Fairing Issue

Discussion in 'Boat Design' started by FirstLight, Sep 4, 2012.

  1. FirstLight
    Joined: Sep 2007
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    FirstLight Junior Member

    Arrrgh. You know it's silly but you still go on!

    I have a question regarding fairing.

    I'm in the midst of rebuilding a 19' Lightning. I've started setting up external frames to support the outer skin (outside of the hull) while I work on the boat.

    I'm setting them up every two feet.

    Something interesting happened when I create templates at several frame locations.

    Where the template fits perfect facing one direction it is off by up to 3/16" when I flip the template around on the boats centerline.

    The boat was built in the 70's and the core is saturated and no longer laminated any longer so I can see why there is a discrepancy.

    My thought was to support the hull before removing the existing core. Does this make sense??

    Also, when all is said and done is 3/16" (over a 4.5' station) critical. It would seem to be a large number. One I'm not interesting in fairing out with bog.

    Any thoughts or suggestions greatly appreciated..

    Cheers,

    T
     
  2. alan white
    Joined: Mar 2007
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    alan white Senior Member

    Absolutely support the hull------ and attempt to make both sides mirror images of each other. While you may have no way to maintain all of the realigned shape permanantly, you will probably reduce the inequality by some margin by some means. 3/16" is a tiny diufference and shouldn't affect performance but it's always good to do what you can to reduce assymetry. Fairing would be a waste of time and even counterproductive, adding weight and lots of money and labor as well.
     
  3. FirstLight
    Joined: Sep 2007
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    FirstLight Junior Member

    Thanks for the input. So you are suggesting making the frames symmetrical. Most likely having to use the larger cutout side versus the smaller cutout side so the skin has room to fall into the cutout versus creating a bump with the smaller cutout side. If that makes sense.

    Also, do you think 3/4" plywood every two feet will support adequately. This would give a total surface area of approximately 3/4" x 4' (frame width average) x 8 frames = 288 square inches which would be about 1.5 pounds / sq inch ( not including inaccuracies in frames wich create mini bumps and hollows).
     
  4. alan white
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    alan white Senior Member

    In case you haven't been there....
    http://www.lightningclass.org/classrules/documents/ilcaBylaws.asp#measurements

    3/4" @ 24" centers is entirely adaquate and might even be overkill, though it's hard to tell not seeing the boat. Make sure the frames are well braced and also lay straight edges (6 ft long or so) one across bow and one across stern and eyeball them for parallel to ensure there's no twist.
     
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  5. Petros
    Joined: Oct 2007
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    Petros Senior Member

    symmetry is over rated, the hull is never symmetrical in the water when under way, it does not matter that much in recreational sailing. However if you expect to enter into class racing, it has to be within the limits specified by the rules.

    One thing that might be possible, is once you remove the the inside frames you might be able to force the hull to be closer to symmetrical, and than install the new frames to hold the new shape.
     
  6. rwatson
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    rwatson Senior Member

    The other posts have indicated that you are dealing with very little to worry about.

    But, just be careful, that by making the 'skinny' side match the 'fat' side, you arent introducing 'ripples' on the hull

    Its the smooth, fair lines that you should be looking for, more than engineering scale measurement tolerances.
     
  7. messabout
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    messabout Senior Member

    Don't sweat the symmetry. If it makes you feel any better you can look at Hobie 14 or 16 hulls. They are deliberately asymetric and they work very well.

    The only reason for troubling yourself about the matter is whether the class rules are so tight as to make the boat illegal. Even if you never intend it as a class racer, having a class legal boat makes it more valuable when you sell it.
     
  8. FirstLight
    Joined: Sep 2007
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    FirstLight Junior Member

    Good thoughts on Symmetry

    Great thoughts on Symmetry. Thanks for talking me off the ledge on that one.
    Another interesting thought that came about was that for the most part only one half of the boat is in the water at a time thus making fairing more important than symmetry.

    For fun are a couple of photos showing part of the extra 350 pounds of weight soaked up by foam in the hull.
     

    Attached Files:

  9. FirstLight
    Joined: Sep 2007
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    FirstLight Junior Member

    While I'm looking at the fairing side of things a few weeks down the road. Wondering who makes a light fairing compound? Just looking at the bumps (or hollows) I'll be dealing with I'm seeing two to three gallons of goop being required.

    Wondering who make a light filling compound (either vinylester or epoxy)?
     
  10. rwatson
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    rwatson Senior Member

    I've always made my own - buy a bag of microballoons ( looks like icing sugar ) and mix it into the goo you are using.
     
  11. alan white
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    alan white Senior Member

    Try West System #410 Microlight. More expensive than Microballoons so build up with the microballoons and finish with #410 Microlight. It's very smooth and fine-textured.
     

  12. FirstLight
    Joined: Sep 2007
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    Location: North Carolina

    FirstLight Junior Member

    Thanks Alan. I'll look into it. I also found a two part epoxy filler called Superfil sold by AircraftSpruce (love those guys). It weighs 3.5 pounds per gallon of volume. Cost is around $60/gallon.
     
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