Prop Shaft Oscillating - Not Bent

scott1956

Member
Oct 16, 2016
87
Lanoka Harbor NJ
Boat Info
2000 410 Sundancer
1999 Bayliner Trophy 1802
Engines
Caterpillar 3126
I have a 2000 410 Sundancer with 3126 CATS and Hurth V Drive transmissions. Over the winter both transmissions were rebuilt, Shafts checked and straightened, couplers refaced, props trued, cutlass bearings replaced and reinstalled with new engine mounts The port side shaft is moving or oscillating quite a bit. I also have what sounds like a prop "singing" only at idle speed in fwd. The mechanic checked the transmission flange, and engine alignment with the boat in the water there is no runout on a dial indicator. The flange coupling was .002 that opens to .008 when the shaft is rotated. Everything was rotated 180 degrees with the same results. This is the only thing that seems strange.

The yard hauled the boat pulled the shaft and had it checked. It is straight and true. I replaced both props re launched and still have the same problem.

Here is a video of the shaft starting at idle speed and going to 2400 RPM.
About 30 seconds in the camera vibration starts sync with the shaft so it looks like it smooths out but it is actually much worse.


https://photos.app.goo.gl/iQFgZdP4UqQJCoss9

There are no noticeable vibrations coming from the running gear and the engines are perfectly still on the mounts. Any thoughts are appreciated.
 
I have a 2000 410 Sundancer with 3126 CATS and Hurth V Drive transmissions. Over the winter both transmissions were rebuilt, Shafts checked and straightened, couplers refaced, props trued, cutlass bearings replaced and reinstalled with new engine mounts The port side shaft is moving or oscillating quite a bit. I also have what sounds like a prop "singing" only at idle speed in fwd. The mechanic checked the transmission flange, and engine alignment with the boat in the water there is no runout on a dial indicator. The flange coupling was .002 that opens to .008 when the shaft is rotated. Everything was rotated 180 degrees with the same results. This is the only thing that seems strange.

The yard hauled the boat pulled the shaft and had it checked. It is straight and true. I replaced both props re launched and still have the same problem.

Here is a video of the shaft starting at idle speed and going to 2400 RPM.
About 30 seconds in the camera vibration starts sync with the shaft so it looks like it smooths out but it is actually much worse.


https://photos.app.goo.gl/iQFgZdP4UqQJCoss9

There are no noticeable vibrations coming from the running gear and the engines are perfectly still on the mounts. Any thoughts are appreciated.
That is not good. That shaft will fatigue and fail and the seal bearing will suffer early wear.
If the face of the gear flange has less than 0.001" TIR then it is good.
Shaft was checked and good.
That leaves two things -
Engine alignment to the log and cutlass bearing isn't correct.
Or, the shaft flange is either bent or the key isn't installed correctly and not allowing the flange to fit the taper.
If you back off the flange bolts just enough to keep the two flanges in place but relax the clamping force, does the TIR on the shaft by the seal come within spec? That is less than 0.006"
 
That is not good.
If the face of the gear flange has less than 0.001" TIR then it is good.
Shaft was checked and good.
That leaves two things -
Engine alignment to the log and cutlass bearing isn't correct.
Or, the shaft flange is either bent or the key isn't installed correctly and not allowing the flange to fit the taper.

the .008 is a bit much for this size coupling right? I think recommendation is around .006 but we were able to get them much tighter (ours is the same boat)

I also went the @ttmott ’s recommendation of centering the shaft in the log with an insert then aligning the engine/transmission to the flange versus trying to work them both into position - it was very important to only move once component and made the job way easier

I’d see if you get them to realign on the hard with this method. You’ll want to check alignment again in the water after a week or so
 
Why were the coupler halfs refaced? Were they 'dinged' up during removal? Did the male/female pilot have to be 'cleaned up'? Are the flange faces now perpendicular to the shaft centerline? The coupler could be an easy place to account for your problems.

As for the 'singing', I hit a rock and had some vibration and 'singing', it seemed to be from the transmission itself. When the new props were installed the vibration and 'singing' were gone.
 
Wow, that's a lot. Coupler (cheap to check), then alignment as mentioned.

Prop singing is common, and can be a problem with new or reconditioned props, but easily fixed by grinding a little material off the training edge of the props with a file or grinding tool. I'd take them back to the shop that did the recent to get that addressed. Should be easy for them.
 
It sure looks like it is at the hub end. At idle it is out of balance and it doesn't get better from there as the engine spools up. As a practical question.....was it doing this before they worked on it or was the work done due to striking an object?
 
The problem was the shaft taper was not properly mated to the coupler. The coupler actually rocked slightly when the shaft was fully inserted. The port trans was replaced prior to my ownership. The port shaft was not original. I suspect it has been like this for a long time at least 10 years. (I purchased in spring 2022). The shaft taper was corrected and the coupler squared to the shaft on a lath. She is smooth as silk and not moving at all. I appreciate all of the suggestions and feedback.
Scott
 

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