How to taper a foam core plywood joint for maximum strength

Discussion in 'Fiberglass and Composite Boat Building' started by Norado, May 16, 2016.

  1. Norado
    Joined: May 2016
    Posts: 8
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Le Havre

    Norado Junior Member

    I am in the process of recoring most of the deck on my Sweden Yachts 34.

    When I cut out the old top skin I made the mistake of not leaving room for a proper bevel between existing skin and the new laminate. The plan is to rebuild the deck with biaxial cloth, epoxy and Divinycell or Airex foam.

    I'm trying to correct the mistake I made by putting a 3" wide marine grade plywood strip along the cabin. I would love some input on how to make the transition between the foam and ply.

    I've attached a picture of the area and a cross section sketch how I envision it.

    Should I glass over the ply before setting the core?

    I'm suspecting that it might be better making a shallower bevel on either side of the ply/foam and glassing over it.
     

    Attached Files:

  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    I don't understand what you are trying to accomplish with the plywood. There is enough width for a good overlap on the new laminate.
     
  3. Norado
    Joined: May 2016
    Posts: 8
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    Location: Le Havre

    Norado Junior Member

    According to the West Systems manual the minimum bevel should be 12:1. The skin is 5mm thick, it should therefore overlap 60 mm on either side. Unless I want to glass over my portholes and cabin, I'll only get a 10-20 mm taper on the existing skin.

    As I see it this will be a stress point, and the foam core won't give me much in terms of tensile nor shear strength. Using ply here would strengthen this area. Am I wrong?

    I'm not worried about the new laminate, I'm worried about the whole side breaking up and departing from the cabin, so to speak.
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Skip the plywood and use all laminate, transitioning from the cored area to the solid 'glass areas. You bulk up the transition areas, so you have enough "purchase" on each side.
     
  5. Norado
    Joined: May 2016
    Posts: 8
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    Location: Le Havre

    Norado Junior Member

    I was thinking of that, it might be a better option. Should be stronger anyways.
     

  6. compo

    compo Previous Member

    some things never change

    put a layer of glass over the foam core then fit 200 density foam core instead of the ply wood ! Its able to be bolted through for tracks etc and totally disregard the ply that will rot over time and you're back to repairing again :D
     
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