Getting Ready to Place the Core - Questions

Discussion in 'Boatbuilding' started by CatBuilder, Jan 9, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    Never ending questions. Sorry, guys.

    I'm getting ready to start placing the core this week. I have a couple pretty simple questions about that:

    1) I was thinking of using a phenolic microbaloons mix to thicken my epoxy for joining and filling Corecell sheets. What do you all think about that?

    2) I am doing the deck in balsa. The deck is done at the same time as the rest of the hull, in a female mold (half hull with half deck comes out). I was thinking of using the same microbaloons mix on the balsa to fill in the large kerf gaps and stuff. Good idea? Bad idea?

    3) To attach the Corecell to the female mold (forms), I heat it, bend it and screw it to longitudinal battens from behind the mold. This holds it in place. The balsa is on the deck, so it's mostly flat with a slight camber. However, it's in all those tiny little blocks. How do I hang the balsa up on the mold so that I can then glass it? Do I drive screws in from behind just like the foam?

    Many thanks in advance for your thoughts.
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    No no... microballons make it brittle, it's just for fairing. Use silica and/or microfibres.. or anything but not balloons.
  3. CatBuilder

    CatBuilder Previous Member

    Thank, Teddy. That's good to keep in mind... microballoons are just for fairing.
  4. rberrey
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    rberrey Senior Member

    cat , is the deck form already part of your mold? I have 5 different hull deck joint types in my foam construction book, I see one more for wood decks in Eds other book, I dont see one for balsa decks.
  5. CatBuilder

    CatBuilder Previous Member

    There is no "hull deck joint" in my build. Here a picture to take a quick look at:


    This is a very small hull - possibly an amma. However, you can see the type of construction. The laminate from the center of the bilge to the center of the deck is one piece on this type of build. You are making a half hull with no joints, only curves.

    To make a full hull out of the two halves, you simply join at the keel, as usual and use biaxial tape to join the deck halves together at the seam. There is no real "hull deck joint"... it becomes a radius curve in these kind of builds. It's just the hull itself curled up and over to form the deck. No leaks! :)

    So, in my case, looking at the picture above, the entire hull layup, from the bilge (on the right side) to the curve where the deck starts is foam. After the curve, I have balsa core I want to put on that more "vertical" section on the left... the deck. I'm not sure how to get the balsa to stick up there so I can glass it or exactly how to fill all the gaps, since balsa is a bunch of small squares that move independently of each other tied together by scrim.

    Here's what the balsa looks like. It's AL600 coated, so it doesn't suck up resin.


    PS: I wanted to answer your rudder question, but was stumped. I can see the advantage in those rudders though.
  6. AndrewK
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    AndrewK Senior Member

    Scrim backed slit balsa is the wrong product for your application, exchange it for solid sheets. These you treat the same as foam, screw from behind.

    Not that long ago you were discussing the merits of precoating the entire surface with a resin and balloon mix, so why would you not add balloons to the glue mix?

    When gluing foam together I deliberately use a high balloon load. Actually only use the cheaper Q-Cell for this, approx 4:1, spheres:silica.
    The reason behind this is that you want softer glue lines that are easier to clean up before you glass. Otherwise if you have very hard glue lines when you sand the joins you end up sanding away the foam either side of the join and end up with ridges. Even at the 4:1 ratio the glue will be stronger than foam, test this out for your self. Glue two pieces of foam together and flex, the foam will fail not the glue.

    Locally the use of single pack foaming polyurethane glue is becoming more common for gluing foam and strip plank timber. Nothing to mix, use a squeeze sauce bottle to apply, expands to fill the gap and easy to sand.
  7. rberrey
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    rberrey Senior Member

    maybe a two sided tape,top and mid, maybe enough to hold it in place untill the glue Andrew is talking about drys.. As for the rudder I want to keep the shallow draft I can get with the kickup, and get the most from an inboard that needs a spade, I thought the daggerboard rudder might be a good compromise. rick
  8. CatBuilder

    CatBuilder Previous Member

    Aww.. damn! Nothing I can buy is local. It all costs hundreds (if not $1000) to ship.

    Thanks again for the input. Much appreciated.
  9. fcfc
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    fcfc Senior Member

  10. rwatson
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    rwatson Senior Member

    The Epoxy should bond very well to the "foaming glue" , it certainly does on strip plank boats. Basically the "polyurethane foam" is the same as you use for filling solid flotation voids. I remember a great discussion on why mixing your own two part foam is a bad idea, but if you slice up commercial, properly manufactured foam - it works much better.

    Maybe thats what you had in mind ? I have never come across epoxy not sticking to foam of any kind.
  11. rxcomposite
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    rxcomposite Senior Member

    Hey Catbuilder. Try asking for a reprint of PB number 33, Hot Weather Laminating Techniques. Includes bonding cores.
  12. SamSam
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    SamSam Senior Member

    Is there any possibilities of a problem in the laminate where the balsa and corecell meet, caused by the different modulus of elasticities? One stiff, one not so stiff?
  13. AndrewK
    Joined: Mar 2007
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    AndrewK Senior Member

    CatBuilder; If you can not exchange the balsa can you use it elsewhere, beams, bulkheads, flat panels?
    Scrim backed slit core requires infusion or vacuum bagging to get resin or adhesive to fill all of the slits. I would not use it with hand laminating full stop, but certain some do.

    "Not that long ago you were discussing the merits of precoating the entire surface with a resin and balloon mix, so why would you not add balloons to the glue mix?

    I don't know. No idea. That's why I'm asking what people usually use. I figured the balloons were proper because they were lightweight and sanded well. But Teddy's comment about "brittle" made me think it might crack in the core and debond over time. "

    My point here is that if your designer is happy to have a layer of spheres under your entire laminate I am sure he will not mind the glue mix having it too.

    I did not glue the foam with the polyurethane glue as I did not know of anyone who was using it at the time. Since then my sailmaker has built a 12m timber strip plank racing cat, he said it was much better than using epoxy in his previous boats.

    I have only used it to join a couple of pieces of foam before infusing on a table, it seems up to the job.
    I think it would make a good separate question on BD to see how widely it is being used, just havent got around to doing so.
    What I did find is that in high humidity the cap on the tip of the squeeze bottle is not good enough. I have had two bottles go of after only a couple of uses. Since then I have been told that if you store the bottle inverted its OK.
  14. AndrewK
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    AndrewK Senior Member

    The transition is at the deck radius so not a problem.

  15. CatBuilder

    CatBuilder Previous Member

    RX: Bought and read PB #33. Great website they have with the PDF downloads. Talked mostly about hot weather work in general, a little less about core bonding the way I'm doing it. It was mostly about using the goo the core manufacturers sell. I had planned to use epoxy and microbaloons. One thing that really caught my eye was the "cool suit!" Now that could make working in FL possible in the summer during the day, which would help greatly with humidity issues in that time of year.

    Andrew: I'm checking on exchanging the balsa. I hadn't planned to use it anywhere else aside from the deck and the roof of the deckhouse. Well, maybe I can just use it there, I suppose? I really don't like the idea of filling lots and lots of slits in the balsa up with resin because I can imagine it would get quite heavy. I am in touch with Nidacore (balsa manufacturer) to see what I can do about this.

    Sam: As Andrew said, it's only going in the deck. Movement of the foam will be 90 degrees out of plane with movement of the balsa. They shouldn't have any interaction at all, and I will carry the foam around the hull to deck radius, since it is a less stiff core. That should do the trick, I think.
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