Shaft vibration

[Neil Gordon]

A buddy at the marina posed this one...

His shaft vibrates when the boat backs only. He's thinking engine misalignment, bent shaft or a worn cutlass bearing. I'm thinking any of those would be the same in forward or reverse. What do you think?

[Phil Shedd]

Neil

If your friend has a folding prop one of the blades may not be completely out when going in reverse. This would cause a inbalance and cause the vibration. Rotation is rotation a line vibration should happen in both direction of rotations.

Phil

[Parfait's Provider]

Might barnacles grow differently on the forward surface of the blades than the aft surface? Might that make a difference when the prop is pulling instead of pushing?

Then, there is the transmission which obviously has additional components in motion when in reverse gear.

[John D.]

In forward, the whole assembly is pushed forward. In reverse, it's being pulled aft. Does that cause slight movement? Is something rubbing in reverse that shouldn't?

[bill2]

I'm thinking along the lines of John D. Without knowing the particulars one could say that in general when there are axles or gears in motion there are what I've been taught are thrust washers/bearings/surfaces . So that being the case it could be the "slack" is being taken up when everything is pushed together but when being pulled apart the axles/gears are no longer being held to factory tolerances by the thrust washers/bearings/surfaces.

Anyways that's my $.02 and I'm sticking to it . . .

Good Luck