plywood foam carbon construction

Discussion in 'Fiberglass and Composite Boat Building' started by Lurch723, Jan 1, 2014.

  1. Lurch723
    Joined: Aug 2012
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    Lurch723 Junior Member

    I want to build a hull that will be fairly simple to construct yet very light and stiff. After much thought (and reading) I have decided to go along the route of a thin Gaboon marine ply pinned & glued to former frames (which I will later remove after layup and replace with foam sandwich ones). I then was going to use a core material like pvc foam or 3d core and finish the layup with a skin of carbon - all vac bagged to the core.

    My question is really one of the final skin layup. My inner 3 ply skin is 1.5mm with a thin glass inner sheathing for sealing protection but do I balance the inner skin with a double outer skin of 240gsm plain weave or go for a thicker single layer of say 375gsm or higher? The hull is a narrow 2 man dinghy of approx 21' in length.
     
  2. PAR
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    PAR Yacht Designer/Builder

    If you're using carbon and foam (or honeycomb, etc.) you shouldn't consider plywood in the laminate, if light weight, stiffness and strength are desired.

    Designing laminate schedules for these types of structures requires a good bit of engineering, if the goal is truly to be light, strong and stiff. As to the actual scantlings for your boat, a whole lot more information would be necessary about the boat, it's use, anticipated loads, etc. Your questions are a bit like asking, how thick should the skin on the home built airplane I'm building should be. Without an idea of what you're doing anything would be just a uninformed guess.

    What design are you building and why are you deviating from the scantlings in the plans?
     
  3. Lurch723
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    Lurch723 Junior Member

    I don't have to adhere to any rules as such but what I am after producing is a construction method that lends itself to a one off design. I could for instance make a plug then make a mold from that plug but the effort isn't worth it or the cost for that matter. No a one off build needs to offer me simplicity in construction, I am familiar with epoxy and vacuum layups but need to arrive at a solution that balances ease of construction with a light weight hull which is essential of performance. I just an amateur and am after rule of thumb type answers or basic principles.

    I could for ease simply build it out of ply well within my experience, but the opportunity to make the hull stiffer for less weight is a welcome one. I can look at others and see what schedules they have used and base my build on those that have already demonstrated workable solutions.
     
  4. PAR
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    PAR Yacht Designer/Builder

    There are very few "rule of thumb" answers in regard to composite schedules and scantlings. You work out how much of whatever, according to loads and the physical attributes each material brings to the table to address them. Basing you schedules on other scantling combinations will not get you anything more then someone else's engineering. Worst yet would be, without understanding what you need and where, you'll be heavier then you need to be, because you didn't employ the materials as economically as you could have, had you known the loads involved.

    If you'd like I can offer a schedule, but without knowing what it's for and the loads expected, it's obviously just going to be heavier than necessary. What part of the currently popular construction methods, do you feel aren't easy enough, nor light, strong and stiff, making you consider a new method? Are conventional cored composites too heavy for you? Taped seam methods too flimsy? What design are you building?
     
  5. redreuben
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    redreuben redreuben

    The only rule is "how light can you afford to spend" ?
     
  6. PAR
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    PAR Yacht Designer/Builder

    This is so true. You can build quite light, but the question is, does your budget permit these approaches. This is one reason plywood is so dominate in small craft home builds. Not only is it easy to work with, but the cost/stiffness/strength ratio is appealing and difficult to beat, pound for pound per dollar. Also coupled with this is the fact that smaller craft, if composite built, have a hard time competing with plywood, because the usually choices for laminate schedules have to have minimum thicknesses, which tends to make them heavier, for the same penetration/strength/stiffness, unless you employ costly, high modulus materials.

    Knowing what the design is would help in regard to laminate schedules with different materials and cores.
     
  7. gonzo
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    gonzo Senior Member

    One off, very light and stiff construction will not be "simple to construct". As for adhering to rules, you can ignore them and engineer it. However, to get the boat registered you will have to certify it. Ignoring the rules makes the process much harder and costly.
     
