Spoke to soon......25 merc on Stumpknocker

Discussion in 'Prop Shop' started by Bigkahuna, Nov 14, 2014.

  1. Bigkahuna

    Bigkahuna Well-Known Member

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    Well me and my big mouth dun did it again...I thought I had my micro good to go but got a problem and need some experienced advice.
    Im having a problem with blow out at about 1/3 to 1/2 throttle. Set up is 14'5 stumpknocker with 25 merc, Vance jackplate 5" setback, and aluminum 4 blade 12 pitch.
    First I thought it was weight..I weigh about 280 and moved to the front to level it out...No good.
    I started with the jackplate all the way down and motor pinned at the lowest setting and tested it at different settings with no improvement.
    Im thinking motor is too high..Cavitation plate is about 2-3" above hull but originally I figured the setback would fix that... I am absolutely baffled any input is greatly appreciated...[​IMG][​IMG][​IMG]
     
  2. No Bait / Lures Only

    No Bait / Lures Only I Love microskiff.com!

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    Aluminum prop with no cup is probably the problem, sra PT should fix the blowout issues. Give Power Tech a call...... ;)
     

  3. DuckNut

    DuckNut Brandon, FL

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  4. Bigkahuna

    Bigkahuna Well-Known Member

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    I appreciate the input...I put this up on iboats and got some good info as well. I'm just trying to get this thing to run skinny....:)
     
  5. HaMm3r

    HaMm3r Well-Known Member

    That isn't too high, you're just not propped right. I agree with the PT SRA recommendation. I ran a 4 blade 15 pitch on my Merc 25 that was jacked up to a similar height. You could also add a hydrofoil which will help with cavitation a lot. The SE200 worked well for me.

    Here's a video showing my current running heights with combo jackplate/TNT, PT prop and hydrofoil on a modified Merc/Hatsu 30HP 2-stroke.
    [media]http://www.youtube.com/watch?v=1fq1oBb_0Vg[/media]
    http://youtu.be/1fq1oBb_0Vg
     
  6. cutrunner

    cutrunner Cert. Yamaha technician

    Its hard to say because every hull is diffe[ch341]ent. In my opinion its way high. I had a somewhat similar flat bottom skiff with even more setback and the highest i could get the cav plate over the hull bottom was maybe 1/2 inch.. but that was with a 3 blade
     
  7. DuckNut

    DuckNut Brandon, FL

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    Agree with cut. The flat bottom and the boats draft is so little that there is not enough water above the prop.
     
  8. No Bait / Lures Only

    No Bait / Lures Only I Love microskiff.com!

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    PT-SRA VS TURBO HOT SHOT 3 Blade props on a flat bottom skif : The turbo is faster, but have to run it very low on the transom as the cupping is very little. The pt-sra is good very high on the transom, a little slower top speed. You have to make a choice higher/lower, 1mph difference in speed..Compromises......... :-/
     
  9. lil_kahuna

    lil_kahuna I Love microskiff.com!

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    Update: lowered it 2" and it will plane..takes a few but getting closer. Set at the lowest hole and cat plate is 1" above pad. That's the lowest I can set it due to tiller clearance. I'm definitely gonna have to reprop or look in cupping mine. Any input on self adjusting tabs? Thanks
     
  10. jwarren

    jwarren I Love microskiff.com!

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    I had near identical set up until I sold my Stumpnocker a month ago and I did extensive testing. I used a homemade jack plate and raised my 18 hp Nissan about 3" with only 1.5" setback in the riser. I put an SE200 on the motor and that was the key. It jumped the boat up on plane in NO time and put the bow down. It dramatically improved handling and turns with no blowout. Did all this with stock prop. Get a tach meter and record your mph and rpms at different heights and trim tab settings.
     
  11. jwarren

    jwarren I Love microskiff.com!

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    I had near identical set up until I sold my Stumpnocker a month ago and I did extensive testing. I used a homemade jack plate and raised my 18 hp Nissan about 3" with only 1.5" setback in the riser. I put an SE200 on the motor and that was the key. It jumped the boat up on plane in NO time and put the bow down. It dramatically improved handling and turns with no blowout. Did all this with stock prop. Get a tach meter and record your mph and rpms at different heights and trim tab settings.