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

On Mon, 19 Apr 2004 19:27:39 +0200, "Anders Lassen"
anders.lassenNEJTILSPAM (at)adr.dk wrote:

Any metal-detectives that can cast (sorry!) a light over the reason?

===============================================

The failure appears to be the result of a crack which developed over
time, and gradually incerased in size to the point where there was no
longer enough metal left to support the structural load.

So the question is, what caused the initial crack, and why did it go
undetected for so long? Dezincification is one possible reason. If
you have inadequate electrolysis protection from sacrificial zincs on
you prop shafts, the zinc in your bronze props will gradually
disappear through electrolytic action, and the metal will be weakened
leading to structural fatigue, cracking, and failure.

Another possibility is that the prop was damaged in some way from
striking a submerged object or accidental grounding. That could have
lead to a small crack which gradually increased to a large crack and
failure.

An ounce (28 g) of prevention is worth a pound (.45kg) of cure.

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

"Wayne.B" skrev i en meddelelse
...
On Mon, 19 Apr 2004 19:27:39 +0200, "Anders Lassen"
anders.lassenNEJTILSPAM (at)adr.dk wrote:

Any metal-detectives that can cast (sorry!) a light over the reason?

===============================================

The failure appears to be the result of a crack which developed over
time, and gradually incerased in size to the point where there was no
longer enough metal left to support the structural load.

So the question is, what caused the initial crack, and why did it go
undetected for so long? Dezincification is one possible reason. If
you have inadequate electrolysis protection from sacrificial zincs on
you prop shafts, the zinc in your bronze props will gradually
disappear through electrolytic action, and the metal will be weakened
leading to structural fatigue, cracking, and failure.

Another possibility is that the prop was damaged in some way from
striking a submerged object or accidental grounding. That could have
lead to a small crack which gradually increased to a large crack and
failure.

An ounce (28 g) of prevention is worth a pound (.45kg) of cure.