It appears that you are quite knowledgeable on the subject. What are your
thoughts on the relative merits of aluminum vs. stainless steel recreational
boat propellers?
"trainfan1" wrote in message
et...
Gene Kearns wrote:
On Sat, 09 Jun 2007 14:48:33 -0400, trainfan1 penned the following
well considered thoughts to the readers of rec.boats:

Steve Barker wrote:
The reason for the step-down recommendation is because the aluminum
flexes
Prove it.

Both do, but aluminum will bend up to about 4 times as much.....

Check the values for "modulus of elasticity:"

http://www.engineersedge.com/manufac...ous_metals.htm
http://www.engineersedge.com/manufac...ous_metals.htm

The chart you are referencing gives (-------), or -0- for shear modulus of
elasticity on cast aluminum alloys until brass is introduced. There is
some tension, of course(as in an application as fastening hardware).

Aluminum alloy props are very rigid & brittle as the chart shows.

Rob


and looses some pitch. Stainless does not.

Jim wrote:
It appears that you are quite knowledgeable on the subject.

Oh please.

I'm just an old outboard mechanic(both me & the outboards).


What are your
thoughts on the relative merits of aluminum vs. stainless steel recreational
boat propellers?

For recreational boats, under 50mph, aluminum is just fine.

I have one AL 19p for one of my 115 Evinrudes that performs just about
as well as as my SSTII 19p prop. I had it double-cupped at rebuild
years ago and am very satisfied. It just blows out a little faster in
hard turns with lots of trim.

At higher speeds, blade shape, contour, & thinness is much more
critical, and SS provides the builder with the ability to make a thinner
blade. The blades can do their work w/o displacing as much water as
with AL.

AL is ~1/3 the cost of SS and is more forgiving to your gear train in
the case of striking a submerged object. You are more likely to lose a
blade or two instead of banging up a SS and putting a little "english"
in your propshaft.

SS is for optimal performance. Some special SS applications do exist,
like if you are running through sandbars often. The SS will get
polished, the AL will get "resized".

4 blade props are overkill unless you have a hull design that can
benefit from the extra surface area to correct a bow-heavy porpoising
situation. The most efficient props are 1-bladed, but they are tough to
balance. Three blades seems to be a good compromise for mid power & speeds.

In all reality, AL is the best choice for recreational boating. Always
carry a spare, & the tools to change it.

Rob



"trainfan1" wrote in message

4 blade props are overkill unless you have a hull design that can
benefit from the extra surface area to correct a bow-heavy porpoising
situation. The most efficient props are 1-bladed, but they are tough to
balance. Three blades seems to be a good compromise for mid power & speeds.

I thought 4 blade helped lift the stern.

Josh Assing wrote:
4 blade props are overkill unless you have a hull design that can
benefit from the extra surface area to correct a bow-heavy porpoising
situation. The most efficient props are 1-bladed, but they are tough to
balance. Three blades seems to be a good compromise for mid power & speeds.

I thought 4 blade helped lift the stern.


It might, but any prop, 2, 3 or 4 blade, with more blade surface area,
will keep a porpoising condition at bay with higher trim or jack plate
settings.

Rob

"trainfan1" wrote in message
et...
Jim wrote:
It appears that you are quite knowledgeable on the subject.

Oh please.

I'm just an old outboard mechanic(both me & the outboards).


What are your thoughts on the relative merits of aluminum vs. stainless
steel recreational boat propellers?

For recreational boats, under 50mph, aluminum is just fine.

I have one AL 19p for one of my 115 Evinrudes that performs just about as
well as as my SSTII 19p prop. I had it double-cupped at rebuild years ago
and am very satisfied. It just blows out a little faster in hard turns
with lots of trim.

At higher speeds, blade shape, contour, & thinness is much more critical,
and SS provides the builder with the ability to make a thinner blade. The
blades can do their work w/o displacing as much water as with AL.

AL is ~1/3 the cost of SS and is more forgiving to your gear train in the
case of striking a submerged object. You are more likely to lose a blade
or two instead of banging up a SS and putting a little "english" in your
propshaft.

SS is for optimal performance. Some special SS applications do exist,
like if you are running through sandbars often. The SS will get polished,
the AL will get "resized".

4 blade props are overkill unless you have a hull design that can benefit
from the extra surface area to correct a bow-heavy porpoising situation.
The most efficient props are 1-bladed, but they are tough to balance.
Three blades seems to be a good compromise for mid power & speeds.

In all reality, AL is the best choice for recreational boating. Always
carry a spare, & the tools to change it.

Rob



"trainfan1" wrote in message



Good post. I can't help thinking that you would get better gas mileage with
a more efficient ss prop. Notice I said mileage not performance.
One blade prop? You mean like an auger?

Semi-interesting discussion. But, essentially meaningless because propeller
selection is impossible to pinpoint except through trial and error. Any
number of computer programs can select the "perfect" prop for your boat
engine combination except they won't except by luck. They'll get you close
but perfection is only acheived by trial and error. Furthermore, the
perfect prop for one set of conditions will not be the perfect prop for all.

Do you want perfect hole shot performance? One prop. Do you want maximum
speed? A different prop. Do you want optimum economy at lowest RPM? Yet
another prop. Whatever prop you choose will be a compromise and provide
perfect performance for only one, if any, operating condition. All we can
do is strive for a prop that provides good all around performance and
economy.

