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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
    ob nut's Avatar
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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

    Woodstuff Sport K&B SS
    HTB290 MAC Lawless
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    Aerotech K&B 11cc Lawless

    In process: PTi PX-300, NX-300, NX-320.

  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
    ob nut's Avatar
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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

    Woodstuff Sport K&B SS
    HTB290 MAC Lawless
    34" WOF K&B Redhead
    Aerotech K&B 11cc Lawless

    In process: PTi PX-300, NX-300, NX-320.

  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
    ob nut's Avatar
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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

    Woodstuff Sport K&B SS
    HTB290 MAC Lawless
    34" WOF K&B Redhead
    Aerotech K&B 11cc Lawless

    In process: PTi PX-300, NX-300, NX-320.

  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
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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

  9. #9
    I also have a vintage tunnel hull boat with a kb 7.5 engine. I recently just got into the boats and I ran the boat for the first time last weekend it ran great after running it I realized that the mounting plate for the outboard came loose I do not know where it originally sat now as it was fine where it was how do you adjust this and what are the guidelines that you go by any help would be great thank you

  10. #10
    ob nut's Avatar
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    After dropping off the face of the earth for 5 years, I myself returned back to running/racing my tunnels. With new boats, had some issues with adjusting the depth. Met a good man at a race this year that gave me some sound advice. Any tunnel, start the prop shaft angle even with the spoons with 1/4" above the line. Since taking this advice, never had a problem with a good starting point. This is for all tunnels and motors.

    RIP Rick....
    Mark Scott - just a nut for outboards

    Woodstuff Sport K&B SS
    HTB290 MAC Lawless
    34" WOF K&B Redhead
    Aerotech K&B 11cc Lawless

    In process: PTi PX-300, NX-300, NX-320.

  11. #11
    I'm new to the nitro boat scene. I recently picked up a vintage nitro boat with a KB 7.5. Its in pretty good shape but seems a little old. Lol. It will scoot that's for sure. What are the best upgrades for the kb 7.5 engines and what maintenance should have to be done on the foot of the boat engine ( lower unit). 20161202_115540.jpeg

  12. #12
    ob nut's Avatar
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    Best for any motor are new bearings, seals/gaskets. Turbo the crank, clean up the cylinder ports for better flow. The lower needs a sharp front and flat bottom. The bottom should be close to how the boat will run. Believe it or not, it is a running surface. Shorten the length of the skeg if the boat turns easily. If it's too short, boat won't turn. Set up an oiler for the lower. Or at least make sure the cable is well greased or oiled for each run. Starting props would be X447 or X448.

    For more power, get a pipe: Silver Bullet, Equi, SPP, Irwin, etc...
    Mark Scott - just a nut for outboards

    Woodstuff Sport K&B SS
    HTB290 MAC Lawless
    34" WOF K&B Redhead
    Aerotech K&B 11cc Lawless

    In process: PTi PX-300, NX-300, NX-320.


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