How does a prop shaft break?

[THECUSCUS]

Discovered my prop shaft sheared off. We motored for about a half hour, no abnormalities . Then sailed about 15 miles, dropped sails and started engine, put into forward and no go! Had a powerboat friend tow me back to mooring ball, where I dove in and verified there was no prop. Got hauled out next day. I find this very odd. Boat is a Pearson 35, which is a full keel, keel hung rudder which has a cutout for the prop. I have not seen the shaft in person yet, but was told it sheared at the end of the key way. The shaft was SS and only about 3 years old and the prop was a 3 blade Michigan. Any ideas?

 

[justsomeguy]

Zincs?

 

[Ralph Johnstone]

Any chance of getting some high resolution, close up photos of the shaft end to see if the failure was the result of mechanical force or corrosion ?

A real head-scratcher.

 

[Maine Sail]

The #1 primary cause of shaft failure is a improperly fit and installed prop. #2 could be corrosion but the bronze prop usually fails first. Was this shaft "stainless" or a true shaft material like Aquamet or Aqualoy?

 

[THECUSCUS]

I will take some close up pictures this weekend. I will have the shaft checked for alloy, hardness and proper maching when I get it out. It was installed by the PO and he said it was SS. Both the shaft and prop were new together, as well as cutlass bearing. Maine, you said improperly fit prop, would this be the fit of the tapers together?

 

[Maine Sail]

Maine, you said improperly fit prop, would this be the fit of the tapers together?

Yes a proper propeller install is always checked for fit against the taper. The bare minimum is 70% contact but even that is a bit sketchy. Fit is checked with Prussian Blue then the prop is "lap fit" to the shaft, using valve grinding compound, in order to ensure the taper is taking the load, NOT the keyway. I aim for 90-95% + in the taper fit... Most yards & especially DIY's simply ignore this very, very important step...

Proper fit also means the prop is not "key bound" as it slides on the taper, that the key fits correctly and that the prop nuts are installed correctly. The head of the keyway should also be "spooned" to minimize stress risers where the slot ends..

You Michigan wheel is most likely manganese bronze which is roughly 40% zinc. So if your prop is not pink, & corroded, it is unlikely it was a electrical/galvanic issue.

 

[Charles Erwin]

Can the prior owner identify the shop that did the machine work?

 

[THECUSCUS]

Here is another picture a friend took for me. Looks like the shaft had some corrosion issues and maybe had a crack for awhile ( darker area of the break?).

 

[njlarry]

Maine sail, I hope some day you will be able to post detail and pics on proper prop fitting.

 

[jibes138]

http://www.pbase.com/mainecruising/fitting_a_prop

This is from Maine Sail's web page.

 

[Maine Sail]

Here is another picture a friend took for me. Looks like the shaft had some corrosion issues and maybe had a crack for awhile ( darker area of the break?).

Looks to me to be clear case of an improperly installed/fit prop. It clearly started to crack, the blackish on the left, and then finally did..

If that key was fit properly there would be damage to the keyway where the key sheared over it. Essentially the key was taking the prop load, not properly installed, and this is not how it is supposed to be.

 

[njlarry]

Thank you Jibes.
Fantastic Maine.
So much to learn so little time.

 

[Blitz]

http://www.pbase.com/mainecruising/fitting_a_prop

This is from Maine Sail's web page.

Although I'd like to see a picture or clarification of what is meant by key bound or spooning the key.

 

[THECUSCUS]

Maine, your conclusion is of no real surprise. The PO of my boat replaced alot on his own, including this shaft and prop. I corrected many installation errors in other areas, but didnt think to check the prop, afterall it was working fine until it didn't. My surveyor checked the prop for bent blades on my short haul, is this something he possibly could have found or should have checked differently?? I will definetely be following MaineSail's shaft and prop installation guidelines as I tackle this job over the winter. If I can ask Maine, who is your preferred vendor for prop shafts and couplings? Also, do you prefer a straight or tapered end at the trans coupling?

 

[Allan12210]

Looks to me to be clear case of an improperly installed/fit prop. It clearly started to crack, the blackish on the left, and then finally did..

If that key was fit properly there would be damage to the keyway where the key sheared over it. Essentially the key was taking the prop load, not properly installed, and this is not how it is supposed to be.

I'd love to look at that fracture surface really closely with a magnifying glass. Not that it matters, but my interpretation of the fracture mechanism was ductile failure propagating from the bottom (keyway). That is evidenced by the arched pattern with no structure definition. Then once the cross sectional area was no longer enough to support the prop, then bang, the failure was brittle fracture. That is the dark area at the left where you can see grains and pullouts. Its hard to tell from the photo if the dark area is corrosion or just that the light is scattered. It just seems that the failure should start at the keyway where the stress intensification is the greatest (stress risers). Never-the-less, as Mainsail says, the root cause is due to prop mismatch to the shaft and the key taking too much load and transferring it to the keyway. Its just interesting to see how that translates to failure.

