Prop sizing done...
 

well; after trying a bunch of props; here's a rundown on the results....
boat needed 4200-4600 RPM at WOT

Original prop was a SS 14x19 3 blade -- WOT 3800 rpm

aluminum 15x17 3 blade - WOT 4950
aluminum 14.5 x 19 3 blade WOT 4250

the 4 blade dropped it 150; so it was too low for for reasonable use; but
getting on plain & staying there was great.

The difference in RPM from the SS to Alum 15x17 is being attributed to the more
agressive cupping on a SS blade.

so I'm much happier with the alum 14.5x19 -- but will try some some 4 blade
15x18 props to see how they do ... but that won't be until much later in summer.

Thought it was interesting about the difference in cupping would make that much
of a difference.

Prop sizing done...
 

aluminum prop blades have more flex then SS blades.



"Josh Assing" wrote in message
...
well; after trying a bunch of props; here's a rundown on the results....
boat needed 4200-4600 RPM at WOT

Original prop was a SS 14x19 3 blade -- WOT 3800 rpm

aluminum 15x17 3 blade - WOT 4950
aluminum 14.5 x 19 3 blade WOT 4250

the 4 blade dropped it 150; so it was too low for for reasonable use; but
getting on plain & staying there was great.

The difference in RPM from the SS to Alum 15x17 is being attributed to the
more
agressive cupping on a SS blade.

so I'm much happier with the alum 14.5x19 -- but will try some some 4
blade
15x18 props to see how they do ... but that won't be until much later in
summer.

Thought it was interesting about the difference in cupping would make that
much
of a difference.

Prop sizing done...
 

ACTUALLY the higher rpm on the aluminum of the same specs is due to flex.
They tend to flatten out the equivalent of an inch or two of pitch.

--
Steve Barker







"Josh Assing" wrote in message
...
well; after trying a bunch of props; here's a rundown on the results....
boat needed 4200-4600 RPM at WOT

Original prop was a SS 14x19 3 blade -- WOT 3800 rpm

aluminum 15x17 3 blade - WOT 4950
aluminum 14.5 x 19 3 blade WOT 4250

the 4 blade dropped it 150; so it was too low for for reasonable use; but
getting on plain & staying there was great.

The difference in RPM from the SS to Alum 15x17 is being attributed to the
more
agressive cupping on a SS blade.

so I'm much happier with the alum 14.5x19 -- but will try some some 4
blade
15x18 props to see how they do ... but that won't be until much later in
summer.

Thought it was interesting about the difference in cupping would make that
much
of a difference.

Prop sizing done...
 

Lost In Space/Woodchuck wrote:
aluminum prop blades have more flex then SS blades.


Try flexing your aluminum prop blades & let us know how that works out.

Rob




"Josh Assing" wrote in message
...
well; after trying a bunch of props; here's a rundown on the results....
boat needed 4200-4600 RPM at WOT

Original prop was a SS 14x19 3 blade -- WOT 3800 rpm

aluminum 15x17 3 blade - WOT 4950
aluminum 14.5 x 19 3 blade WOT 4250

the 4 blade dropped it 150; so it was too low for for reasonable use; but
getting on plain & staying there was great.

The difference in RPM from the SS to Alum 15x17 is being attributed to the
more
agressive cupping on a SS blade.

so I'm much happier with the alum 14.5x19 -- but will try some some 4
blade
15x18 props to see how they do ... but that won't be until much later in
summer.

Thought it was interesting about the difference in cupping would make that
much
of a difference.

Prop sizing done...
 

Steve Barker wrote:
ACTUALLY the higher rpm on the aluminum of the same specs is due to flex.
They tend to flatten out the equivalent of an inch or two of pitch.


Prove it.

Rob

Prop sizing done...
 

The OP just did.

--
Steve Barker







"trainfan1" wrote in message
et...
Steve Barker wrote:
ACTUALLY the higher rpm on the aluminum of the same specs is due to flex.
They tend to flatten out the equivalent of an inch or two of pitch.


Prove it.

Rob

Prop sizing done...
 

