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  #21  
Old 04-25-2012, 11:39 PM
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Originally Posted by TRBenj View Post
The 2 pieces of the puzzle that you need to supply are:

1) Where does your boat make its peak hp? (I would expect a NA 6.0L to peak in the 5400-5600 range... not sure if/how the SC affects that).

2) A known data point (WOT RPM vs. GPS speed with a known prop).

Then you prop it to turn at or near the peak hp level at WOT. That will maximize top end. If youre currently overpropped (which may very well be the case) then if you go to a prop with less pitch, you should gain holeshot, midrange power and top end as well.
Yes but you also told me a few months back that the blades can flex causing pitch change. How do we factor that in. My speed doesn't change past 4700-4800 so there is 300-400 rpm that are not doing anything.

Does a higher pitch flexing compensate for the lower pitch flex?
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  #22  
Old 04-26-2012, 07:00 AM
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Sorry for the thread jack

Eric, I got a '93 ps190 and I'm turning a 13.7 x 17.5 four blade at 5100 ish rpm at 47 mph on gps stargazer. Can I turn a 19.5 or what do you recommend.
Higher pitch will decrease the low end as well as drop RPM relative to speed, you may see a slight increase in top end, 1-2 MPH. I would not suggest a 19.5 with a 5.7 engine as it may not allow the engine to see max RPM.
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  #23  
Old 04-26-2012, 10:50 AM
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Higher pitch will decrease the low end as well as drop RPM relative to speed, you may see a slight increase in top end, 1-2 MPH. I would not suggest a 19.5 with a 5.7 engine as it may not allow the engine to see max RPM.
Thank you Eric.

I guess I will save my money then. Just figured I would ask since nothing happens after 4700 rpm speed wise but the engine can turn another 400 rpm without change. I was thinking that a 19.5 would lower my rpm by 300 ish rpm but I would still get performance to 4800. The prop I'm running now is a monster out of the hole and I figured that a little less hole shot would not matter that much.

Again Thanks for the advice.
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Last edited by Kyle; 04-26-2012 at 10:55 AM.
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  #24  
Old 04-26-2012, 11:56 AM
TRBenj TRBenj is offline
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Originally Posted by Kyle View Post
Yes but you also told me a few months back that the blades can flex causing pitch change. How do we factor that in. My speed doesn't change past 4700-4800 so there is 300-400 rpm that are not doing anything.

Does a higher pitch flexing compensate for the lower pitch flex?
I have seen worn out props that act like youre describing. They turn consistent speed vs. rpm at skiing speeds, but flex at WOT, causing extra RPM's that dont translate to speed. I consider props like that to be either worn out (if old and springy) or simply inefficient (meaning they dont turn RPM's into speed very well at the top end).

I have little experience with the newer XMP OJ's out of the few Ive tried for the 1.23's (lower prop RPM), but I know someone who saw the exact same behavior youre describing with a new XMP on a 1:1 boat. The last few hundred RPM didnt speed the boat up.

In his opinion, the Acmes arent as susceptible to this phenomenon... they turn RPM's into speed a bit better. You may want to try a comparable sized Acme... my guess is that it will turn similar RPM's at skiing speeds, but will drop your revs slightly at WOT, if it bites and pushes the boat instead of flexing.

I would not expect a steeper prop of the same design youre using now to help performance in any way.

Last edited by TRBenj; 04-26-2012 at 12:03 PM.
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  #25  
Old 04-26-2012, 12:48 PM
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Will I get more speed out of a 3 or 4 blade?

I don't want to have a footin prop and a slalom prop and I need one that holds speed in a course like my 4 blade.
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  #26  
Old 04-26-2012, 01:13 PM
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Originally Posted by TRBenj View Post
I have seen worn out props that act like youre describing. They turn consistent speed vs. rpm at skiing speeds, but flex at WOT, causing extra RPM's that dont translate to speed. I consider props like that to be either worn out (if old and springy) or simply inefficient (meaning they dont turn RPM's into speed very well at the top end).

I have little experience with the newer XMP OJ's out of the few Ive tried for the 1.23's (lower prop RPM), but I know someone who saw the exact same behavior youre describing with a new XMP on a 1:1 boat. The last few hundred RPM didnt speed the boat up.

In his opinion, the Acmes arent as susceptible to this phenomenon... they turn RPM's into speed a bit better. You may want to try a comparable sized Acme... my guess is that it will turn similar RPM's at skiing speeds, but will drop your revs slightly at WOT, if it bites and pushes the boat instead of flexing.

I would not expect a steeper prop of the same design youre using now to help performance in any way.
Tim - Our blades are thicker than that of the other brand. They will not however flex to the point of loosing enough pitch to change MPH. A thinner prop will run a bit better top end, 1-2 MPH, than a prop with a heavier root. While I understand the need for speed, we have always defered to durability and longevity and had thicker blade sections. I can thin a prop down for MPH but it will bend at the slightest impact, which most guys do not prefer to gain 1-2 MPH.

