Alternative framing for discussion

Discussion in 'Boatbuilding' started by 2farnorth, Jul 21, 2009.

  1. 2farnorth
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    2farnorth Junior Member

    Will it work? Maybe its been discussed before. PVC pipe for framing with plywood panels. Cheap, readily available, strong, easy to construct, waterproof, filled with expandable foam for flotation. Attached is a quick overview of a proposed frame for a simple design. Looking for devils advocates and open minds.
     

    Attached Files:

  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Maybe on small boats only, not for an ocean-going boat.

    Eric
     
  3. 2farnorth
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    2farnorth Junior Member

    How about a 40 foot LOA with a 12 foot beam for use on rivers and ICW?
     
  4. 2farnorth
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    2farnorth Junior Member

    Eric, Nice website by the way. Thanks for any input from true professionals such as yourself.
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    PVC is not a very strong material, nor is it very stiff (has a low modulus of elasticity, bends easily), particularly compared to plywood. Also, a pipe shape is not necessarily the best shape for a frame where strength and stiffness are required, at least in your suggested application. Frames are usually made with edge-on flat panels and perhaps even with T or L flanges for very good reason--more strength and stiffness for the least amount of weight and cost. For plywood construction, you would be able to build a stronger, stiffer, cheaper, and lighter boat simply by using plywood frames. These can be easily cut to shape, even in pieces and glued together to maximize material usage and minimize waste.

    Eric
     
  6. 2farnorth
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    2farnorth Junior Member

    Thanks for the input Eric. So plywood frames, epoxyed (sp) to the panels huh? Lightweight is very imortant for this project as it will be electric propulsion.
     
  7. PAR
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    PAR Yacht Designer/Builder

    2farnorth, you can't design a boat just yet, as much as you'd like to, you just don't have the knowledge required.

    There are several good books that will get you started understanding the concepts, principles and engineering involved. Try the library here at BoatDesign.net.
     
  8. apex1

    apex1 Guest

    Ohh gawdelpus.................
     
  9. 2farnorth
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    2farnorth Junior Member

    Are we feeling a little high and mighty? Look, I just threw something out there for discussion. I got the idea from a very accomplished boat designer in Australia at Compu-Craft. They are using round and square tubing as framework on some of thier boats. So gawdelpus if we all just stick to the norm and never think there may be a better way to do things.:)
     
  10. 2farnorth
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    2farnorth Junior Member

    I can design a boat. Whether it floats or not is another story :confused:
     
  11. daiquiri
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    daiquiri Engineering and Design

    I don't see any structural advantage of using PVC pipes for reinforcing this composite pannel. A reinforcement needs to have a better mechanical behaviour under a given load then the structure it is supposed to reinforce, or otherwise it is just a useless weight on board. The elastic modulus of PVC is only 30-50% of plywood's so it is a more probable candidate to become a reinforced part of the structure, not the reinforcing one.

    That said, I have some objections on your actual pannel/foam setup.

    What you are trying to create is a sandwich of plywood/rigid foam, and you have correctly concluded that the two outer plywood skins need to be connected transversely in order to obtain a more efficient structure.
    But thru-bolts are not a correct way to obtain that goal. Instead, those holes are a good way to introduce humidity in between plywood pannels, creating conditions for their rapid decay.
    A hull structure is in continuous movement, and sooner or later a leakage through bolt-wood interface would occur. And once a leakage has started, it can only get worse with time.

    A more correct way to connect those ply pannels would be to glue them tightly to the foam core. In that way you give a sense to the presence of the foam pannel in between, which is now not very clear.
    Glueing the foam to the ply pannels provide at least three important functions:
    - first, under bending loads it would stabilize the compressed ply pannel, giving it much higher resistance to buckling.
    - second, it would keep ply pannels separated by some distance, which would give the sandwich structure a higher rigidity. A bigger separation between the outer skins means higher bending rigidity.
    - third, it would prevent humidity form condensing on internal plywood walls, which could create conditions for bacterial and fungal growth and wood rot.

    Once you have created a correct sandwich structure, it will have much more strength than a practical PVC framing can provide.

    What remains as a problem is the thickness of the outer ply skin. It takes a relatively thin plywood sheet to make a rigid sandwich structure, but a thin sheet can be easily perforated by floating objects. So a (much thicker) ply-on-frame (but not a PVC frame) becomes competitive again here...

    If something else comes up on my mind, I'll get back. ;)
     
  12. 2farnorth
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    2farnorth Junior Member

    Daiquiri, Thats what I am talking about. Good discussion that will inform more than just myself. From what you are saying, after the sandwiching of the skins, the pvc would just be useless weight. Thank you for your expert opinion.
     
  13. apex1

    apex1 Guest

    You had already expert opinions mate! No, you had ONLY expert opinions, because all the contributors by so far are professionals in the boatbuilding business. ALL

    And not just to contradict you. YOU CANNOT DESIGN A BOAT! period............
    Maybe you can draw a picture of a boat, but that is another sandbox to play in.
     
  14. 2farnorth
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    2farnorth Junior Member

    Apex1, I didnt hear your expert opinion, and I am sure you have one as I looked at the pictures in you public profile. Very nice, but why do you have to be such an ash hole to someone just looking to discuss different ways of doing things. Also I guess you didnt notice the sarcasm when I stated I could design a boat.
     

  15. apex1

    apex1 Guest

    Well, my comment was´nt really supportive, touchez! Your a..hole in this case (not nice but) understandable. Pari...

    I will skimp on comments about the structure shown in your drawing, Eric, PAR and daiquiry said it all.
     
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