, are. With the computer aided machine tools of today all of the shafting, couplings and so on should be close to perfect. Someone looked over your propulsion system and decided it was good enough.
I'm an old guy and been on or around boats and ships my whole life. When shafts and couplings were measured in my time is was done to the 10,000", not 1000". Shafts, keyways, couplings were as close to perfect at possible. I was taught and continue to make my alignments to within .002". More than that causes wear and vibration. On the West Coast, back when we had many ocean salmon trollers, it was found that any vibration hurt the fishability. So among trollers and their servicers, .002" became a kind of standard in the better yards.
I've seen several "inventions" come along to make alignment easy. Most (probably all) don't work. Besides bearing wear, misalignment is hard on the transmission and engine. In vessels going long distances, it shows in fuel usage.

Also, aligning engines is "easy" if you have the correct wrenches, set of feeler gauges and some patience. You will also likely need knee pads. Every comment above is a good one. Also, get into the rudder compartment and check for play in the linkage, it doesn't get lubricated as often as it should and loose bushings can cause a vibration, I had a vibration and thinking it was a cutlass bearing, got my diver to check. He said there was play in the rudder. I changed out the linkage gudgeon pins to fix it, plus it gets a shot of grease every year now.

Check the rudder stock stuffing for play and lubrication too. Mine was rock hard and smelled like a dead fish. You can change the stuffing in the water usually.

Lepke, how about a photo of your boat? Sounds really interesting.

To the OP. Does your boat have stabilizers?

Thanks! for all the responses!
Follow Up;
Well I met with the yard yesterday to discuss the issues and to check the work they did. We disconnected the couplings and checked what the yard was responsible for (from the coupling on the shaft back to the propeller). The run out on the face of the new coupling was .002 and the deflection on the coupling rim was .004. Then we checked the shaft deflection and it was .004. So all of the work done by the yard checked out as good.
The next check was the output coupling at the transmission. This is where it got nasty. We could not get an accurate reading because as we were attaching the dial indicator the coupling was moving (see video below). I was able to grab the transmission coupling and move it 18-20 thousandths. Going into this I never thought that this could be a possibility. Looks like we have a bad rear bearing without the seal leaking. Guys the next time you have vibration have the yard check the transmission coupling for run out and deflection. Anyone know of a shop in the New Orleans area that rebuilds Twin Disc Trans.

Capt.Bill11 said:

To the OP. Does your boat have stabilizers?

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No.

So far on this years winter trip (now about 200 engine hrs and 1100 miles), the biggest vibration contributer has been the alternator belt...after that is my rudder flutter....after that it is the sliding doors in a good breeze.


Most alignments are no where near what people claim them to be because of the guesses involved with weighing and raising the shafts...then the coupling mating.....blah blah blah....


Also I have read some respected expert opinions that alignment is a moving target as engine isolation mounts allow engine movement and some engines on softer mounts are way out of the "recommended" alignment allowances soon after throttle is applied. Even without that I have seen two respected mechs check each others work and disagree on the final alignment.


I am with Capt Bill on this as far as getting a vibe analysis to keep you from chasing the ghost.

Could the bolts holding the output flange to the output shaft be loose? Not sure if these have nut(s) or bolts.

Congrats on the find....

....lots of places it could have been and not always the obvious...but its the simle you always should check first and keep a black book of all the not so obvious problems we all have...