Building foam sheets over MDF frames. Best way to secure?

Discussion in 'Fiberglass and Composite Boat Building' started by mrybas, Aug 16, 2019.

  1. mrybas
    Joined: Jun 2017
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    Location: East Coast or Cruising

    mrybas Junior Member

    Hi,
    I’m building a 14’ catamaran dingy out of Divinycell H80. What is the best way to secure the foam sheets onto the MDF frames while laminating the exterior of the boat? Stainless brads? plastic screws?
     

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  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Trim screws work well. You can pull them through the foam after the exterior is laminated.
     
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Normally you'd have longitudinal battens over your frames, and the foam screwed to that, from beneath.
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Typically, you would cleat the foam from underneath.

    But you can also use a wood screw and a flat wood washer about 2" square.

    MDF will require predrilling or it will shatter.

    For my build in a female jig, I used screws into battens running for and aft; sadly had to penetrate the laminated panels. I wasn't crafty enough to create a method that didn't. However, some of my panels were held in tape seams and this s much preferred. Same for you when it works....
     
  5. fallguy
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    fallguy Senior Member

    Also, for your hard chine design, you can just screw; then glue the seams; then remove the screws. The boat should hold the shape pretty well at that point or you have erred.

    If you had a trouble spot; you can also hot glue the foam to the jig temporarily.

    I used hotglue a lot in my foam build as a temp hold.
     
  6. Chuck Losness
    Joined: Apr 2008
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    Chuck Losness Senior Member

    Take strands from woven roving and tie the foam to the mold. After glassing the exterior all you have to do is cut the strands to remove the hull from the mold. As suggested above it would be best to have ribbons to tie the foam to.

    Have fun with you project.
     
    fallguy likes this.
  7. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    If thats the extent of your support structure,you have an unpleasant surprise ahead.Ordinarily I would expect some definition of the stem(s) and sheer and a liberal covering of battens.You can sew or tie the foam to any part of the framework and you need to take some care to make the joints between sections of foam fair.
     
  8. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Fishing line or back screw. Might be worth pre laminating sheets and leave one skin off near the bow.
    Jeff
     
  9. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    A fair assessment. There needs to be a minimum of a batten at the gunwhale.
     
  10. mrybas
    Joined: Jun 2017
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    Location: East Coast or Cruising

    mrybas Junior Member

    I got a chance to dry fit the pieces to the frames today. They fit pretty well except where there’s a lot of direction change close to the bows. I’m thinking about drilling small holes and dry stitching with leftover carbon scraps to pull the panels together (or maybe small zip ties)?

    Next step is to bog the joints, sand joints, then tape.
     

    Attached Files:


  11. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Looking good.
     
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