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  1. #1

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    Pitching the stock prop (record speed on stock prop?)

    Today, I got sick of waiting for IMPROS to return my prop (been 3.5 weeks since I sent it) so I decided to take some measurements and re-pitch one of the stockers I have sitting around. I learned a few things and thought I would share.

    I measured the blade height a 8 evenly spaced locations along the tip of the blade, then used trigonometry to calculate the angles.

    Most seem to agree that the stock prop is a 10/21, but my measurements say different.

    This is what I got on the untouched stocker:
    Blade 1: 13/18.6
    Blade 2: 12.8/19.7
    Blade 3: 12.2/21.1
    Blade 4: 12.6/18.4
    Average: 12.7/19.5

    I then took a large crescent wrench and cheater pipe to it. At first, it was very difficult to keep the blade shape uniform and consistent, with no sharp turns or waves. The first blade took me 2 hours, but I actually got the last one in 1 try (10 minutes).

    When I got done, it measured at:
    Blade 1: 16.1/24.5
    Blade 2: 16.2/24.9
    Blade 3: 15.9/24.5
    Blade 4: 16.2/24.1

    I was shooting for 16/23 or 16/24, but will take a little extra on the trailing edge. I'd say it's not too bad, considering the measurement range on the stocker and what I've heard about the Skat Trak props.

    I learned that it's pretty easy to tweak the front and back corners to get a prop to measure the desired result. However, by doing that, you're leaving most of the blade untouched. You have to start close to the middle of the blade and work your way back, bending it one way, then work your way forward from the middle, bending it the other way. At the trailing edge, I bent it on average 4 mm down, and the leading edge 3 mm up. In the middle, it gradually transitioned from one to the other. I then plotted all the measurements in Excel to make sure all the blades were the same, and had to tweak one of them that didn't quite match.

    I attached my Excel file in case anyone is interested in the plots of the stock vs. modified blade profiles, plots of the 4 modified blades, or what any of the measurements or calculations look like.

    Mike
    Last edited by engineermike; 04-20-2008 at 03:00 PM.


  2. #2
    kp04rxp's Avatar
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    damn mike your always taking the extra step and really digging into this stuff... very interesting and thanks for the info!

  3. #3
    ou812's Avatar
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    I believe the stocker is a 12/19.
    Did you send impros the 15/20 to tweak on?

  4. #4

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    Quote Originally Posted by ou812 View Post
    Did you send impros the 15/20 to tweak on?
    Long story. I still have the 15/20 I got from you and it's on my Stage 1+ ski working fine. I had a another 15/20 that I tweaked 1 mm for the turbo ski. It worked reasonably well, but I wanted more speed on the stock 8100 rpm ECU. I sent it to IMPROS and he bent it to 17/22. The ski lost speed and pulled the same rpm. No one is quite sure what went wrong, but the general consensus amongst board members is that the leading edge is too steep and there's not enough curve in the blades. I sent it back to IMPROS and requested him to make it into a 15/24 and am still waiting to get it back.

    The whole goal here is to get 83 - 85 mph on the stock ECU. I got the power, if I could ever get he prop straight (er, bent).

    Mike

  5. #5
    rxpdoo's Avatar
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    PROP

    I Would Think That Bending The Prop This Many Degrees Would Impact The Overall Diameter, Increasing The Clearance At The Trailing Side To The Wear Ring And Decreasing The Clearance On The Leading Edge To The Wear Ring.

  6. #6

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    I got to test it today and I have bad news and good news.

    The bad news is that, at 76.1 mph at 8100 rpm, it didn't perform like you would expect a 16/24 to perform. I guess all 16/24's aren't created equal. The root angle must affect performance after all.

    The good news is that this may be a record speed for a stock prop on a stock ECM.

    It looks like I effectively extended the speed limit at 8100 rpm from ~69 mph up to ~76 mph, thereby adding 7 mph. Not too bad I guess. By the same token, I hope I can extend the max speed of the 15/20 by 7 by doing the same. I believe it's accepted that you can do 77 mph at 8100 rpm on a stock 15/20. If I can pitch it, or get it pitched, to allow 84 mph at 8100 I'd be happy.

    Mike

  7. #7

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    Spreadsheet didn't attach the first time:
    Attached Files Attached Files

  8. #8
    performance enthusiast AD4342's Avatar
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    mike, Im really interested in your work and in bending. You said that you start bending from the middle of the blade, so after you finish bending does the midpoint stay at the same height or does it change? What about the root angle ever changed it ? what does it effect?

  9. #9
    Matthew K ReDevilRXP's Avatar
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    I remember reading way back when. Peteshauling said he went 83 on a pitched stock prop.

  10. #10

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    Quote Originally Posted by BadGP1300 View Post
    mike, Im really interested in your work and in bending. You said that you start bending from the middle of the blade, so after you finish bending does the midpoint stay at the same height or does it change? What about the root angle ever changed it ? what does it effect?
    On the ones I did, the mid-point didn't change.

    The root angle doesn't change easily. You would have to cut and re-weld. As we re-pitch props more and more extreme (example: stocker bent to 16/25), the prop loses efficiency. It may get you more speed and any given rpm, but it's going to take a butt-load of power to get there.

    I'm not sure about Peteshauling going 83 on a stock prop. I coincidentally am running the hull that he went 83 on, but that's a story for another day. I bent the stocker to the point of killing the efficiency and only went 76 and it took a lot of boost to get there. I later switched to a 15/20 +1mm and went 77.5 mph at less boost. When you try to go fast on the stock prop, you need lotsa power and he was supercharged, so limited on boost.

    Mike

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