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  1. #1
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    Altitude vs prop slip

    I know altitude effects horsepower, but can it also affect prop slip?

    I am running a 118c with a 150 show 21P yamaha prop. I am running at 4500 ' in elevation. I'm 4" prop to pad with a manual jackplate....
    I get 44 mph at 5800 RPM. Basically 20% slip. Takes forever to get on plane. Doesn't seem like there is much bow lift either.
    I think I should be able to get better than that, either by changing props, playing with set up etc. Thinking of going to a 4 blade...

    Any suggestions?

    Thanks

  2. Moderator 21XDC's Avatar
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    #2
    Less HP makes the boat go slower and drags more hull in the water. This causes more slip. The extra speed helps pack air under the hull and lift the bow. Reducing wetted surface always increases speed and efficiency. Creating less slip.

    Marks Props 317-398-9294, 1850 East 225 South, Shelbyville, Indiana 46176 propellerman59@gmail.com http://www.marksprops.com/index.html

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    #3
    Thanks, that makes sense. We drop pitch to get RPM's up but boat doesn't move forward as far. Doesn't sound like a prop will help me much..

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    #4
    That's Mike's take. My take is that since water is not compressible, and viscosity doesn't change, prop slip doesn't change. You do lose HP at higher altitude, so the outcome is the same. You need to drop pitch in higher altitudes.

  5. Moderator 21XDC's Avatar
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    #5
    Quote Originally Posted by John Jackson View Post
    That's Mike's take. My take is that since water is not compressible, and viscosity doesn't change, prop slip doesn't change. You do lose HP at higher altitude, so the outcome is the same. You need to drop pitch in higher altitudes.
    John... Check your slip at 44 mph... I'm sure it's more than you running 90 mph.

    More boat in the water increases drag/slip.

    Marks Props 317-398-9294, 1850 East 225 South, Shelbyville, Indiana 46176 propellerman59@gmail.com http://www.marksprops.com/index.html

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    #6
    So I guess my next question would be why altitude isnt factored into to all the slip calculators. ? I only found one that did on the mercury website. When I plug all my numbers in, it shows a vengence or trophy plus prop with a 21 pitch would get me to 57 mph... This is figuring in the 4500 feet of altitude... I have seen other posts with the same hull as mine having the same issues... I guess at this point I dont have anything to lose.

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    #7
    You loose about 3% HP per 1000' of elevation. So your 150 is making about 130 at most. Slower speeds equils less lift. Also how the boat is loaded is another factor. If you have everything crammed into the bow storage it will cause more speed loss.

  8. Moderator 21XDC's Avatar
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    #8
    Too much emphasis is put on prop "slip" ... If my best prop slips 30%, Who cares?

    Marks Props 317-398-9294, 1850 East 225 South, Shelbyville, Indiana 46176 propellerman59@gmail.com http://www.marksprops.com/index.html

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    #9
    Quote Originally Posted by 21XDC View Post
    John... Check your slip at 44 mph... I'm sure it's more than you running 90 mph.

    More boat in the water increases drag/slip.
    And that would be true at any altitude. 93 at 1000 ft vs 96 at 330 ft won't show any difference.

  10. Moderator 21XDC's Avatar
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    #10
    Quote Originally Posted by John Jackson View Post
    That's Mike's take. My take is that since water is not compressible, and viscosity doesn't change, prop slip doesn't change. You do lose HP at higher altitude, so the outcome is the same. You need to drop pitch in higher altitudes.

    Quote Originally Posted by 21XDC View Post
    Less HP makes the boat go slower and drags more hull in the water. This causes more slip. The extra speed helps pack air under the hull and lift the bow. Reducing wetted surface always increases speed and efficiency. Creating less slip.

    So now you agree to "Mikes Take"

    Altitude makes the boat go slower and increases slip.

    Marks Props 317-398-9294, 1850 East 225 South, Shelbyville, Indiana 46176 propellerman59@gmail.com http://www.marksprops.com/index.html

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    #11
    Slower at altitude, yes. Slip no. And I know it doesn't affect slip as the bass still fight just as hard. They ain't slipping.

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    #12
    I agree altitude will slow down a boat, which increases drag as the bow starts to drop. But I also believe I am not getting the best performance from the prop as I could. 22% slip I think is a lot, that is 1/4th of the props total efficiency. I think dropping a pitch would get me another 200 RPM till I hit the limiter, and adding some cup or rake should get me a little better lift.... possibly adding speed..... Just seems to me like 22% slip and 44 MPH is not what the best this boat should do...... Am I wrong?

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    #13
    I'm 5000' and have always had a hard time getting bow lift. I've wondered if the thinner air, less dense, has an effect on bow lift from the air packing under the hull. Aircraft have reduced lift at higher altitudes so it would seem this would also apply to boats?

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    #14
    The boat does not generate lift like an airplane wing. Airplane wing generates lift by the air moving faster of the top of the wing than the bottom. Boat lift is generated by hull design and engine and boat set up. Air does not "pack" lifting the boat, the hull climbs up on top of the water.

  15. Moderator 21XDC's Avatar
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    #15
    Pack may have been the wrong word as we don't have tunnel hulls... But air does lift the bow at higher speeds. Trust me, I know.

    Marks Props 317-398-9294, 1850 East 225 South, Shelbyville, Indiana 46176 propellerman59@gmail.com http://www.marksprops.com/index.html

  16. Member lpugh's Avatar
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    #16
    Quote Originally Posted by Rich1 View Post
    The boat does not generate lift like an airplane wing. Airplane wing generates lift by the air moving faster of the top of the wing than the bottom. Boat lift is generated by hull design and engine and boat set up. Air does not "pack" lifting the boat, the hull climbs up on top of the water.
    Apparently you have not watched race boats fly and become airborne, they most defiantly do get air lift, I feel this start to happen in my slow ass cougar really gets started at about 75 requiring neutral trim (no rooster tail at all) and gets a lot of lift, that is not all from just water pressure, The faster the speed the lower slip numbers are, thats a given
    Last edited by lpugh; 07-06-2020 at 08:21 PM.
    Thank You Leon Pugh

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    #17
    So would a different prop help? Seems a prop with better bite would reduce slip, making the boat go faster therefore reducing the wetted surfaces, even decreasing the slip more, reducing yet more wetted surfaces etc etc,..... Or should I just pump nitrous in the intake...

  18. Moderator 21XDC's Avatar
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    #18
    A 3 blade bow lifting prop would be your best bet, In my opinion. If your prop is a 21M Pro Series, I'm not sure there is much speed left. Try a known good, 21P Tempest Plus if you can.

    Marks Props 317-398-9294, 1850 East 225 South, Shelbyville, Indiana 46176 propellerman59@gmail.com http://www.marksprops.com/index.html

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    #19
    Quote Originally Posted by Invader186S View Post
    I'm 5000' and have always had a hard time getting bow lift. I've wondered if the thinner air, less dense, has an effect on bow lift from the air packing under the hull. Aircraft have reduced lift at higher altitudes so it would seem this would also apply to boats?
    I fish at 6800 feet and ran a fury 25 from my old boat. Couldn't get any lift and would top out at 69 mph. I then ran a bravo 1 XS 22p and obviously I hit the limiter. But the lift is unreal at 66mph.

    The bravo XS is the best prop hands down at higher altitude. Just my opinion.
    I also just ordered a 25 bravo XS and can't wait to see how that does.

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    #20
    Im thinking about trying a bravo as well. Looking at the XS it looks like lowest pitch is 22 and 15.25 diameter. I think I would be better off with the FS at 20P