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| Tony's Tips |
ARTICLE DATE: 10/01/2000 |
| So what do all those propeller
shaft numbers really mean? |
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If you go back about 30 years, there was a very popular
product on the market called "Sealoy" that became the alloy of
choice for propeller shafting (kind of like "Kleenex"). Armco
Corp. bought the trade name and rights to manufacture this particular high
grade of stainless shafting and renamed it "Aquamet 19." Since
then "Crucible" bought Armco(or (?) whatever it's called) and
presently owns the trademark and proprietary manufacturing rights to this
alloy. Also, way back, besides Aquamet 19 alloys with those particular properties,
they also marketed Aquamet 17 (basically a 17-4 PH stainless), Aquamet 18
(I think this was a low grade/cost marketing failure) and Aquamet 22. From
all the reading I've done, the best I can ascertain is that Aquamet 19 is
a nitrogen strengthened 304 stainless with a few additional "secrets"
put into it, and Aquamet 22 is a nitrogen strengthened 316 stainless, again
with added super-secrets. Aquamet 17 is their highest strength alloy, but
it does not have the corrosion resistance needed for some applications.
Many companies have jumped on the bandwagon over the years and used names
like Aquatec 19, Aqualoy 19, Nautaloy 19, Aquashaft 19,(all of them sometimes
now just called "19" alloy) etc., etc., etc. .....These other
companies all have brochures suggesting and comparing their products to
Crucible's 'Aquamet' line. From personal experience I can tell you that
(from strictly a machinist's point of view) the original Sealoy or Aquamet
19 always proved to be more of a challenge for my tooling when cutting keyways
and threads than the newer copy-cats like Aquatec 19 which I've used just
about exclusively for the past 10 years. There were many reasons for my
shop switching to Aquatec and obviously, cost was one of them. Whether or
not the slightly easier machining for Aquatec alloys translates into decreased
overall longevity due to strength and/or corrosion resistance, I cannot
answer. But I can tell you this with reasonable certainty......most all
shaft failures I've seen can be traced to many reasons: a propeller coming
loose on the shaft; a propeller installed incorrectly on the shaft; a shaft
overhang past the rear strut bearing that caused excess reaction forces;
too large a propeller in relation to shaft diameter; too small of a shaft
in relation to HP and/or prop size; or the boat/propeller/shaft striking
an object. Of course, one could say that w/ 22-grade material that possibly
some of these failures would not have occurred, but I'm not going to argue
that point.
Also, one the big claims to 22-grade shafting other than strength is the
resistance to crevice corrosion when sittings still inside shaft logs, rubber
bearings, etc., for a long time. But again, someone has to way these potential
benefits against the increase in cost (typically about 50%) for this alloy.
Personally I'd put my $$ elsewhere and use slightly larger shafting if I
was putting together a boat and was on the borderline as to strength. |
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