How bent does a prop shaft have to be to see it?

[MolsonCanadian]

Little back story. Selling my Sea Ray. Potential buyers hired a surveyor to give them a "market evaluation". Works for me. Surveyor does his thing, then at the end says "the drive has taken a big hit, prop shaft it bent so much I can see it"

I could be wrong, but I'm pretty confident that if it was bent that much, I would have seen it, and felt it for the year that I used the boat. I'm going to put a dial indicator on it tonight.

Unless he has xray vision, I can't see it being that bent. Wouldn't a shaft that had a visual bend in it vibrate like a SOB?

 

[gm280]

Re: How bent does a prop shaft have to be to see it?

I would think that if he was able to detect a bend prop shaft that it would have caused much trouble as well. I'm waiting to see what your dial indicator shows... And if the dial indicator shows it is in tolerance, I would be giving the inspector a talk to also...

 

[Don S]

Re: How bent does a prop shaft have to be to see it?

Wouldn't a shaft that had a visual bend in it vibrate

Surprisingly no.

Take the prop off and turn the shaft, it's really hard to tell sometimes with the prop on.
Even a stick with a mark on it next to the shaft and stuck in the ground whileyou turn the shaft will pick up a bend that can't be seen by eye.

 

[Bondo]

Re: How bent does a prop shaft have to be to see it?

Surprisingly no.

Take the prop off and turn the shaft, it's really hard to tell sometimes with the prop on.
Even a stick with a mark on it next to the shaft and stuck in the ground whileyou turn the shaft will pick up a bend that can't be seen by eye.

Ayuh,... With the stick in the ground ya don't need to pull the prop, just zero in on the shaft's center....

 

[MolsonCanadian]

Re: How bent does a prop shaft have to be to see it?

Well we put a proper dial indicator on the shaft and the runout was 0.006-0.008. So he was full of crap.

 

[Fun Times]

Re: How bent does a prop shaft have to be to see it?

Depending what drive you have and what your actual dial indicator runout truly was, the numbers shown are pretty close to being considered bent according to the book.

GEAR HOUSINGS MR/ALPHA ONE/ALPHA ONE SS Pre-Disassembly Inspection,
1. Check propeller shaft for side to side movement,
as follows:
a. Position dial indicator on propeller shaft.
b. Push propeller shaft to one side and zero the
dial indicator.
c. Move propeller shaft to opposite side while
observing dial indicator. Without rotating
propeller shaft, reposition dial indicator and
check up and down deflection. A shaft
deflection of more than .003 in. (0.08 mm)
indicates one of the following:
• Worn propeller shaft bearings
• Improper propeller shaft preload

2. Check for a bent propeller shaft, as follows:
a. Rotate propeller shaft while observing dial
indicator. If deflection is more than .005 in.
(0.127 mm), a bent propeller shaft is
indicated.

Newer model Bravo & Alpha Gen 2 drives allow a little more room for tolerance vs older drives.
Check for a bent propeller shaft, as follows:
a. Rotate propeller shaft while observing dial indicator. If deflection is more than .007 in.(.178 mm), a bent propeller shaft is indicated.

 

[MolsonCanadian]

Re: How bent does a prop shaft have to be to see it?

Good to know, thanks Fun Times. From our, and the mechanic's oberservations, we'd surmise the bearings are a little worn. Regardless, it is not "bent so much I can see it just by turning the prop"

 

[gm280]

Re: How bent does a prop shaft have to be to see it?

WOW those are awful close tolerances for determining a bent prop shaft in my opinion. I would be interested in seeing how that same prop shaft would read on a true lathe and dial indicator myself. I would think you could get that much tolerance differential in bearings alone...and the further you get from the last bearing support the more pronounced the out-of-round tolerances will get too...