tunnel boat

Discussion in 'Boat Design' started by Nodded, Aug 22, 2014.

  1. Nodded
    Joined: Feb 2014
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    Nodded Junior Member

    just completed my 3 year project boat. took her for the shakedown cruise and all was good, except:

    since there is a tunnel now where the keel used to be, at speed (on a plane) the boat slides around side to side.

    I think I would like to install a skeg on either side of the tunnel and i remember bolt on aluminum skegs like are used on hydroplanes, but can't find any.

    Ideas?

    thanks (i'd insert an image but can't figure out how to do it)
     
  2. Nodded
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    Nodded Junior Member

    i think i just figured out how to post image
     

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  3. kapnD
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    kapnD Senior Member

    Maybe visit a surf shop to look at surfboard fins and boxes. Plastic fin might be a good idea as it would breakaway on a grounding instead of coming up through the hull
     
  4. Nodded
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    Nodded Junior Member

    That's what I'm looking for but I think surfboards skegs would be too small to be effective.
    I also would like ti stick to a material a little tougher than plastic. If I ground the skegs the prop and rudder would already be buried. Skegs would be up on the deadrise and about 12" above the wheel.
    You can see the tunnel in this early pix.
     

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  5. Mr Efficiency
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    Mr Efficiency Senior Member

    When you say, slides around, is this only occuring at certain angles to the wave train ? Is the boat trimming a lot flatter than before ?
     
  6. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Nice job!
    How do you steer this boat? Rudder(s) in front of the prop? I have used two tunnels with four rudders on either side of the tunnels. The boat behaved like it was on rails, turning was very slow unless there was a large difference in rpm.
     
  7. Nodded
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    Nodded Junior Member

    I'm upretty sure a skeg on either side of the tunnel will solve the problem but I can't find any to buy so will probably build my own.
    I have the build ( the early part) on you tube. 2 videos , search for " dream boat project" part 1 and 2
    The propeller/rudder arrangement is typical with rudder behind wheel and not protruding past transom.

    If I cross a wave or wake that is running nearly parallel to boat the boat slides off and it is difficult to maintain a heading. Slow speed maneuvering is fine
     
  8. hoytedow
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    hoytedow Wood Butcher

  9. PAR
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    PAR Yacht Designer/Builder

    I think I see a rudder there, in it's usual location CDK. I think we need some at speed photos both profile and while turning to really get an idea of what's up. I think skegs could cause new problems in turns, if not placed or sized properly.
     
  10. CDK
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    CDK retired engineer

    Yes, thank you, my old eyes had trouble finding it....
     
  11. Nodded
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    Nodded Junior Member

    It will be about 2 weeks before I can get som pix at speed., but will post them when I get back to FL
    THANKS
     
  12. tom28571
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    tom28571 Senior Member

    There appears to be adequate lateral plane on that boat to avoid sliding around without any additional appendages. Are you perhaps having some chine walking issues? That may be a stretch, but it doesn't look like sliding should be an issue at all. The fact that it maneuvers well at low speed makes me wonder about other things other than lateral plane which is the only thing skegs or fins would add.

    Was the boat originally a inboard/outboard? Taking that off would result in a definite reduction in steering effectiveness and tracking.
     
  13. Nodded
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    Nodded Junior Member

    Boat was an IO, however, because of new tanks location, weight distribution is the same. Attitude of the boat is exactly wher it should be at all speeds.
    When I say slides, I mean the transom (or the last third of the boat) seems to "swivel" on the bow. Maybe another way of putting it is; it does't track
     
  14. tom28571
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    tom28571 Senior Member

    That is what I meant. The foot of the IO provided quite a bit of longitudinal area, and the small rudder that replaced it is likely not adequate compensation. Making the tunnel removed more of the tracking provided by the keel that you cut off.

    If the problem is bad enough to warrant it, you probably should add some lateral plane in the form of skegs. The skegs should terminate at least 18 inches forward of he prop to avoid ventilation in turns. That is what works on most boats but I don't know about tunnels.

    The boat probably does not bank inward as much now as it did with the IO.
     

  15. gonzo
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    gonzo Senior Member

    Skegs may make you track straight, but will prevent the boat from turning; particularly at higher speeds. A larger rudder or multiple rudders is probably what you need. Basically, if the boat run well before, you need to figure out how the modification screwed that up.
     
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