Bent prop shaft, repair or replace

[Thistle]

The boat yard caught the end of the shaft with the sling when hauling out. Now we have a slightly bent shaft and tube. The shaft is 40mm x 72", and is bent close to the prop end. There is an internationally recocognized shaft company (France Helice) in Cannes that says they can fix the shaft, but I'm concerned, and want the yard to spring for a new shaft (1209Euro-vs-665Euros--$1.55=1Euro). Has anybody had a bent shaft reconditioned? What are the pros and cons? The engine guys here think I should get a new shaft even if I have to pay the difference, but thats mostly because the old shaft is 20 years in service.

Any thoughts?

BTW, Great site! Thanks, Ed on sy Thistle

[Pblais]

You would want the shaft within 0.001 inches of straight. Alignment of a bent shaft is a problem down the road. Someone good can align a shaft to with .0015 inches. If it isn't aligned and straight it will make a terrible noises and eat out the stuffing in the stuffing box.

If you are dealing with a reputable engineering firm then they may be able to straighten it properly. The fact that it is 20 years old may not matter except that it is now a bent 20 year old shaft. Had it not been bent we would not be reading the post. I think it comes down to how well the shaft can be straightened. People don't change out a shaft when it gets 20 years old so I'm not sure that argument is valid.

Doing a proper alignment is going to be the real issue. Not everyone that does this is also good at it. Specs are usually .003 to .004 inches but if you can get it better it matters. If it can be aligned properly it has to be straight or the person doing it should be able to tell it was not. Since you have a pro doing the straightening I think you can assume they will do the job right or tell you it can't be done. Any honest vendor would. Personally, I would be more concerned about the mechanic aligning the shaft. getting that wrong will cost you far more money. You might better spend the money on new engine mounts instead of a new shaft.

[Thistle]

Okay, point well taken on 20 yr.s not being an issue. We have an Aqua-drive, so normal alignment is not the real issue.

We also have a bent shaft tube that extends about 18" from the keel and carries the cutless. We (the engine/drive train shop--not a part of the boat yard, but hired by them) have to try and straighted that as well. If we get it right the shaft SHOULD slide right in the cutless and right into the coupler. The France Helice shop is huge ( FRANCE HELICES - CONTACT US ) and say they can recon the shaft to 'tolerances'.

So, the real question: If a good shaft shop says its fixed, do I trust it, not just to be straight, but is there a chance that the re-bending causes stress cracks and eventual failure?

Ed, Thistle

[Pblais]

So, the real question: If a good shaft shop says its fixed, do I trust it, not just to be straight, but is there a chance that the re-bending causes stress cracks and eventual failure?

Ask them how straight it will be. Seems like a good question. How they do it will matter. Not done right and the metal fatigue will cause it bend under load. I don't know how to do it but you can't just heat it and bang it until it looks good. I would assume the bend is actually quite slight and not bent into a circle. the shaft tube needs to be as straight as the shaft is so you are a bit stuck.


I would bet the shaft comes out OK but the rest of it may be the bigger problem. bending shafts is a some what common industrial job. A lot of machines need straight shafts that spin so that part probably can be handled. They remove the whole shaft and can do what they will. The shaft tube and the rest are not so easy since they are attached to the boat.

You may want to hold out for some limited warranty so you can splash the boat and load test it for a number of hours then recheck the tolerances. Once done you need to do something get feel you can trust it was done right.

[Therapy]

Originally Posted by Thistle

The boat yard caught the end of the shaft with the sling when hauling out. Now we have a slightly bent shaft and tube.

Any thoughts?

I would be soooooo PISSED!

AND,

I would expect everything to be replaced with new.

BECAUSE,

I have ruined a couple of other folks things over the years. I have always replaced with new.

BUT,

I have never been treated that way by others, ever!

SO,

Best of luck.

[Alan Wheeler]

Surely the yard has a Public Liabilty Insurance? I would be getting them to replace the shaft only because I also think that is the moral thing to do. But in regards to straightening the shaft, it should be simple and accurate enough. However, It is not a cheap job to do though especially factoring in freight and I would be suggesting the yard look at the differences between new and straightening.

[GordMay]

See ABYC’s P-6, Propeller Shafting Systems, at:
https://www.abycinc.org/committees/P-06.pdf

Take particular note of the tolerance tables on pages 4-6, & etc.