Should I Straighten or Replace Bent Prop Shafts?

3

320Bob

Active Member
Nov 2, 2009
1,314
AZ
Boat Info
2012 Chaparral 267 SSX Sold
Engines
Boatless
Unfortunately this past weekend I tore up my underwater gear on a sandbar or other submerged object. The props are most likely bent beyond repair but I will problably find tomorrow if that is the case. However, the marina service manager said both prop shafts were bent out of alignment by .040" on the starboard and .025 on the port. I talked to my surveyor/captain and he recommended replacing over staightening. especially if the bend is at the tip of the shafts. He said in this case machining will only be temporary and they will deform back to the bend over time.

I have no experience what so ever in this area so I would like some expert advice as to what option I should take. All but $1,200 of the total repair cost will be covered by insurance but I may be required to go with machining if it is cheaper. Is my surveyor correct or not?
 
It would depend on the experience of the person doing the work - if they know what they are doing - follow their recommendation - some friends of ours grounded near a marina that they knew nothing about and they had the shafts "straightened" by them and then they had to be redone by someone else.
 
If you were boating on a Great Lake or the ocean, I would definitely replace them. Looks like you do mostly river or maybe small lake boating and shaft failure would not likely be life threatening. Given your locale and assuming you can get good advice from someone with experience in your area who has done this repair before, I could see going either way. I'm too compulsive however to repair a shaft. Personally, I would replace.
 
There are two issues:

1) can the shafts be straightened?

2) will they stay straight?

I can't see the condition, but the general answer to question one is yes.

The answer to question 2 is harder to answer. Because the shaft has bent, the metal has exceeded the elastic strength of the metal. Typically, once the elastic limit has been reached, metal can be bent, or deformed, permanently at substantially lower levels. This means that if the shaft was bent in an area that is under load, then the risk exists that the operating loads will re-bend the shaft.

That being said, somebody who has experience in this area really needs to see the shafts and make a determination based on the exact conditions.

I would also suggest that if you feel strongly that the shafts should be replaced, get the opinion in writing that the shafts can not be repaired. Provide a copy as part of your claim and the insurance company will probably prefer to buy the new shafts rather than run the risk of future damages caused by the repaired shafts failing in the future.

Henry
 
Henry Boyd said:
There are two issues:

1) can the shafts be straightened?

2) will they stay straight?

I can't see the condition, but the general answer to question one is yes.

The answer to question 2 is harder to answer. Because the shaft has bent, the metal has exceeded the elastic strength of the metal. Typically, once the elastic limit has been reached, metal can be bent, or deformed, permanently at substantially lower levels. This means that if the shaft was bent in an area that is under load, then the risk exists that the operating loads will re-bend the shaft.

That being said, somebody who has experience in this area really needs to see the shafts and make a determination based on the exact conditions.

I would also suggest that if you feel strongly that the shafts should be replaced, get the opinion in writing that the shafts can not be repaired. Provide a copy as part of your claim and the insurance company will probably prefer to buy the new shafts rather than run the risk of future damages caused by the repaired shafts failing in the future.

Henry
Click to expand...

Thanks Henry,
I remembered just enough of my undergraduate materials science course to have concerns about deformation and elasticity. I will wait until I get the estimate from the marina as to what repairs/replacements are recommended. My marina is recommending to the insurance company that the prop shafts be replaced due to the memory issue. The marina owner said that prop shafts are made of spring steel with a hollow core that contributes to the bend memory. He said that if this were an older boat he has the press in the shop to straighten them but not for my boat. Now I'll have to see if the insurance company will go along with his recommendation. If not, I'll pay the difference to have them replaced.
 
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Replace. Your surveyor is correct. A friend of mine elected to have one shaft straightened instead of replaced and he always detected the vibration on that side. A slight reoccuring vibration can really get on your nerves after a while - particularly when you know that it could have been eliminated by paying a few extra bucks! He always always worried that that vibration (which only he could really detect, I couldn't feel or hear it myself) was damaging his trans or engine or shaft seal or strut, etc., etc.

I wouldn't think twice, replace!
Warren
 
I would disagree with the surveyor's assessment based on the facts you presented.
.040- four one-hundredths of an inch....you can't even see that amount of deformity.

Jeez, it's not like you bent it halfway over.

I could certainly be wrong- but having straightened a shaft that was visibly bent before (at a cost of $70) and having had no problems with it- I say send it off to a QUALIFIED prop shop (I'm talking about a bigger outfit that deals primarily with inboard boats, not some guy that straightens outboard props by hand) and they'll tell you straight up if it can be repaired, or not.

You might save yourself a grand.
 
In all seriousness, if the OWNER of the yard that is going to repair your boat actually told you that propshafts are hollow, you REALLY should find another boatyard to do the work...

I forgot that this is an insurance claim...heck, if the adjuster OK's new shafts go for it.
Keep the old ones that aren't too far outta whack for spares.
 
tobnpr said:
The marina owner is an idiot.

BTW, you posted this in the diesel section...
Click to expand...

Sorry about posting in the diesel section, my bad. But my marina owner may or may not be an idiot but he seems to be doing right by me by requesting replacement over repair. I believe the alignment tolerance is 0.003 and .040 is a magnitude larger so I would rather pay out of my own pocket for new shafts than take a chance they might deform in the future. If I had an older boat or had the knowledge and skills to do the work myself, I might not feel the same way.
 
I just replaced the SB shaft on my boat. We didn't need to measure as the bent portion was right at the taper and clearly visible. My boat yard manager, who has earned my trust over several years, told me that SST if straightened will eventually bend again in the same spot. A new shaft was $425 and seemed like a good deal on a 21 year old boat.

Remember: There is never time, or money, to do it right but always time, and money, to do it over.
 
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