Drive train Vibration - Alignment or Balance?

[gspeckma]

After replacing the cutlass bearing, dripless shaft seal, shaft zinc and prop shaft a year ago I experienced a drive train vibration but only at certain RPM/Speeds. I did my best to align the shaft coupling to the transmission flange when all was re installed so I am now wondering if perhaps this is a balance issue. I get a serious vibration at about 4 knots, but it goes away a 6 knots. Slower speeds (less then 4kts) everything is also fine. A friend suggested this sounds more like a balance issue vs. an alignment issue. He recommended putting a hose clamp around the shaft coupling and experiment with moving the screw part around on the coupling to see if this has any effect. I wont be able to do this until spring as the Boat is currently on the hard in upstate NY. Has anyone tried this, or are there any other suggestions to determine if this is a balance vs alignment problem? Obviously all the new components that were installed had not been balanced prior to installation, a lesson learned.

[Dave Neptune]

g, yes this approach can work if you stay at it. I have smoothed many an engine by placing extra washers under flywheel bolts hen moving them/it around until it gets better. You can rotate the prop in neutral while on the hard to see if it is bent past on either side of the strut~usually the prop end.

Can you see much of the shaft when running, if so it is probably a bend that will not smooth out well with "counter balancing"~straightening is the probable answer.

Note, I have only seen this a few times but a prop that has blades at a slightly different pitch can create a harmonic vibration at various RPM's and little or none elsewhere in the RPM range~~very rare.

Dave Neptune

[ndutton]

I hate what I'm thinking.

Is there any chance the aft lifting sling was positioned on the shaft, even slightly, when relaunching the boat after all the driveline work?

If it were me, while the boat is on the hard I'd remove the shaft and take it to a shop to check straightness. If it's straight it can be eliminated as a cause. If it's not straight, all the other stuff you might try will be futile. With the boat on the hard anyway the opportunity is now.

[gspeckma]

Thanks for the replies. The shaft was checked for straightness prior to installation, it was a new SS shaft, replacing the original brass one, so I don't think that is the problem. I also visually checked the rotation last summer, from in the engine compartment and under water with goggles, etc. and saw no wobble. The boat is taken out of the water using a hydraulic trailer so there should be no issues that might be associated with using a travel lift/sling. My plan is to recheck the alignment first, ( I can do that on the hard), then next spring experiment with the balance.

[ndutton]

Good luck to you. Please keep us updated.

[Cool Beans]

You should do the alignment in the water after the coupler has been disconnected for a couple days to let everything relax.

[Easy Rider]

You should do the alignment in the water after the coupler has been disconnected for a couple days to let everything relax.

I'm with Cool Deans with this one. An alignment check should always be done again after the boat has sat in the water for a few days.

[Easy Rider]

I'm with Cool Deans with this one. An alignment check should always be done again after the boat has sat in the water for a few days.

Pardon me Cool Beans. I didn't mean to misspell your handle.