  8. PAR
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    PAR Yacht Designer/Builder

    4 mm plywood skinned kayaks and canoes are light, stiff and simple. A Kevlar/carbon single skin, popped out of a mold is relatively simple, certainly light, strong and stiff, once the mold is built. It all depends on what you want (one off, production, etc.) and can afford. The scantlings (hull shell thickness, reinforcements, material choices and locations, etc.) are of course application specific.

    Class rules only apply to those appropriate designs. Proving certification only applies to production runs, not back yard one offs. Without knowing what design he's building this is all just fodder.
     
  9. Lurch723
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    Lurch723 Junior Member

    Now this is more like it, I could get quite excited about producing a plywood hull - maybe with a foam core then sheath the ply in a thin layer of carbon or glass for protection. Attached is a crude 3d image of something similar I am planning - it's really a monohull with outriggers added purely for roll stability.
    The rig compression loads are taken by an alloy space frame that's bolted into the hull.

    I really don't want to get hung up on any particular method as this is for me and it's meant to be a bit of fun and a learning experience all at the same time.
    I can build it, sail it and devlope it - change mast step configurations add foil assist lift etc.. you get the picture. Nailing it in one go is never on the cards but coming up with a suitable construction method is high on my list of things to crunch
     

    Attached Files:

  10. Ike
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    Ike Senior Member

    Have you considered cold molding a hull with wood veneers? It is simple, light and relatively inexpensive compared to composites. It doesn't require any advanced resins technology or even vacuum bagging. (but you can vacuum bag it if you want) I have built a dinghy this way with four of us laying on the veneers. It took about three days to complete the hull. Of course you need a mold but the mold can be built out of junk lumber, even press board or scrap wood. This technology has been around for decades and is still used to produce beautiful highly competitive racing sail boats and sailing dinghies.
     
  11. Lurch723
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    Lurch723 Junior Member

    Thanks for the suggestion Ike, I haven't tried or even looked at veneers before.
    I'll have a little dig about to look at what is involved verses the marine ply core marine ply route. I think I am going to go ahead and start building a jig for the frames what ever happens. I'm glad PAR and yourself have talked me out of doing the the usual approach, I can use more traditional skills to achieve pretty much the same result.
     
  12. capt vimes
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    capt vimes Senior Member

    i was thinking something very similar, but my approach would be completely different...
    take a plywood design for a mini 650 like the one from dudley dix:
    http://www.dixdesign.com/didiminiMk3.htm
    and now replace the inner support structure with carbon cut out from prefabricated sheets... we leave the question about money aside.
    from the material list and this describtion i know that the 9 mm and 12 mm plates are only used for the bulkheads, frames, backbone, transom and such...
    3mm (1/8") - 7 sheets
    6mm (1/4") - 20 sheets (23 sheets for Cruise-Mini Mk3)
    9mm (3/8") - 15 sheets
    12mm (1/2") - 2 sheets (4 sheets for Cruise-Mini Mk3)
    all of the 3 mm plates are used for the radius chine and the 6 mm are mainly used for hull and deck skin.
    if we replace the 9, 12 mm with say 6 and 9 mm carbon sheets - what would be roughly the weight saved and is it possible?
    if possible and even if the weight advantage is only 50 kg, it still could be an advantage...
    one could add 25 kg to the bulb and invest in a stiffer rig to account for the gain in righting moment - or something... ;)
     
  13. SukiSolo
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    SukiSolo Senior Member

    Following on from Ike suggestion. You could use very light veneers to make your own ply core (by cold moulding) and wrap carbon each side as another alternative. I am tempted to go this route on something I plan to build in the relatively near future. It will stay stiff in a different way to foam whilst constructing it even if slightly heavier.
     
  14. redreuben
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    redreuben redreuben

    Keep it simple, cold moulding is lots of hours.

    Strip plank in Paulownia, DB carbon external, uni at 90degrees internal.
    If carbon is too pricey then basalt or Sglass.
     

  15. JRD
    Joined: May 2010
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    JRD Senior Member

    International Canoes may have a similar shape to your craft as best I can see. They also have a whole variety of construction methods you may be interested in, from carbon and foam to tortured ply.

    There is an interesting thread on SA you may also find useful http://forums.sailinganarchy.com/index.php?showtopic=65915&page=4

    Good luck
     
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