Building a ply design with balsa instead?

Discussion in 'Boatbuilding' started by Newickspark, Aug 27, 2019.

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  1. Newickspark

    Newickspark Previous Member

    The boat I’m looking to build is a Dix 29 retro.
    Allot easier then the cats I currently looked at.

    I want to build the boat on the original plan, but use balsa core with carbon skins.
    DD sent me an email with a basic idea how to do it. But I need more detailed opinions,
    Figured this is the best place to discuss.

    Here is the questions I have, I think it will make for a good convo.

    Should I use core cell with carbon skins
    Balsa with carbon skins
    Or maybe use Sglass instead of carbon

    I’m quite sure the ply would be just fine, but it’s 2019, and just won’t use it with all the other options avail.
    This is not another which core debate, this is which core and skin for a Dix 29 retro, that’s spec’d With ply.

    He said that i could use the original bulkheads frames ect, just lose the stringers and skin
    The inside first, then outside. I THINK I have that right. How is this done?


    Which would you use and why?

    Thanks all

    Here are two photos, the builder did a little custom job on the cabin, which I like
     

    Attached Files:

  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    When you say "ply", you meant veneer strips ? That couldn't be built with ply sheets. You would need a female mould to be using balsa as the core, so it is basically down to foam core, and male mould, unless you have the female mould.
     
  3. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    That not a true round bilge hull, it is radius chine. Topsides and bottom have no compound bends and are ply sheets on stringers. The rounded chine is cold molded with two layers of vertical ply strips. The joint between chine and topside and bottom is a staggered butt with a ply doubler underneath and glass on top.
    You can see the construction here: Didi 29 Retro https://www.flickr.com/photos/23039635@N03/albums/72157631074824814

    In theory one could build the same way with cored panels, laminate the inside skin on the core, trim full panel to size, install on framework and tab in place, then fabricate the curved chinepanels and glass the outside in one shot. Chinepanels could be stripplanked in place, removed for inner glassing and reinstalled, or a male or female mold could produce them outside the boat. This would leave him with the options of balsa or foam.

    Wich core and skin depend on the designer and what he is willing to acomodate without needing to redesign the whole boat and taxing you a big fee. You could also inquire about single skin carbon on stringers like some of the big Open 60's are buildt.
     
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  4. Newickspark

    Newickspark Previous Member


    Thanks for the reply, that is basically how he described it. The boat is 9mm ply core,
    He mentioned 12mm core cell. This boat is NOT begin built to a class rule, so the extra thickness is a non issue. He said I could use carbon instead of glass and use a thinner core.

    So what I’m shooting for is the exact same thickness in the hull as if I used ply core.
    So 9mm balsa with carbon skins.

    What am I saving in weight:

    9mm ply with a glass skin
    Vs
    9mm balsa with a inner and outer carbon skin? Approximately?

    Here is what I’m thinking for this one.

    Hulls:
    9mm balsa with in/out carbon skins
    I will make sure there is no,none,zero screws going into the hulls
    From above that can get into that core.
    I don’t mind using balsa here in the hulls, and it will be isolated 100%

    Decks:
    Corecell with carbon skins.
    All screws and attachments into solid g10 board, laminated into the deck.

    On the right track??? For anyone interested, here are a couple build photos.
    (Not my build, I haven’t started yet.

    Cool note, the guy standing on his hull claimed 14 days build time too that point!
    The kit was pre cut
     

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    Last edited: Aug 28, 2019
  5. Newickspark

    Newickspark Previous Member

    Want to clarify

    “What could work is to laminate a layer of glass or carbon onto one side of the foam to form the inside surface of a composite skin for hull side and bottom panels. Do the radius in foam strip. Then add a layer of glass or carbon to the outside and to the inside of the radius. This would allow you to build over permanent bulkheads using sheets of composite sandwich.”

    If someone could put that into a boat building for dummy’s format, it would be greatly appreciated. Ha!

    I think I get the jest of it, but a more detailed how to would help

    Here is how I see it now

    You take your balsa, cut it to shape needed
    Then test fit
    After it’s the right size, you laminate the inside layer of carbon, while ***still wet*** You attach it.

    Once all the hull is done, you just one shot the whole hull,(outside laminate) with carbon.

    I understand there is more to it, but is this the basics of how it’s done?
     
    Last edited: Aug 28, 2019
  6. Rumars
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    Rumars Senior Member

    The original boat is not "ply core", it is ply on stringers. The only glass on that boat is for abrasion and some reinforcement at the chine joint, the ply is structural. Total thickness is 9mm ply+0.Xmm glass+fairing+paint. If you want to stay at the same thickness you must tell this to the designer, he is the only one capable of telling you if that is possible. He should also be able to tell how much weight savings there are. It will not be much, if the savings are significant he needs to redesign the boat, add ballast etc., after all it must float on its lines and behave as he envisioned.

