Blow out!

Discussion in 'Boat Yard Basics' started by firecat1981, Mar 30, 2010.

  1. firecat1981

    firecat1981 BBA Counselor

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    Well I put a jackplate on and now I think I'm experiencing prop blow out (cavitation) :(

    ok heres the deal, the whole reason I built my jack plate was because I didn't feel I was running shallow enough and even when on plane the cavitation plate was still a few inches below the surface even without a loaded boat. With the plate installed at it's lowest setting I am sitting about 2 inches above where I was and set back is about 4.5 inches. I also have a SE200 hydrofoil on my motor (2009 yamaha 20hp 4-stroke)
    When I was testing it in the lake I first started with the second to lowest setting which gave me about 3 inches of lift. As I got on plane and speed increased it started feeling like the motor was choking, or someone was tapping the brakes, I tried adjusting the trim setting and it was best when the motor was at a negative angle (hole closest to the boat) but still not perfect. I went back with the JP now adjusted to the lowest setting, 2 or so, and it was a little better, but still very much there.
    I decided to take the SE200 off and see what happened. The blow out seemed slightly less, but it was harder to get on plane and stay there and I had some porposing. When I look back as I'm on plane now I can see the top of the SE200 slightly out of the water.
    So any suggestions? My thoughts are first trying to redrill the JP, I think I can get maybe another .75" lower. The only other thing I can think of is maybe a new prop. Any suggestions there?
    I think if I were running loaded down with buddies, ice, and full livewell then this would be less of an issue, but I fish alone alot too, so.......
     
  2. Brett

    Brett > PRO STAFF <

    So much fun, tweaking a set up for the absolute best performance.
    Type of prop used? Stock? Cupped? Stainless or aluminum?
     

  3. firecat1981

    firecat1981 BBA Counselor

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    prop is the stock aluminum piece, not sure of the size. Hey Brett maybe it's a design flaw :eek: ;D ;)
     
  4. Brett

    Brett > PRO STAFF <

    Stock aluminum props are designed to be run with the
    cavitation plate 1/4" to 1/2" below the surface of the water.
    In order to run in the turbulent, aerated water found at the wake surface,
    you're going to need a cupped, stainless, performance prop.
     
  5. Tom_C

    Tom_C Well-Known Member

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    First if your motor is to high, you are ventilating, not cavitating. I don’t think you are ventilating, because taking the SE200 (Anti-ventilation Plate) off would make it worst not better.

    Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

    Anti-ventilation Plate: Large plate cast into gearcase housing directly above propeller. Helps reduce surface air from being pulled into blades.

    Do you know what your RPMs are? With everything you are saying it sound to me that you are hitting your motor over revving limit setting. When you hit the over revving limit your motor will cut out.

    Lowering your jack plate make help out or a larger prop.
     
  6. firecat1981

    firecat1981 BBA Counselor

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    I don't know what my RPM's are, I was thinking of getting a tiny tach so I can find out. The over reving would make sense, but it starts choking at about 2/3 to 3/4 throttle. Is there a local shop to the bay area where I can buy a tach?
    I hope I don't need a SS prop cause thats a bit out of my price range right now.
     
  7. firecat1981

    firecat1981 BBA Counselor

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    Also how does the motor cut out when it hits the rev limiter? is it a studder, or does it just lose power, cause this is more like a studder.
     
  8. Tom_C

    Tom_C Well-Known Member

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    My johnson will studder.
     
  9. mark_gardner

    mark_gardner I Love microskiff.com!

    most motor will stutter or act like they are kinda misfiring when hitting the rev limiter. sounds like a ss 4 blade prop might be in your future :eek: but you might try getting the motor back down a little more too in the mean time
     
  10. Tom_C

    Tom_C Well-Known Member

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    An aluminum prop with more pitch will reduce your RMPs. No need to go to a SS. I have gone from a 10.5 X 10 to a 10.5 X 12 aluminum prop.
     
  11. mark_gardner

    mark_gardner I Love microskiff.com!

    true that he doesnt actually need a ss prop but if he's hitting the rev limiter from blowing out how would more pitch keep that from happening? sounds like he needs to get it back down in the water a bit more or a 4 blade to give it more grip and a little less slip :D ;D
     
  12. firecat1981

    firecat1981 BBA Counselor

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    ok before you guys get me all freaked out about SS props and such, where can I go buy a tiny tach today around tampa/lakeland?
     
  13. firecat1981

    firecat1981 BBA Counselor

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    after calling around all morning I have a line on a tiny tach locally so I may go pick it up. But it's gonna cost me :eek:
    That's just the price you pay for instant gratification I guess.
     
  14. Gramps

    Gramps Living &amp; Dying in 3/4 Time

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    It should only take 3 days or so to arrive if you order online.
     
  15. firecat1981

    firecat1981 BBA Counselor

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    too late, picked it up a few minutes ago while running around, I'll wire it up after lunch and see what I get on the lake before continuing my height adjustments.
     
  16. firecat1981

    firecat1981 BBA Counselor

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    ok it's definately not over reving. I bought the tiny tach this morning and have spent about 3 hours figuring it out and running the lake. First I wrapped my upper cylinder and it was showing an idle of 1050-1100 RPMs, it seemed to read better the more wraps I put on it. Then I tried the lower cylinder and it read an idle of 600-650.
    Anyway the whole time I was running around the lake the motor whould never get above 3000 RPMs no matter how I set up the tiny tach. Can this be? The range of the motor is 5000-6000 RPM's how can I only be able to reach 3000 at full throttle?
    I'm not exactly impressed with the tiny tach, I called to make sure I have the right one and the tech said I did. I don't get why everyone raves about these, there has to be a better product out there to read RMP's, The thing that annoyed me most was the updates every 1/2 second, which really is more like 1 second. I like to be able to see my RPM fluctuations in real time.
     
  17. topnative2

    topnative2 Well-Known Member

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    DITTO!
    I can not get past 3600rpm on a 20hp 4str 10p--called em and they advised maybe more raps --kinda hard to do after cutting the wire accord. to instructions

    go lower or cup it
     
  18. firecat1981

    firecat1981 BBA Counselor

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    Well I think I may just scrap the whole jack plate and return to my original setup. I'll set it to the side for the next project maybe. I'm thinking that I may have the prop set as high as I could have with the pocket drive and higher transom already. Building the Jack plate was fun and all, but it's causing me more trouble then it is worth.
     
  19. Brett

    Brett > PRO STAFF <

    The purpose of a jack plate is to get the last bit of performance from an outboard.
    Trial and error to find the point where the propeller starts to ventilate,
    then adjust back down a hair to where it functions properly.
    Or, if you're like me, you get it close enough and go fishing.

    ;)
     
  20. mark_gardner

    mark_gardner I Love microskiff.com!

    well i was talking to capn' ron today about my situation and with my ss prop i'm hitting 7000 as per my "new" tiny tach, he's says thats crazy as it should be bouncing off the rev limiter :eek: so i switch props to a steeper pitch and i'm down in the low 5000's and the motor sounds a little healthier :) so we're chatting about the accuracy of the tiny tach and he says he's seen to many hits and misses with them and his going a different route, i'm with you though as i wasnt overly impressed when i got mine, gonna try a few different set up to see what happens :cool:
     
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