Rebate or Taper - Which is Strongest?

Discussion in 'Boatbuilding' started by CatBuilder, Nov 22, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    I was wondering...

    There are two ways to do a lap joint in a piece of glass laminated to a foam core:

    1) Leave the foam surface completely flat and just stack the two edges over each other leaving a "stair step", which you then hit with a grinder (to taper it) and fairing compound (to make it fair in to the smooth hull shape).

    2) Use a belt sander to cut a little groove into the foam, so the bottom layer of glass dives under the top layer at the overlap, creating a completely fair surface, but no taper.

    Which one is best to do?

    Which is stronger?
  2. tunnels

    tunnels Previous Member

    think about it!! the second will naturally be stronger and is how you should do all your joins . :D
  3. CatBuilder

    CatBuilder Previous Member

    Thinking about it, I come up with this:

    You use a taper so the glass goes from full thickness to nothing over a distance. That taper allows the edge of the glass to flex more and lets it bend, avoiding stresses that could cause a delmination.

    The choice #2 leaves the glass at full thickness everywhere and leaves a square end on the lap. My understanding is that this square end could have a tendency to start a peel in the lamination.

    I have been doing #2, which makes work a lot easier since you have almost no fairing, but was getting worried that I should be doing #1.
  4. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    I reckon both can be good in regards to stitched fabric(but not to woven), although if you have a plain lap there's a step in the layer on top where it passes over the first...... here it is very important never to sand or grind the glass so as to compromise the fibers at this "cross over" point, if its set in a "groove" that's very unlikely so a good plan. If you were laying up wet on wet there wouldn't be the choice to grind a taper anyway & perfectly acceptable at less dust & labour( & what I would do). If your laying wet onto dry obviously that option of tapering the first layer is available & possibly the "best" transition if done with very great care & taking extra time . This question is a good one & kinda similar to debate on tabbing or repair laminate wide to narrow or from narrow to wide, on this here in Aus the standard is narrow to wide. I think it's an "it depends" kinda question. Regards from Jeff.
    1 person likes this.
  5. Submarine Tom

    Submarine Tom Previous Member

    Tapering, fairing, it's all good compared to square edges. It all depends on how you grind and finish. By the sounds of it, you already know the answer CB. I hesitate to comment further as it may get "lost in translation". This is something you could Google for further clarification complete with drawings and graphics.

    Last edited: Nov 24, 2011
  6. AndrewK
    Joined: Mar 2007
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    Location: Australia

    AndrewK Senior Member

    There will be no difference providing you only fair the free edge in option 1 and dont make the rebate too deep in 2.
  7. david@boatsmith
    Joined: Aug 2008
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    Location: Jupiter Fl USA

    david@boatsmith Senior Member

    I like to cut a rebate in the foam for the glass to get the benefit of a full overlap while not inducing any humps that require lots of bog to make go away. I have let one layer kick and tapered the edge by sanding. It is a lot of tedious work to get satisfactory results. JMO
    1 person likes this.

  8. CatBuilder

    CatBuilder Previous Member

    Nice! Thanks, guys. So the rebate is the way to go. That's good to hear since that's what I've been doing.

    I should probably add (just for clarity) that I'm now infusing everything I can get my hands on (at least 2/3 of a 45' hull at a time, both sides of connective beams at once, rudders, boards, etc...) and the overlaps are now inside infusion jobs. This changes things just a little (no sanding).

    But, I could do the overlap without rebate, leaving the hard edge to grind down and fair, or do like I have been doing and put a rebate in the foam and have a nice, smooth seam right out of the bag with little or no fairing.

    If they are both equivalent on strength, "little or no fairing" has to win! :)

    Thanks again for the input.
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