As to the original question from the OP.... as Clams pointed out, it's about
enertia.

Butch
"jamesgangnc" wrote in message
nk.net...
The pressure created by the prop pushing the boat is orders of magnitude
greater than anything you could attribute to spinning the weight of the
prop. The different between aluminum and ss would not be noticable.

"Josh Assing" wrote in message
...
I purchased 2 props (both alumininum) to reprop with.
The guy agreed with me that if I hit stuff; it's better to destroy the
prop than
the outsdrive -- but for performance, a ss is the way to go.

he said the weight issue is silly because it's underwater and doesn't
weigh as
much and that no one would use ss if it wore out the outdrive faster.

So -- Here I am; for curiosity sake; asking you if the weight of a ss
prop
really does wear out the drive faster than an aluminum one.

-j

Jim wrote:
"trainfan1" wrote in message
et...
Jim wrote:
It appears that you are quite knowledgeable on the subject.
Oh please.

I'm just an old outboard mechanic(both me & the outboards).


What are your thoughts on the relative merits of aluminum vs. stainless
steel recreational boat propellers?
For recreational boats, under 50mph, aluminum is just fine.

I have one AL 19p for one of my 115 Evinrudes that performs just about as
well as as my SSTII 19p prop. I had it double-cupped at rebuild years ago
and am very satisfied. It just blows out a little faster in hard turns
with lots of trim.

At higher speeds, blade shape, contour, & thinness is much more critical,
and SS provides the builder with the ability to make a thinner blade. The
blades can do their work w/o displacing as much water as with AL.

AL is ~1/3 the cost of SS and is more forgiving to your gear train in the
case of striking a submerged object. You are more likely to lose a blade
or two instead of banging up a SS and putting a little "english" in your
propshaft.

SS is for optimal performance. Some special SS applications do exist,
like if you are running through sandbars often. The SS will get polished,
the AL will get "resized".

4 blade props are overkill unless you have a hull design that can benefit
from the extra surface area to correct a bow-heavy porpoising situation.
The most efficient props are 1-bladed, but they are tough to balance.
Three blades seems to be a good compromise for mid power & speeds.

In all reality, AL is the best choice for recreational boating. Always
carry a spare, & the tools to change it.

Rob



"trainfan1" wrote in message

Good post. I can't help thinking that you would get better gas mileage with
a more efficient ss prop. Notice I said mileage not performance.

That is another consideration. The difference is going to be negligible
over the range of the fuel capacity of a recreational boat(18'-21' w/ a
27 gallon tank being on the large side).

To make an accurate comparison, you will need a selection of props, a
GPS, & fuel flow meter(and of course your tachometer).

One blade prop? You mean like an auger?

PWC jet pumps typically have 2 or 3 overlapping-blade
impellers(different dynamics - the water is captive) almost resembling
an auger, but I meant an actual 1 blade prop! That's why it is so hard
to balance.

1 blade makes less ancillary disturbance/interference in the water than
2 or 3 or 4.

Props are always a compromise of blade thickness, materials cost,
balancing ease. Not to mention blade area, shape, rake angle, linear,
regressive, or progressive rake, overall pitch, cupping profile, hub
hydrodynamics, leading & trailing edge profiles.

I have several props that purport to be the same, they never perform the
same. Aside from a few props that are CNC machined from billets, you
would be hard pressed to find two identical wheels from a production
run. They're almost like snowflakes.

The lower the horsepower & speed, though, the less the differences are
noticeable.

Rob

On Mon, 11 Jun 2007 10:46:14 -0400, trainfan1
wrote:


I have several props that purport to be the same, they never perform the
same. Aside from a few props that are CNC machined from billets, you
would be hard pressed to find two identical wheels from a production
run. They're almost like snowflakes.

The lower the horsepower & speed, though, the less the differences are
noticeable

Do you notice a difference between performance in sal****er vs fresh?

Why? Because you get your panties in a bunch when dissagreed with? It's
theory, and I have enough practical pratice (in the real water, not fixing
propellers) to dissagree.

All things being equel, you can drop a pitch when going from aluminum to SS,
because the SS blades are thinner and cut water better.

Perhaps aluminum flex works against fact that in some theoretical
high-horsepower environment - but not in day to day operation with normal
horsepower ranges.
-W

--
CC Marine

Lake Hartwell SC
"Where the fish are always biting, and the swimmers are always nervous!"


"Steve Barker" wrote in message
...
It's not MY theory, and it's not theory. thanks for the input. I
remember
now why I left this group 3 years ago.

--
Steve Barker

for the spam bots:






"Josh Assing" wrote in message
...
On Sat, 9 Jun 2007 07:31:36 -0500, "Steve Barker"

wrote:

The reason for the step-down recommendation is because the aluminum
flexes
and looses some pitch. Stainless does not.


flexes - I think that's generally accepted; but flexes to the point of
loosing
2" of pitch? I find that hard to believe.

On this theory; I'm giong to be way under propped as I had a SS 19 and I
went to
an aluminum 17; so on your theory; this will be like a 15 pitch SS....

Errrrr, I meant go UP a pitch. - not drop a pitch. sigh

--
CC Marine

Lake Hartwell SC
"Where the fish are always biting, and the swimmers are always nervous!"


"Clams Canino" wrote in message
news
All things being equel, you can drop a pitch when going from aluminum to
SS,
because the SS blades are thinner and cut water better.