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  1. #1
    Vern's Avatar
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    Prop trailing edge measurement

    I know many of us try different props, and tweak the trailing edges for best rpm/speed. What I wondered was if we posted the average measurement, what it would be ... in general a larger impeller, with the trailing edge pulled back a little seems best for high speed. When I measured with a flat edge sitting on the hub, the very outside trailing edge of my modded 15/20 is approximately 6 mm below the plane of the hub. I think this is close to stock, but might be a little on the 'tight' side compared to some stockers.

    Anybody else? I would love to hear what most stock 15/20s measure in this manner, stock or not. Not trying to figure pitch, only the distance of the trailing edge to the hub, using a straight edge on the impeller hub.


  2. #2
    Moderator The Bandit's Avatar
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    Vern, not quite sure how you are measuring and from where.
    If you can show in a pic, I will give you some numbers from my prop collection in the garage...

  3. #3
    Vern's Avatar
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    Got it, sorry, I have no pics I can post right now, but what I mean is when you have the prop sitting on its nose, put a straight edge across the hub, then take a micrometer and measure from the straight edge down to the very outside corner of the trailing edge. Typical measurements done this way should be anywhere from 5 - 10mm usually.

    Does that help? I know a picture would be better ...

  4. #4
    Moderator Franko's Avatar
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    Are y'all using digital micrometers? If so, what kind and where do you get them. I want one to make my measurements more accurate.

  5. #5
    You r on to what I have already done and they have been doing this for years n racing. Here is theory! Move prop back closer to hubs gives you bottom and mid. Move prop forward pulling away from hubs gives u all top. I have done the bottom end trick and found it is true. They do this in Europe all the time. I took a 13/18 3blade and cut 3mm out of the back. Leaves great but motor is straining so hard to get to top. Call me with any other ?a 804-502-5515

  6. #6
    Top banana's Avatar
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    6.88mm 14/19 from outer edge to hub its the way i bend them

  7. #7
    Vern's Avatar
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    Thanks Beachboy, that confirms what we have seen with other skis in the past too. This is why running a bigger prop with the trailing edge pulled back seems to give the highest top speed. Just FYI, last year my 15/20 trailing edges were around 8mm from the hub ... I then bent them back this year, at 6mm now, but I think that is too close for my setup and I need to bend it back to 7mm at least.

    What I wish I knew was what the optimal distance from trailing edge to pump vanes is ... primarily for best top speed on my RXP. I think too that once you push the prop too close, it really bogs down the motor on top; it seems better to have more pitch and keep the trailing edge a little farther away from the vanes. Since many props are different (Skat, Solas styles are different, then 3 blade vs 4 blade, etc), optimal distance might have to be determined for each style of prop. Also, 'stock' distance from vanes likely varies among different prop styles, contributing to 'hit or miss' results with different stage skis.

    If only we could test all day ... we could figure this out. But maybe by us all measuring different impellers and doing some experimenting we can come up with what is the most common optimal measurement. Sharing that might help some people who are struggling with low speeds ...

  8. #8
    Vern's Avatar
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    Quote Originally Posted by Top banana View Post
    6.88mm 14/19 from outer edge to hub its the way i bend them
    Hmm, you posted while I was scribbling ... but we both kinda pointed to 7mm. A good start I think ...

  9. #9
    Red Devil's Avatar
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    Quote Originally Posted by Franko View Post
    Are y'all using digital micrometers? If so, what kind and where do you get them. I want one to make my measurements more accurate.
    Yep, I picked one up at Harbor Freight for about $25 and have been using it for 2 years. It is a 6" one and works great. No need to spend a bunch of cash on one.

    Check the site:
    http://www.harborfreight.com/cpi/cta...emnumber=95846
    Last edited by Red Devil; 07-25-2007 at 09:50 PM.

  10. #10
    Vern's Avatar
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    Yup, what Red Devil said, digital micrometer from Harbor Freight, works great.

    FWIW I measured 3 impellers trailing edge tonight. Stock, untouched RXP prop, averaged 6.7mm, stock impeller redone by Impros, averaged 5.7mm, my 3 blade 15/19R averaged 7.6mm. I have not tweaked any of these myself. Comparing the 3 blade is unfair tho, the design is so different ... on the 4 blades, the trailing edge is roughly 6.5mm right where it joins the hub ... on the 3 blade, the trailing edge is roughly 3.9mm where it joins the hub. So even at 'stock' measurements, the 4 blades trailing edge is pretty horizontal, whereas the 3 blade trailing edge rises going away from the hub.

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