On Mon, 2 Jul 2007 23:52:57 -0500, "Steve Barker"
wrote:

The OP just did.


no I didn't....
the SS had a more agressive cupping to the blade; the props do not have the same
physical prop design; so even tho they're the same pitch; they are different
props...

Prop sizing done...
 

You all can twist the facts all you want. Pitch is pitch. In theory, a
given pitch goes xx inches through the water in one rotation. Given this,
the stainless and the alum will do the same inches in one revolution.
Until, that is, you put some power behind it and the aluminum flattens out a
bit. Thus the higher rpm for the same pitch. You can cup this and cup
that, but the fact is the aluminum flexes and loses pitch. Period.

--
Steve Barker







"Josh Assing" wrote in message
...
On Mon, 2 Jul 2007 23:52:57 -0500, "Steve Barker"

wrote:

The OP just did.


no I didn't....
the SS had a more agressive cupping to the blade; the props do not have
the same
physical prop design; so even tho they're the same pitch; they are
different
props...

Prop sizing done...
 

Steve Barker wrote:
You all can twist the facts all you want. Pitch is pitch.

No, it isn't. A thinner blade will travel further each revolution. A
fatter blade(aluminum) of the same will disrupt the water flow and not
travel as far.

In theory, a
given pitch goes xx inches through the water in one rotation.

Now you're mixing your "facts" with theory.

Given this,
the stainless and the alum will do the same inches in one revolution.

No, they won't. You're leaving out too many variables.

Until, that is, you put some power behind it and the aluminum flattens out a
bit.

No, it won't. The slippage you "see" is due to the blade cross-section
shape & design, not the blades flexing(until you get into composite
props). Props would be breaking all the time from fatigue alone if they
gave enough to even slightly affect pitch.

Thus the higher rpm for the same pitch.

The slippage you "see" is due to the blade cross-section shape & design,
not the blades flexing(until you get into composite props).

You can cup this and cup
that,

What about rake, leading edge & trailing edge designs, blade surface
area & thickness, diameter, blade contour & # of blades? Do you really
think pitch is all there is?

but the fact is the aluminum flexes and loses pitch. Period.

Prove it. What are your sources?


Rob

Prop sizing done...
 

And to add to the fire -- I tried an 4 blade 18pitch.
My goal is fuel consumption #1, #2 is getting & staying on plane at lower
speeds.

#2 was achieved, but 20 mph "on plane" with the 4 blade used more fuel than 30
mph wiht the 3blade. it also used the same amount of fuel at 20 mph with tabs
shoved all the way down -- so to me; the 4 blade was actually worse all around
than the 3 blade.

( I have a 4 blade 14.5x18 aluminum prop for sale with approximately 2 hours on
it if any one is interested-- I also have that 14x19 SS prop.... I'm keeping
the 15x17 and 14.5x19 prop for pulling & cruising)

I never understood why people are so adament about "aluminum flexes" .... it
just isn't logical to me.

-j


On Wed, 04 Jul 2007 09:04:26 -0400, trainfan1 wrote:

Steve Barker wrote:
You all can twist the facts all you want. Pitch is pitch.

No, it isn't. A thinner blade will travel further each revolution. A
fatter blade(aluminum) of the same will disrupt the water flow and not
travel as far.

In theory, a
given pitch goes xx inches through the water in one rotation.

Now you're mixing your "facts" with theory.

Given this,
the stainless and the alum will do the same inches in one revolution.

No, they won't. You're leaving out too many variables.

Until, that is, you put some power behind it and the aluminum flattens out a
bit.

No, it won't. The slippage you "see" is due to the blade cross-section
shape & design, not the blades flexing(until you get into composite
props). Props would be breaking all the time from fatigue alone if they
gave enough to even slightly affect pitch.

Thus the higher rpm for the same pitch.

The slippage you "see" is due to the blade cross-section shape & design,
not the blades flexing(until you get into composite props).

You can cup this and cup
that,

What about rake, leading edge & trailing edge designs, blade surface
area & thickness, diameter, blade contour & # of blades? Do you really
think pitch is all there is?

but the fact is the aluminum flexes and loses pitch. Period.

Prove it. What are your sources?


Rob