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  1. #1

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    7.5cc O/B tunnel hull set-up questio

    I've used the search feature but can't seem to find the right thread. Does anyone have a link to photos or text describing basic engine setup for a 7.5cc tunnel hull? I mean basic stuff, like the height to set the engine on the transom relative to the tunnel bottom and a starting trim angle (I understand these are unique to each hull but there must be some basic guidelines). I have such a boat bought used and the engine seems to be too high on the transom, i.e. the anti-ventilation plate is set much higher than the bottom of the tunnel.

    Thanks.

  2. #2

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    RE: 7.5cc O/B tunnel hull set-up questio

    Most of the current "race" boats out will always start around the prop line at 1/8" +/- above the spoon bottoms. With that, about 1 degree neg on the thrust angle. Start with that and work the setup. Also, the CG is around 27% to 33%. A X447 prop is a good starting point.

    You might want to look at this thread: 7.5 tunnel thread
    Mark Scott - just a nut for outboards
    A.D.I.D.A.S...

  3. #3

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    RE: 7.5cc O/B tunnel hull set-up questio

    Thanks for the thread link. I'll read it in its entirety. I'm not racing- just want to have a little fun with this boat in my pond. So, I'm a little behind on the phrases and nomenclature. Are you saying the centerline of the prop should be 1/8" above the bottom of the sponsons? I'm accustomed to full-sized boats where the anti-cavitation plate is normally even with the bottom of the hull. (These are not racing boats either)

  4. #4

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    RE: 7.5cc O/B tunnel hull set-up questio


    ORIGINAL: Mogrits
    Are you saying the centerline of the prop should be 1/8" above the bottom of the sponsons?
    Yepper.. It's a starting point. Might need to go up a bit.
    Mark Scott - just a nut for outboards
    A.D.I.D.A.S...

  5. #5

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    RE: 7.5cc O/B tunnel hull set-up questio

    Okay then. Instinct tells me to set this motor far too deep. I understand having it too deep will create too much drag but I'll start about 1/8" above the sponsons at the rear of the boat, with a slightly negative trim (bow-down). I'll take it from there. I did read the entire 7.5cc thread but apparently it assumes a level of knowledge I don't possess. Thanks for your help.

  6. #6

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    RE: 7.5cc O/B tunnel hull set-up questio

    Prop to deep - boat will "plow" thru the water. prop to high - boat will be "flighty".. The full size racing tunnels also run the prop half submerged. This allows the motor to rev up..

    Good luck..
    Mark Scott - just a nut for outboards
    A.D.I.D.A.S...

  7. #7

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    RE: 7.5cc O/B tunnel hull set-up questio

    After taking sometime off I'm returning to the sport. Most of my experience has been with K&B's. I still have a 3.5 and a 7.5 both stock with SS sharpened and balanced props. Engines stock but with polished lower units. I always ran them on Deep Vee's.....

    I decided to try something differant. I ordered a Rossi outboard with Tuned pipe from Hyperformance and a TX45 hull. I'm sure I will haev some Questions after I get the custom paint work done.

    If someone can hook me up with a link to better inform me on where to mount the engine that would be awesome. Any info on desired Slip % How to achieve max RPM and potential out of this setup?!

    I do not believe there is a local group of racers here. Back in the day there was but I don't think they exsist anymore! Does anyone knows of a group near Albany NY ? If not it will just be a recreational play boat like the rest.

    Thanks

    Kris

  8. #8
    Ron Olson's Avatar
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    RE: 7.5cc O/B tunnel hull set-up questio

    The club near Albany might be a NAMBA club. Check www.namba.com , if not try www.impba.net .
    IMPBA 17238 S
    269 Racing Team


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