Polycarb vs. AL5083

Discussion in 'Boat Design' started by truecougarblue, Apr 27, 2012.

  1. PAR
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    PAR Yacht Designer/Builder

    I don't believe he suggested a build from PC as much as was attempting to rationalize the concept, with the concrete comment. Neither material is well suited as such, though you could do it, the obvious question of why arises.

    Simply put Truecougarblue, to an engineer the idea bears little merit, though not explained well here, some very elemental research into the physical properties of PC, will reveal these short comings. You'll also find the same sort of comments directed at folks interested in building concrete canoes (not that you are interested). These seem obvious to us, but we can forget that most aren't as fluid in material dynamics and should take an easier tone.
     
  2. MikeJohns
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    MikeJohns Senior Member

    Hi Upchurchmr
    You sound so incredulous.

    If we stick to shipbuilding materials it often comes as a shock to people to find that you can produce a slightly stiffer lighter structure in relatively cheap medium carbon steels, weight for weight, than say in 6061 T6 Aluminium Alloy. It can also have the advantage of using much smaller sections.

    Both the Aluminium alloys and the Iron alloys (steels) are close in Specific Modulus ( stiffness/Density ) and without considering the necessary overheads of Al construction wrt fatigue and weld strength loss (which do add weight). Consider simply that a medium carbon steel has a higher specific modulus than say 6061 T6. So any stiffness criteria will be met with a lighter structure with the steel .

    Where you are coming unstuck is in considering extreme fiber stress related to material density or a strength criteria rather than a stiffness criteria. But then again if you need to consider design fatigue limits, Aluminium alloys can lose out again on strength to density as well.
     
    Last edited: Apr 30, 2012
  3. upchurchmr
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    upchurchmr Senior Member

    Truecougarblue,

    This is not tough, this was honest, from some people who spent a lifetime getting the information you don't have.

    They could have been very nasty and said "yes, you go try this - have a good time" all the time knowing the issues you will face, and the expense you will have for a failure.

    It is always true that a knowledgeable, clever, persistent person MIGHT come up with a unique and successful boat. The old farts sitting at their computer might be wrong. If you can learn enough to understand and find a way around their/ our objections you deserve to be right and recognized for it.

    That is a challenge. When can we see your plan?

    BTW, why don't you make a concrete canoe as practice? Everyone I know who did that just laughs at the thought of using the one they made. The usual comment is "be very sure to have your life jacket on". Its obvious you can make a concrete canoe - why would you ever want to? The civil engineering courses that do this are trying to teach students about the limits and requirements of materials.
     
  4. MikeJohns
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    MikeJohns Senior Member

    Upchurchmr

    FYI
    Specific modulus steel (medium carbon steel) 26.3
    Specific modulus Aluminium alloy 6061 T6 25.5

    And to keep within the thread topic for everyone:

    Specific modulus of Polycarbonate is close to 2

    So any stiffness criteria ( the main criteria in elastic instability ) will result in structural weights with around the following proportions

    Steel and Aluminium alloy are about on parity but the steel will have much smaller sections and be slightly lighter (that's why steel masts are so much more practical on large sailing vessels).

    Polycarbonate will be about 13 times heavier for the same stiffness and such large sections as to make it totally impractical for structural use. Where it could be used is in tension like a fabric cladding over a stiffer materials frame.
     
  5. PAR
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    PAR Yacht Designer/Builder

    This is precisely how it is being employed in all the efforts I've seen and appropriately so. The canoe linked previously in this thread, is a PC on carbon composite frame build and a perfect example of PC's application.
     
  6. MikeJohns
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    MikeJohns Senior Member

    Hi Paul I'd feel very exposed in a clear kayak ! Do you know how they joined the seams ?
     

  7. PAR
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    PAR Yacht Designer/Builder

    No, but I'll bet they "capture" them in the composite frame during assembly, likely against a gasket.
     
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