As I said earlier, a higher pitch prop will not help the overall performance, no matter who makes it.
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  #27  
Old 04-26-2012, 01:31 PM
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Eric, I do not believe the blade thickness was the concern. The speculation was that the shape of the XMP blade may have been more sensitive to flex at higher (prop) rpm's.

Im not sure what else would explain the phenomena that we're describing here, but feel free to enlighten me! Like Kyle, he saw the last 300-400rpm add little to no speed. Where are those extra RPM's going if theyre not pushing the boat any faster?

Kyle, in theory, the fewer the blades, the more efficient the prop. Ive found the 3-blades Ive run to be a bit faster than the 4's. I think that the large blade surface area found on the modern CNC props largely levels the playing field between 3 and 4-blades, at least for applications with relatively low loads (skiing and footing). If youre show skiing or running a wakeboat with 2k lbs of ballast, I hear the 4's still do better for the big loads, especially out of the hole.
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  #28  
Old 04-26-2012, 01:37 PM
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Originally Posted by TRBenj View Post
Kyle, in theory, the fewer the blades, the more efficient the prop. Ive found the 3-blades Ive run to be a bit faster than the 4's. I think that the large blade surface area found on the modern CNC props largely levels the playing field between 3 and 4-blades, at least for applications with relatively low loads (skiing and footing). If youre show skiing or running a wakeboat with 2k lbs of ballast, I hear the 4's still do better for the big loads, especially out of the hole.
Tim, it's been a while since I have inspected one up close, but the large blade surface area thing is not necessarily a function of CNC machined props, but rather an Acme design feature. When I compared two similarly sized props, one an Acme and the other an OJ XMP, the Acme had more surface area per blade. This was several years ago. But as far as I know, OJ has not varied the design of their 3 blade props from a blade area perspective. You can see the measurements I took and a side by side picture of the two props in the link below. Both companies make very good props. But, as I'm sure you know, they are different and perform differently.

Acme vs. OJ XMP 13x12 3 Blade Prop Comparison

From the article:

The Acme has an approximate blade size of 8 3/16” across the blade (vertical
in the picture) and 5 1/8” from hub to the edge of the blade. At first glance,
the surface area of the blades appears larger on the Acme. From my
measurements, it appears that the Acme has about 1” on the OJ on the vertical
blade measurement. From hub to edge, they are about the same.

The OJ has an approximate blade size of 7 3/16” across the blade and 5 1/8”
from hub to the edge of the blade.
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Last edited by east tx skier; 04-26-2012 at 01:44 PM.
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  #29  
Old 04-26-2012, 02:29 PM
TRBenj TRBenj is offline
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Doug, youre absolutely right- so far as Im aware, the CNC manufacturing process used on the Acmes and OJ XMP's is not the reason for the large blade surface area. Nonetheless, both CNC props do have a lot more blade surface area as compared to their conventional hand finished counterparts. Youre right that the Acme has more surface area, but the XMP isnt that far behind.

This is a new-ish hand finished OJ for comparison. As you can see, the XMP vs. Legend props are very different in shape, with the hand finished prop having quite a bit less surface area.
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Last edited by TRBenj; 04-26-2012 at 02:46 PM.
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  #30  
Old 04-26-2012, 02:45 PM
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Tim - I see the last 200 - 300 RPM not gain much MPH with any prop regardless of who built it. The blade shapes are much different. The other is a more tradtional work boat type shape while ours is not that style. There are may other differences as well, rake and pitch distribution to name two.

Blade thickness is a major factor in top end. A thinner cross section IS faster. I used to thin my tunnel boat props to the point of breaking, in fact they would break and not last very long at all but I could thin a few more MPH put of it. When one of those props breaks you can end up upside down under water. Thin to win we used to say. Last I looked these boats were nowhere near 100 MPH nor do they have numbers on the side. As I said we prefer to build a good all around propeller that will last.

The RPM we see on the ski boats should not cause deflection at all. We still run our "old style" 3-blades on flatbottom v-drives with overdrive transmissions, prop turning faster than engine RPM, turning 5500 to 6500 RPM with no flex issues. You will see more load and pressure on blades with the wakeboats loaded to the gills than you do with a 1:1 turning 5000 wide open

Our current XMP 3-blades do have a different shape, even from our 4-blades. I built the prop pictured, 13 X 13 RC, in 1990 specifically for the CC boat. The blade shape I used was from a wooden pattern originally designed by my great grandfather Oscar in the late 1940's for racing applications.
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Last edited by EJ OJPROP; 04-26-2012 at 02:55 PM.
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