Bent prop shaft repair- will it be good as new?

[Onemoreproject]

My prop shaft might be bent and I am wondering if that is the case am I better off with a new one or should I try to have a machine shop straighten this one?
I’m wondering if a straightened shaft is weaker than a new shaft and might be less strong and susceptible to bending again from vibration, etc.

I look forward to learning about this from people knowledgeable about prop shafts.

[capt jgw]

No guarantee it will stay straight is what I was told when I bent one. You know when you bend a piece of metal back and forth it weakens and eventually breaks. That's called metal fatigue. When a shaft is bent then straightened that's two bends in the same spot. It's weakened and can bend again. Replacing it is probably not a whole lot more expensive than straightening it.

[wsmurdoch]

The shaft will have to be mounted and rotated between centers (perhaps in a lathe) and the eccentricity measured at several locations with a dial indicator to see if there is a problem. Straightening a shaft is a bit of an art, and I have seen it done with shafts much smaller and much much larger than the typical sailboat shaft. Force and heat were used to bring the shaft back into alignment. I have never heard of a problem with loss of strength, but I have had machinists look at a job and recommend replacement rather than attempting to straighten the shaft.

[Statistical]

It will never be the same. It might be good enough depending on your risk tolerance and usage. If this is a daysailer I would probably get a quote on bending it back. If you are sailing around the world time for a new shaft.

[markxengineerin]

If it has bent, then it has plastically deformed. The effect of that on future properties would depend on the material- do you know the alloy? Since no shaft is going to bend "from vibration", if it's bent, something else happened- probably prop collided with or became tangled in something, subjecting it to abnormal load. There is enough safety factor in these things not to worry about strength in normal operation. They seem to fail from corrosion before anything else, so I'd focus more on checking for that, especially if it's stainless.

The practical answer would be to evaluate the cost of a new one vs. how bent it is and how confident your shop is in straightening it for less than that, and to what tolerance, and if you trust them to measure it to that tolerance and tell you the truth. If you're not saving more than 50% by straightening and it's not over $1000 for a brand new one, my own decision would be to go new- and use the old one for something else. Decoration, as stock for small parts, festivus pole, etc.

[markxengineerin]

The shaft will have to be mounted and rotated between centers (perhaps in a lathe) and the eccentricity measured at several locations with a dial indicator to see if there is a problem.

Much easier method and the one I use- just roll it around on a granite plate and make sure a 0.0005" feeler gauge will not slide under it, at any location. This assumes the shaft is a constant diameter the whole way, which mine was.

[markxengineerin]

That's called metal fatigue. When a shaft is bent then straightened that's two bends in the same spot. It's weakened and can bend again.

Or is it just an additional cycle of work hardening? I think it depends on the alloy, but for the level of bend we are likely discussing, negligible impact on strength either way relative to the safety factor present.

[Bellinghamster]

Yes, they can oftentimes be straightened and be "good enough" for continued use. I'd hit a log with my 32ft Tollycraft back in college (1989-ish) and put a slight corkscrew in my shaft. Local shops said it couldn't be fixed but I had very little money and access to a shop with a lathe and press and plenty of time. With careful measurements, marking and pressing using a dial indicator to determine precicely when "yield" was met, repeat many times, turning slowly in a lathe between centers. I was able to get it true to better than .002 runout anywhere along it's length. Afterwards it ran smoothly behind a 327cu in (about 275hp) and 1:1 transmission (up to about 4000 rpm). I think it was 1" diameter shaft, but it was a long time ago.

A slower turning sailboat prop shaft should be significantly more forgiving.

Here's a table showing acceptable limits vs. length of shaft: https://www.pacificmarine.net/engine...nce-tables.htm

[Recy]

Originally Posted by markxengineerin

Much easier method and the one I use- just roll it around on a granite plate and make sure a 0.0005" feeler gauge will not slide under it, at any location. This assumes the shaft is a constant diameter the whole way, which mine was.

Interesting, where do you find such a granite plate.

[NYSail]

Bring it to a quality prop shop and let them tell you.

[RedneckRedcoat]

Originally Posted by NYSail

Bring it to a quality prop shop and let them tell you.

This is the best advise you could get in this thread !

We just had ours looked at and straightened, a new shaft is about $1k having it repaired a few hundred dollars and could last you a god while longer. If it’s so bad they will tell you as the don’t want the liability if it breaks and damages your boat. Whilst your there take your prop and coupler to get assessed and cleaned whilst your at it and you might as well change the cutless bearing.

[markxengineerin]

https://www.ebay.com/b/Granite-Surfa...47/bn_55189569
Any machine shop or prop shaft shop will have one of these- ideally the long dimension is as long as your shaft. I got one cheap at an auction, comes in handy in the shop for many uses.

[AKA-None]

What I learned some time back was to use v blocks and a dial indicator to check runout

[nwdiver]

Replace it and while it is out, replace cutlass bearing and realign the motor….. I also did my motor mounts at the same time.

[Cheechako]

Depends on cost. How much are prop shafts now days? How much to straighten? It must take a pretty good hit to make a permanent bend. I wonder how far you have to deflect a 1"-1.5" bar for plastic deformation...?