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  1. #1
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    Is my engine too high? Rigged on a 30" gill bracket

    So I wanted some help with an issue I seem to be having. Boat is a 23' Concord LS-7 with with 30" gill bracket and 250hp Merc 25" shaft. 27 pitch 4 blade prop. The boat seems to have tons of cavitation and what feels like the prop is slipping. I have to have engine trimmed almost all the way down to even be able to take off the boat. I'm getting to full rpm however only getting 46mph out of her. She should hit mid 60s I would assume. I measured some heights as I heard an inch rise for every foot back but this seems like engine may be too high?

    Another theory was maybe it could be the prop is missing a ring in the front of it? Or the hub is shot but I feel I couldn't do that kind of speed
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    Last edited by LIGlastron; 05-25-2020 at 09:32 PM.

  2. #2
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    Does the boat porpoise?
    LETS GO BRANDON.... LETS GO BRANDON

    Sometimes I talk to myself...then we both just laugh and laugh

    '84 Checkmate Convincor

  3. #3
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    It does porpoise when trimmed flat at full speed and slight chop but mainly just feels like unless I'm very easy on gas accelerating it will start to slip where I have to stop the boat and allow the prop to grip again. It's hard to explain. I mean engine is a 250 efi and I'm getting 5000rpm out of it however I feel I'm losing alot of bite with the prop? Does this seem like I need the engine lower? I'm 4 holes up so I would get about 2" lower if I stopped all the way

  4. #4
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    I would definitely check the prop too... what diameter is it...are all the vent holes open?
    I would start by lowering it a couple/few holes and see how it handles.
    And the wot rpm range on motor is,? 5000-5500?
    Whats the gear ratio on the lower? 1.87?
    Last edited by mrichartz; 05-25-2020 at 10:36 PM.
    LETS GO BRANDON.... LETS GO BRANDON

    Sometimes I talk to myself...then we both just laugh and laugh

    '84 Checkmate Convincor

  5. #5
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    I'll have to check the diameter however it does appear that I couldn't put a larger diameter prop, it looks to be the largest for the gearcase however I'll double check. All pvs holes are plugged, I did try opening two up a week ago but it only hurts performance. I believe Max rpm is 5000-5800 I'm finding . Im pretty sure gear ratio is 1.75

  6. #6
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    Engine does not seem too high to me. I am suprised that you can even turn a 27 4 blade prop. Try to borrow a Bravo 1 or a rev4 prop and start from there. I would start with a 22 or 24.

  7. #7
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    Quote Originally Posted by Turborr View Post
    Engine does not seem too high to me. I am suprised that you can even turn a 27 4 blade prop. Try to borrow a Bravo 1 or a rev4 prop and start from there. I would start with a 22 or 24.
    Way over propped could make sense to me too, this is how I got the boat already set up like this and the guy claimed 68 MPH. It does feel like it could be over propped that was one of my first theories

  8. #8
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    So after doing some research today of just searching for what props do people recommend for the LS7 I've been seeing a lot in the 22 pitch range like a 22 pitch raker at 15"in diameter. I'm just curious what made the previous owner throw a 4 blade 27 pitch, seems way over propped from what I'm reading.
    So to give a idea of what I'm going through, take off I have to be trimmed all the way down and put the throttle to the floor, bow goes up and it takes a decent amount of time to get on plane I'd say about six seconds or so just to start leveling out then I have to ease off throttle and be real easy on it so I don't get the prop to start spinning. Then I can slowly trim it up. like I said max speed is around 46 miles per hour at 4900 RPM oh, it does feel very stable but it's going away slower than it should so I think I'm going to try a prop next before I keep tinkering with the motor thinking something's wrong

  9. #9
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    Do you have a water PSI gauge? And does the lower unit have low water pick ups?...

  10. #10
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    Oh you bought the boat that way...and your just figuring what you have...
    Guy probably threw that on and kept the other prop..?
    LETS GO BRANDON.... LETS GO BRANDON

    Sometimes I talk to myself...then we both just laugh and laugh

    '84 Checkmate Convincor

  11. #11
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    Quote Originally Posted by mrichartz View Post
    Oh you bought the boat that way...and your just figuring what you have...
    Guy probably threw that on and kept the other prop..?
    Just about sure that is what happened. No way that boat will pull a 27 4 blade to top rpm. Hopefully you can borrow couple props to see where you need to be. I don't think you are too high. If you were loosing water pressure or overheating the engine will tell you.

  12. #12
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    Quote Originally Posted by Turborr View Post
    Just about sure that is what happened. No way that boat will pull a 27 4 blade to top rpm. Hopefully you can borrow couple props to see where you need to be. I don't think you are too high. If you were loosing water pressure or overheating the engine will tell you.
    Good.point on the prob theory maybe he had a prop he wanted to keep and gave me that, I just figured he sold.me boat rigged properly. I do have a water pressure gauge , I'll have to look at full throttle wide open how much pressure it's pushing, however the temp gauge is within check and I always have a steady piss stream. Also has lower water pickups in front of cone, and upper pickups. Given I'm doing 46mph would I really possibly gain a possible 20mph from a 22 pitch prop which is approx the speed people seem to run on this setup with a 250?

  13. #13
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    With the 27prop 1.75 gears 46mph @5000 you have slip of 37 ..
    If you use a 22 prop 1.75 gears @5500 would be 65.. BUT THATS w no slip...so guessing I'd say about 60 w a 22 w slip and if everything else is ready to run
    LETS GO BRANDON.... LETS GO BRANDON

    Sometimes I talk to myself...then we both just laugh and laugh

    '84 Checkmate Convincor

  14. #14
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    Quote Originally Posted by mrichartz View Post
    With the 27prop 1.75 gears 46mph @5000 you have slip of 37 ..
    If you use a 22 prop 1.75 gears @5500 would be 65.. BUT THATS w no slip...so guessing I'd say about 60 w a 22 w slip and if everything else is ready to run
    Thanks that's definitely more in the range for this boat, it is a heavy hull. Going to try a new prop and see what I find. Also in one of the pics I added in the thread, is there supposed to be a ring for the front of the prop to prevent air passing through? I'm wondering if it's also missing a piece?

  15. #15
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    too me I would say your running too low, not too high. the prop dont fit the gearcase and is getting way too much exhaust flow over the blades for a start.. this will cause a load of performance on that type of prop in your situation.. but the 1 inch up in every foot is a crazy thing as it would mean a 6 inch jack plate you only go up 1/2 inch ? and a 12 inch jack plate you only go up 1 inch ? seriously, who the hell came up with that daft formula. most people buy a 6 inch jack plate and go up 4 inch straight away. to be back that far on the gill bracket I would be going up higher..... look at the way the bracket bottom is and compare that to your ventilation plate because thats where the water is coming from.. get rid of the gap between prop and hub will get rid of a lot of your cavitation.. just my thoughts on what I see.

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