wobble or shimmy between 1500 rpm 2000 rpm

[Cinken007]

We have a 1990 Hatteras 40 double cabin, with Detroit Diesels (6v53ti) and Twin Disc transmissions. My problem is there’s a wobble or shimmy that starts at 1500 rpm and stops at 2000 rpm. It’s more evident on the starboard side, less evident on the port side. The props are new/reconditioned and the shafts have been checked with a dial indicator. The cutless bearings are okay with very little or no play. I’m uncertain about alignment and I’m assuming motor mounts are original. Any ideas of what you think I should check or do next is very much appreciated. Thank you, Bruce And Cindy Nelson

[rustybucket]

Do you get the vibration in neutral at the dock?

[kelpy]

Were the new props fitted and checked to the shaft taper during initial installation?

[Pascal]

I would check the alignment if it hasn’t been done in years

[motoryacht lover]

No expert but have been down the vibration shimmy road on 2 inboard boats. I had a very similiar situation on a 42 Bertram. You could see the ladder shake as it was getting on plane and then it cleaned up. After much to do it ended up being the prop bore on one side was not “true”. I think it was required machining to fix not just lapping with a compound. Also if the shafts were checked with a dial indicator while in the boat and hooked up to the gear I don’t think the results of test can be trusted. I think to get a proper reading you would have to pull the shafts and check them out of the boat. Good luck with persistence you will figure it out.

[Red Hatt]

Mine was the transmission. The bearing had gone bad.

[rustybucket]

A loose prop will also cause a wobble. Worth a dive under to check.

[dsharp]

Are you running a 3 blade or 4 blade prop? A 4 blade will tend to run a little smoother. The cutlass bearings will generally not cause a vibration. To check alignment you will need to unbolt the coupling and check the clearance between the shaft coupling and the flange on the reduction gear with a feeler gauge. You want less than .003. You don't have to use the .003 blade you can use a .010 and .013 or .012 would be better. Pull the coupling up tight with a pair of channel locks with the .010 gauge between the coupling. If you can't get the .012 gauge in at the points around the coupling you are good to go. Obviously there's a little more to it so you may want to let the yard do it if you have a good relationship with them. You may have to rig up an a-frame to lift the back of the engine for shimming. If you have the mounts that have the vulcanized rubber holding them together they can shear and let the engine sag.