    I suggest asking for a comparative weight study and his opinion on best option, plus a check for the incured work. Just shoot him a mail with: "Mr. Dix how much for a detailed study of all options, when can I get it and how do you prefer payment?"
     
  7. Newickspark

    Newickspark Previous Member

    Thanks, already did that before I posted. And I have the price for the changes, he gave a fair estimate on doing the conversion.

    trying to learn the techniques better before I get started. He said 12mm was a guesstimate, and until I pay him to do the conversation, that’s fine for now.

    I guess what I’m missing

    How the heck can a boat built of ply and epoxy Be almost the same weight as a boat built with balsa and carbon?
    I’m not sure I buy that....I researched on west systems site,
    A 2x4 section of ply epoxy and glass is almost three times as heavy
    As balsa epoxy and carbon...

    Only thing I can figure is the laminators are using Old cheap balsa that sucks up to much resin And uses way to much epoxy for the job.
    If the ply is 3/4” and the balsa is 1/2” with carbon skins

    The balsa panel has to be lighter? Unless mistakes where made allowing tons of resin to soak into the balsa
     
  8. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    As per RUMARS hinted at.
    If the new vessel is to perform similarly to the original design, then its total weight must be similar to the original design weight. Any savings in hull weight should be made up with increased ballust. A hypothetical example: original 7 ton plywood hull hanging 3 tons of ballust vs 6 tons of ballust hanging from 4 tons of carbon/balsa. Both boats total weight is 10 tons.

    Often there is no weight savings when converting from plywood to lighter cores. The additional thickness of glasswork required to achieve the same panel stiffness outweighs the plywood which would be replaced.
     
  9. Newickspark

    Newickspark Previous Member



    Thanks so basically, your saying the ply is so much in sheer and compression stronger, too achieve the same values, I need to use so much carbon, it ends up around the same weight.
     
  10. Blueknarr
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    Blueknarr Senior Member

    Sometimes the cored glass version can out weight the plywood version.
    Either way, the ballust is adjusted to compensate for any difference.
     
  11. Rumars
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    Rumars Senior Member

    There are threads here about ply to foam conversion, here one, there are others.
    plywood and foam sandwich comparison https://www.boatdesign.net/threads/plywood-and-foam-sandwich-comparison.40533/
    Also search for flat panel building and/or infusion and foam strip planking and foam core male form building to see different techniques.

    Basicly you have to match strenght, stiffness and everyday useability. It makes no sense to have a very thick core (stiff) and very thin high strength skins if every scratch breaches the hull. Then there is only so much to be done if significant weight savings are made before you have to redesign the whole boat. First you increase ballast, then you redo the framing because you need to support a bigger keel and then you redesign the rig to match the ballast. Then repeat until you have a balance you like. But that is a custom design and costs as such.

    Duplicating plywood is difficult in small boats. Gets simpler the bigger the boat is until you get to the point where cored is better. Where that point is depends on the design and the design goals.
     
  12. Newickspark

    Newickspark Previous Member


    Thanks again, I get it now. The key begin, larger boats see more weight decreases switching to a different core. Makes sense, this boat is only 29 ft

    So next question I have,

    Building to Dudley Dix design with ply, how can I

    Assure I’m building it as light as possible? Building tricks and techniques
    And more importantly, what Building techniques can I use to assure the ply is best protected from Water saturation in the future.

    I would like to use an inside layer of Kevlar down low to help with small impacts
    Like putting the hull up on a beach, ect.

    What I’m thinking is

    Below the waterline
    Light skin of Kevlar with sglass ***over*** i read Kevlar is better as a skin under something, in this case sglass
    Rest above the waterline str8 sglass.
    I’m thinking sglass for it’s stronger property’s.
    I already understand not screwing through the ply, so any place that requires a screw will be G10 inlayed into the ply or a solid something. Ha

    Thanks again
     
  13. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Your boat could be either way, depending on how it was initially concieved. It's not about lenght, it's about the design goals. Pay Dix for the weight study and explain to him why you want it (weight, no rot, etc.) Tell him if you want the boat to be all foam or balsa core or if he can use plywood for the frames and bulkheads and furniture. He probably has enough leeway to give you a lighter boat without doing it all over.
    Forget kevlar and S-glass, just build the boat how Dix says (regardless for what material you decide) and be carefull with the epoxy and fairing. If you want reinforcement tell him that and do what he puts in the plans. Most common mistake is to "reinforce a bit" here and there. The pounds add quickly on a light boat.
     
  14. Newickspark

    Newickspark Previous Member

    Now your contradicting yourself, by saying first you can’t really save weight
    On a smaller boat, going to balsa and carbon, from ply.
    Then To, ask him to do look at saving weight.
    I’m more interested now in how to build and protect the ply for years of use.

    Im sticking to a light ply build with skins to help protect the top quality ply.
    Hardwood ply

    Thanks
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    hardwood is heavy, though ?
     
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