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Old 07-01-2022, 08:17 AM   #1
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Shaft Vibration Mainship 400

Need some help,
Mainship 400, single Cat 3126: We did a sea trial with an excellent surveyor. This was the first run after a new shaft, cutlass bearing, and reconditioned prop were installed (shaft replacement was a finding from the previous sea trial). The shaft vibration could still be felt when running at full speed. We could visually see the shaft seal assembly violently wobbling.
I find any information on the prop shaft assembly layout.
We put a dial indicator on the shaft near the shaft seal. We had 0.025" runout on the shaft however, when we place a little force on the shaft we had > 0.050" movement.
Does anyone know what the runout tolerance is on this type of shaft?
Is there a bushing in the haul of the boat between the shaft seal assembly and the cutlass bearing near the prop?
Any suggestions on where to go from here? We really like the boat!!
thanks
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Old 07-01-2022, 08:38 AM   #2
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Given that your prop, prop shaft and cutlass bearing was just replaced, unless some of this work was done wrong, there are only a few things left to check. BTW should be a few thousandths of an inch not the hundredths that you have.

Check the coupler, the prop shaft needs to be concentric and aligned with the transmission output shaft. That is about the only thing left that might be causing the vibration that wasn't part of the recent work.

Other things to check that should already have been done:

Make sure the prop is seated on the shaft and isn't riding on the key. Check the prop blades for alignment. Check the prop shaft for straightness. Check for shaft alignment.

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Old 07-01-2022, 08:50 AM   #3
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Quote:
Originally Posted by Hydrospud View Post
Need some help,
Mainship 400, single Cat 3126: We did a sea trial with an excellent surveyor. This was the first run after a new shaft, cutlass bearing, and reconditioned prop were installed (shaft replacement was a finding from the previous sea trial). The shaft vibration could still be felt when running at full speed. We could visually see the shaft seal assembly violently wobbling.
I find any information on the prop shaft assembly layout.
We put a dial indicator on the shaft near the shaft seal. We had 0.025" runout on the shaft however, when we place a little force on the shaft we had > 0.050" movement.
Does anyone know what the runout tolerance is on this type of shaft?
Is there a bushing in the haul of the boat between the shaft seal assembly and the cutlass bearing near the prop?
Any suggestions on where to go from here? We really like the boat!!
thanks

Something is obviously out of round, or specs, or wasn't balanced or adjusted properly. Suspect it's not the shaft, if new ... unlikely to be bent but easy to check with your dial indicator.

Was the shaft properly centered with the engine mounts? I suspect not, as something is wiggling. Check the engine mounts and check the alignment of the shaft where it connects to the transmission. There's 6 bolts that attach, loosen them enough to get a feeler gauge in between the shaft flange and the transmission flange. You want less than .003" difference between any of them. Can fix that with adjustment of the engine mounts. Easy to check.

I'd bet that .025" is too much and for sure .050! I don't show the specs in my book. (I have Yanmar, not that it makes much difference).

You've replace or overhauled most everything, but didn't mention the alignment to the tranny... I'd start there.

And, did the prop shop balance the prop and give you a paper on the specs of the balance?


The MS 400 is a great boat, worth the effort to salvage the deal. And the Cummins is a great engine for that boat. Every bit as good as the Yanmar.
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Old 07-01-2022, 09:55 AM   #4
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Spud
The purpose of a sea trial is to ascertain if issues of concern are found prior to purchase. Just guessing, but the current owner pulled the boat to “fix” things due to a grounding, prop strike or ??

As a prospective new buyer it is safe to assume the vibration is caused by one or several things from the motor mounts to the prop - about five areas. Why was the boat pulled and some of these areas fixed? Was it a insurance claim? Obviously the current owner is responsible for addressing the issues and making them right.

Good luck and be patient.
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Old 07-01-2022, 10:31 AM   #5
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Unfortunately all the work that was done is suspect. How do you know that it was done correctly? I would not buy the boat like this. Have the seller fix it before you buy and do another sea trial. Good luck.
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Old 07-01-2022, 06:08 PM   #6
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Check the engine mounts. I had a similar issue and only discovered the problem when I stepped on the engine (floor access only) when it was running, and noted a change in pitch. The rubber on one mount was split in the middle and you could not tell looking at it that there was damage.
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Old 07-02-2022, 08:55 AM   #7
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You have some good suggestions above.
One thing not mentioned. Was the shaft coupler faced off while connected to the new shaft?
Common practice is to do this to insure the face of the coupling is square to the shaft.
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