Shear-web ?

Discussion in 'Multihulls' started by BobBill, Feb 28, 2020.

  1. BobBill
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    BobBill Senior Member

    A shear web is a somewhat stablizing material as form for various projects. In this case, the web is the loa center stiffener of a foam hull.

    Normally, 1/8-inch or 2mm ply can be used and foam attached with non-epoxy (too hard) adhesive. I have perused a number of items and always come back to cheap ply...

    At the risk of havoc, I ask, what say you swabs? Any noto so spendy, items I can replace cheapo ply with?
     
  2. gonzo
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    gonzo Senior Member

    I say that cheap ply won't last long in a marine environment. You should use better quality materials. You can buy 3/16" underlayment at Menard's for less than $20.
     
  3. BobBill
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    BobBill Senior Member

    Gonzo...thanks.

    The ply exposed might not work, but completely encased...and sort of why I laid out the plan.

    I know I can use cuts of some composites as a shear, but the ones I have encountered have been very spendy.

    A foam-hull with ply shears are common enoough...I have even thought of using the flat side of a starboard Hobie 16 hull (which I have). But may be too weighty, though cheap...

    And I understand it is not wise to "go cheap" when dealing with boats and aircraft, which can fail dramatically, with dire consequences...
     
  4. gonzo
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    gonzo Senior Member

    If you are talking about a small solid foam boat, it is like building a surfboard. I can't think of anything cheaper that would do the job than water resistant plywood. Also, the side of a Hobie is not quite flat and the surfaces of the foam would have to be shaped to fit; a really hard job. All the surfboard I've built were first shaped and then cut where the stringer will be. One of the halves was laid on the plywood or plank and the edge traced to make it fit.
     
  5. BobBill
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    BobBill Senior Member

    Gonzo, Dig. I live in small town and marine ply is far too spendy here, so figured other stuff should do, moveover, voids in 2-3mm ply are few are far between and can "proof wood."
    Not my first trip doing boat, just looking for more help and looks like this site is once again the place to find it.
     
  6. gonzo
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    gonzo Senior Member

    As an example, I built a dinghy with the bottom of underlayment from Home Depot. It only got one coat of house oil paint outside and one coat of fence stain inside. It lived outside on a rack, sometimes in the water for a couple of days. Three years later, when it was stolen, it was still fine.
     
  7. fallguy
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    fallguy Senior Member

    Boil test exterior ply 30 minutes pass fail.

    I also screw test. Put 10 # 7 screws in the panel no predrilling. If more than one cause interlaminar shear; reject.
     
  8. BobBill
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    BobBill Senior Member

    So, it is ply, or the salvaged half of the H-16 aka I am using.
    Makes more sense to me...both as shear and as hotwire guide...makes perfect sense to me, considering most of rig is hand-tooled salvaged parts and working experiments...(e.g. synthetci stays, hook and loop sail-to-carbon spars. Remember the rig's motor is 220 squqre feet dacron, ala MO plans. Appreciate the input. Will apply, test and return to report results.
     
  9. BlueBell
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    BlueBell "Whatever..."

    When?
     
  10. BobBill
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    BobBill Senior Member

    Bluebell, it is basically Winter in MN, so likely spring, unless garage is clear, two car g with two cars inside...but plenty of material on line and not hurrying affords precluding error a bit. (If I had made two years ago, would have used epoxy and found fairing the hull tricky (For example hard epoxy; softer foam...messy sanding too. Now will use hot wire to cut and shape foam, and mess of sanding will be minimized.
     
  11. BobBill
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    BobBill Senior Member

    Gonzo, I dig. Ply is way heavier than foam...and shaping will be new and interesting with wire. The Gorilla Glue/adhesive, is soft and stong, per some who have used instead of epoxy. Will post comments.
     
  12. gonzo
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    gonzo Senior Member

    Ply is denser than foam, but it can allow for a lighter structure; two different things. Gorilla glue is a good adhesive. However, soft and strong don't go well together when describing an adhesive.
     
  13. BobBill
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    BobBill Senior Member

  14. fallguy
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    fallguy Senior Member

    Does Dierking use a shear web in his amas of comparable size?

    I have some smaller ones foams glued and glassed and would neber consider needing a backbone.

    We are some kind of neighbors, btw.

    A great shear web would be some 12mm marine foam.
     

  15. BobBill
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    BobBill Senior Member

    Fallguy, you lucky, getting warmer there am thinking.

    Gary Dierking, am only guessing here, likely has a shear in most of his foam rigs...not sure, but judging from his work, I think so. Certainly cannot hurt.

    There is a blue hull in the link pics which shows the wood shear...for what I am doing, I figure the web is needed as stiffener and as a guide for the outrigger hull or aka? If I get it right, I mix ama and aka up all the time, Dang it.
     
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