Forensic metal detectives

Hi,
Had a really bad experience yesterday: A blade on my propeller came right off - really serious damage to my boat... :-(

http://www.kroglassen.dk/skrue1.jpg
The fracture itself looks like this::
http://www.kroglassen.dk/skruener.jpg
Any metal-detectives that can cast (sorry!) a light over the reason?
Anders Denmark
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Anders Lassen wrote:

Nice fatigue-crack. Looks like long-growing.
Michael Dahms
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Michael Dahms wrote:

Originating at a corrosion pit on the surface or other flaw.
If you aren't going to rebuild the propeller, please provide it to your local university for failure analysis classes.
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Who manufactured it? Some of those cast "bronze" propellers coming out of China have a large content of iron, which they do to save money. I have even heard that they will pass off a cast iron, copper plated part as a bronze casting, although I don't think that is the case here.-Jitney
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It's Italian - from http://www.elicheradice.com /
Seems ok....but must have been flawed in some way
Anders

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Anders Lassen <anders.lassenNEJTILSPAM (at)adr.dk> wrote:

I don't think it was "flawed in some way" it looks like what we used to call on the railroad a "detail fracture" where something happened to the surface, and then later a crack "grew" from it.
I read a welding book once where a brand new, big-ass tractor axle broke because a welder accidentally touched the live electrode to the axle. A crack that looks just like yours, developed accross the 5"-6"? axle and it snapped right in two. And generally speaking almost nothing in this world could break that axle like that otherwise. :)
It was already mentioned, and I want to ask about the rusty looking spot/flake that appears in the starting point of the crack. Need to figure out what that is to be sure... a closer-up picture of it? It could easily be the reason for the crack... could have started with a rock ding or something iron you ran over and thought nothing of it at the time.
"whew... thought that was going to break the prop or something... glad it didn't.. hardly scratch it" <- famous last words? ;)
Alvin in AZ
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All,
Just collected some more bits of information regarding my broken propeller that I'd share with you:
Seems like the propeller is NOT made from aluminium bronze which I thought it was! The material should be Manganese Bronze (which supposedly is the same as Hi tensile brass?)
This material should be well suited for propeller use - BUT not for high powered crafts. My engine has 300 hp and a torque of 700 Nm - which is over the limit of what a (die cast?) manganese bronze propeller is suited for. (all this according to one of the leading Danish marine propeller-experts)
Also, the company that imports the Italian "Radice" (manganese bronze) propellers have had problems with them operating in arctic waters near Greenland. Seems as the cold water increases the risk of cracks. Interesting, because my boat has been in the water all winter at temperatures of down to -1 degree centigrade.
All in all it seems as if the boat-company wanted to save the extra 30% that a NAB-propeller costs and installed this type instead. Pity what some people will do to save a buck....
Again thanks for your valuable information - which I think played a part in the fact that the insurance-company decided to pay for the damage. AND - install a NiBrAl-propeller instead!
Kind regards
Anders Lassen
Elsinore, Denmark
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Alvin,
To answer your question:
This was as close as I could get to the "root" of the problem:
http://www.kroglassen.dk/hulner.jpg
Notice the small "crater" to the left of the picture. A "sister"?
Anders

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Oh yes, and see this also:
http://www.tc.gc.ca/tdc/projects/marine/d/5199.htm
"Anders Lassen" <anders.lassenNEJTILSPAM (at)adr.dk> skrev i en meddelelse

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Cool thanks. Looks like you've got it settled. :)
Alvin in AZ
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