 

[Charles Erwin]

Although I'd like to see a picture or clarification of what is meant by key bound or spooning the key.

Spooning is finishing the key slot in the shaft by forming a dish at the end of the cut. The keyway then resembles a spoon with the slot as a handle. Spooning removes the sharp edges and so reduces the chances of stress cracks starting to form. Maine Sail published a picture of his newest propeller installation a couple of weeks ago and that picture shows a spooned keyway.

To assemble the key stock must slide easily in the slot cut in the propeller. The key is just pinched in the slot in the shaft. The propeller is said to be key bound if the key lodges in the propeller slot and so cannot slide. This is possibly what caused the problem here. Even if the propeller bore had been prepared with proper lapp fitting the load was concentrated where the key was jammed in the propeller slot and the lapp fit surfaces could not come into proper contact with each other.

 

[njlarry]

So if I understand this correctly, the key has to slide in the keyway? Can it also be too loose? I always thought that the key was there to transfer all the torque and that the taper kept the prop aligned.

 

[jibes138]

I'd love to look at that fracture surface really closely with a magnifying glass. Not that it matters, but my interpretation of the fracture mechanism was ductile failure propagating from the bottom (keyway).

Hard to tell from the picture but do you think the fracture might have started at the root of the end of the thread that holds the prop on? Looking at that black area on the left it is some distance from the keyway. That would likely be the worst stress riser and subject to stress corrosion since the nut covers the area preventing fresh water from getting in. The thread likely was machined long enough to be inside the face of the prop, then a washer and two nuts would cover that area up. Add some marine growth further restricting access and you have a nice corrosion environment. Given the relatively smooth surface of the fracture with a lot of area, it must have sounded like a gun going off, from the original post it seems like there was no notice of any loud event, just stopped sailing, turned on engine and no thrust? Very odd.

 

[jibes138]

So if I understand this correctly, the key has to slide in the keyway? Can it also be too loose? I always thought that the key was there to transfer all the torque and that the taper kept the prop aligned.

The torque would be transferred by the friction between the prop and the tapered shaft in addition to the key. If the prop slips on the shaft, that would be likely if the tapers are not perfectly matched, then most or all of the load would be on the key. Here's a pretty good article on keyways and key design for consideration.

http://machinedesign.com/mechanical-drives/shaft-keys-revisited

Another thought is if the prop had been removed in the past, I have seen huge loads applied to prop shafts by pullers trying to get a stuck prop off. Usually a torch is then applied and the prop expands and pops off. If the end of the nuts that the puller tool is applying force are not square to the shaft then the tool is also trying to bend the shaft, with that bending stress concentrated in the threads since the root is a sharp corner and the smallest cross section diameter.

Of course an inclusion or other material defect is not out of the question in this case either, the shaft may simply have had a material defect leading to failure. I've seen bars with seams running the entire length of 8 or 12 feet right from the rolling mill.

 

[Maine Sail]

So if I understand this correctly, the key has to slide in the keyway?

The key should fit snugly in the shaft. This is why a properly machined shaft comes with the key already snugly fit. It should slide in the prop but not be sloppy. The idea is that when the prop is sliding onto the taper the key does not move and stop the prop from seating fully on the taper. This is why you always install the prop first, without the key, and mark where it sits on the shaft with a Sharpie marker, then remove it and re-install with the key. If it does not hit the Sharpie mark then it has become key bound and the fit is not correct. All of this occurs after properly lap fitting the prop to the taper.

Can it also be too loose? I always thought that the key was there to transfer all the torque and that the taper kept the prop aligned.

Yes a key can be too loose and if it is, and the prop not properly fit, it exacerbates the shock loading of the keyway when going into gear. With a properly fit prop the taper is what is taking the load, the key is added insurance. I once removed a prop that had been installed for approx 7 years and it had no key at all. What it did have was over 85% contact and a good taper fit.. Someone probably went to lunch and when they came back never removed the prop and re-installed the key.. The prop had not budged and was still extremely difficult to remove even with a beast of a puller.

Here is a tapered coupling end cut-away. Picture this as a prop, as opposed to a coupling, and allowing the key to slide UP the shaft while fitting the prop. If it does the taper will NOT mate as it should and the key will be taking the vast majority of the load, which it is not designed to do.

This is another failure due to improper fit. The diver was able to recover the 3K prop at a cost of about 1.2k.....

This is spooning of the keyway..