Broken prop shaft - saved by the zinc

[harbin2]

I was returning home from a week's sail when I heard a clunk and lost all forward and reverse power. A check of the engine compartment showed nothing. I had been motoring along at 1,800 rpm. I figured I either lost the prop or broke the shaft. After sailing 45 miles to my marina, I took a quick dive and found the shaft dangling with the prop about an inch in front of the rudder. It was dumb luck that I had placed the zinc where I had that it allowed water to the cutless but kept the broken prop from interfering with the rudder. The boat came out two days later and I'm having a new shaft made. I'll be replacing the PYI seal and the cutless. The break was inside the SS collar for the PSS seal. Still not sure what the cause was but might be related to the seal overheating (was an earlier design that had no vent) combined with some corrosion at the point where the set screws met the shaft. Below are a couple photos.
Any thoughts on a possible cause?
harbin2
Islander 30

[Gene T]

That is troubling. I find it hard to believe the shaft would fail from no cooling. It would be difficult to see a scenario where the shaft would get that hot and the bellows would not melt. But it definitely has something to do with the PSS seal. You should send pictures to PYI for their evaluation. The shaft could be tested for heat stress. It doesn't look like electrolysis either.

Gene

[harbin2]

Yes - it is puzzling. The bellows doesn't really come in contact with the shaft. It just contains the seawater. The carbon collar comes close but only the stainless collar really contacts the shaft.
I know I have overheated it myself - have had the boat for just one full season and it took me a several hours of run time with the seal screatching before I learned that they need to be burped (to let the air escape). I have been on the phone with PYI and sent them photos. They have been very good to deal with but they have not seen anything like this. The think the shaft might have been a low grade steel (304 or 316). The new one is Aquamet 22. I hope to put it in tomorrow. The maker of the shaft thought it might have been the coupling to the transmission. The current shaft, seal, cutless, etc. were all replaced in 2002 by the PO. I'm still trying to find out why he replaced the shaft then.

[Gene T]

It looks like it broke right where the set screws are. Interesting that if it were hot and got quenched rapidly when cooling water hit it could have carbonized. A simple hardness test would tell if the shaft is harder where it is broken. Your machine shop doing the new shaft could check it quickly for you. At least then you would know something. I am glad you posted this and I believe this failure mode is worth exploring.

Gene

[paul323]

I think Gene's post is very plausible. Where do you sail?

If the shaft was heated, and then rapidly cooled, it could indeed have made the steel brittle to the point of failure. For example, a lot of engine use (heating) in very cold water (rapid cooling when you stop). I would guess that repeating this cyle would increase the hardness of the metal to such an extend that the torque driving the propeller would break it.

You could probably do a 'quick and dirty' test yourself using a file. I am no metallurgist, but I know that different alloys of steel vary in their susceptibility to this kind of failure.

[harbin2]

Thanks for your thoughts
There were several times last summer that the seal would start squealing. It would usually take several minutes to a half hour for me to get to it and burp it. I recall once that the unit was hot enough that it produced steam when I did. I'm sure you are correct about the quenching and I do believe this is the "root" cause. It is interesting to note that the seal design was changed some time ago to include a vent. I plan to put everything back together tomorrow and/or Wednesday. I'm considering supplying water to the vent but I'm reluctant to do that because my raw water pump pumps very little water and I'm not sure there will be enough left to cool the exhaust etc. I guess for now, I'll just vent it.

[donradclife]

The broken ends look like fatigue failure, probably caused by some misalignment. When you put things back together, be sure to check all alignments, especially that the face of the coupling is perpendicular to the shaft.

[tommays]

Seafevers strut was out of line for 40 years and it only took about .085 to move the shaft 1" 4' away by the motor

[Gene T]

All you need to do is run the vent hose above the water line and leave the loose end open. I managed to find an area above the engine between the companion way and bulkhead to run mine. Forcing water in there is only for power boats at higher RPM.

Gene

[SoulVoyage]

My system is more traditional...a standard stuffing box forward of the cutlass bearing and hooked up on the engine side with a flange.

My flange set-screws came loose one time because I'd forgotten to through-wire them together. Same result for me, lost all way; just different cause.

BUT, I was more cocerned about the possibility of completely losing the shaft. No outer struts on this shaft. Full keel with a rudder aperture. A loose flange COULD result in the shaft spinning itself away and dropping from the boat if you were turning at the time (the rudder would normally stop the prop/shaft from backing out altogether). Resulting in a "hole" that could quickly sink your boat.

To protect against such misadventure, I put a SECOND zinc on the inside of the engine compartment....just a couple inches forward of the stuffing box. The zinc stops the shaft from moving more than a couple inches if the flange comes loose...it certainly won't back out!

Protect your boat from sinking....put a second zinc on the inside!!