Shaft vibrating.

Hi All. First time posting. Had bent shaft which was replaced along with the transmission and all cutlass bearings, and motor mounts Still vibrating. Had prop re conditioned. Better, but still vibrates at 2000 rpm and above. I am at a loss. What else can it be ? The damage was caused either by tranny seizing or old motor mounts giving way. Shaft was bent pretty good. Broke all bolts on coupling. Did not hit anything. Thanks Paul

Did they do a proper alignment? Start there.

Welcome aboard. Start first at aligning the engine. But it may have also damaged the cutless bearing. But the engine alignment is the first and easiest thing to do.

Thanks guys. Yes boat was realigned

forgot to mention that

still vibrates

Did they lap / seat the prop to the new shaft taper?
Beyond that my next point to check would be condition of damper plate.

But did they do a good job aligning it? It is so easy to double check it. KISS principal. It is real easy to run past the easy simple stuff and dive into the difficult stuff. Go back and start over with the easy and most likely stuff and work your way up to the difficult stuff. It will probably take 10 or 15 minutes to check the alignment if it is in good alignment. Then you have a solid base to build on. Good luck.

I am assuming the shaft itself is straight. If not then it does not matter if all else is good.

The coupler between the shaft and the gearbox needs to be faced in a lathe so the coupler clamping face is square to the shaft centreline and the spud on the coupler face is not wobbling AND fits the recess in the gear output flange properly. Any excess slop here will allow/force the coupler parts to be out of line.

It sounds like the damage was bad enough it likely bent the strut. THey can be straightened. Was it checked, straightened and realigned?. Even with new cutless bearings if the strut is tweaked it will cause a misalignment.

Even if all is well with the coupler faces it needs to be checked to ensure there are no dings or scratches which can tilt the faces out of line. A fine stone used lightly will clean that up. Not to remove a lot of material , just any burrs.

Any burrs on the part of the shaft that the coupler clamps on must be removed/ dealt with. Just the burrs if any.

Put a dial indicator on the gearbox output flange and ensure it is not wobbling.w.

Of course the engine mounts, even though now new, will need an adjustment after some run time. Get them rechecked as they will settle with the weight on them now. Settling will throw the alignement off.
Also the hull itself will shift somewhat once back in the water so that needs to be done.

First would be to realign the engine mounts so each one is carrying as close to the same weight as the others.

Then realign the engine/shaft.

WOW Great info Thanks Guys. Shaft alighn done recently. 2000 all around. Was better after that but still vibrates at 2000 rmp. Tamd 40 b diesel. Star engine the prob. cutless bearings new.after damage. After trip home I put on a spare prop and had original recondtioned. Dove and put it back on. Seems tight. no wiggle. Will double check it. I dont know if it was lap/seated when they put it back on at marina on land. I see they put a new key in. Damper plate is newly replaced also. Shaft brand new. Coupler looks great centered and tight I will double check anyway Thanks. Struts are good.according to marina when they did job.Nice new coupler but I will double check for dings and scratches. Re alighnment was done after new motor mounts. same day. About a month ago. I cruise at 1900 on that engine and 2200 on port with no problem.

Thank you all. I will re check all. Will dive again see if prop or cutlass problem. Boating is FUN Haaaa. Thank you guys. Happy cruising !

_______Re alighnment was done after new motor mounts. same day._______

I think you missed my point. When were the new mounts installed, after splashing or while on land?
The new engine mounts will compress once the engine weight is on them meaning the alignement must be rechecked after a week or so once in the water. The hull will change shape also once back in the water.
What was perfect alignement on dry land will almost certainly change and must again be checked.
In your case from two reasons, the hull shape changing once back in the water
and the engine mounts compressing.

C lectric said:

_______Re alighnment was done after new motor mounts. same day._______

I think you missed my point. When were the new mounts installed, after splashing or while on land?
The new engine mounts will compress once the engine weight is on them meaning the alignement must be rechecked after a week or so once in the water. The hull will change shape also once back in the water.
What was perfect alignement on dry land will almost certainly change and must again be checked.
In your case from two reasons, the hull shape changing once back in the water
and the engine mounts compressing.

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Yup.

Hi Comodave. Yes all was done in the water.

When you say shaft was aligned, do you mean they pulled the shaft and did an alignment on the cutless bearing or just an engine alignment to the shaft? Since you put the prop on in the water it could be off a bit, maybe a bit loose.

Hi Comodave. The engine alignment was done in the water with the new shaft in. I motored back home. Had vibration (prop was on). Shut that engine down and motored home on one engine. Mechanic came to my home dock and adjusted motor mounts and re aligned the shaft. Took her for a spin on another day still felt vebration but less (1700 rpms) Got back took prop off another day had it tuned up. Was better but still vibrating at 2000 rpms. Mystery.

Well it sounds like you have done the simple things. Did they do a shaft alignment? Not an engine alignment but checking that the shaft is aligned properly in the struts up to the transmission coupler. It is a much bigger job so I would talk to an experienced yard and see what they suggest.

No one has asked whether the coupling includes a DriveSaver. Getting an accurate alignment with a DriveSaver in place is near impossible save blind luck.

Hi Comodave. Yes Shaft was also aligned while in the water.

Hi Catalinajack. I will double check that. Interesting. Thanks

something has obviously been missed or dismissed as good enough. it sure doesn't take much to cause vibration. even a small machining defect on the mating surfaces of the coupler or output flange can be a problem. very careful checking with precision measuring tools should reveal the issue. maybe enlist a different mechanic who might have a fresh perspective?

rocky6450 said:

Hi Comodave. Yes Shaft was also aligned while in the water.

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The shaft can’t be aligned in the water. They have to pull the shaft and sight through from the stern to make sure everything it lined up.

Hi Caatalinajack. There is no drivesaver.

Good point Bmarler Thanks

Got it ! May have to haul again. Thanks

HI COmodave. May have to haul again. Took them 8 months just to get to me here in Fla with Covid and lots of people buying boats. Crazy

If you can’t find and cure the vibration you could install one of these. I installed one and it smoothed out my driveline considerably.

https://www.bruntonspropellers.com/sigmadrive

Kewl ! Thank you cardude01

Comodave said:

The shaft can’t be aligned in the water. They have to pull the shaft and sight through from the stern to make sure everything it lined up.

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you would have hoped that when they replace a bent shaft they'd check the alignment. any hit that bends a shaft can surely displace a strut too.
disappointing if this turns out to be the culprit. easy to see how it could happen with yards being so busy though, shortcuts happen.

Thank you Bmarler. More will be revealed.