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