Polycarb vs. AL5083

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

  1. truecougarblue
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    truecougarblue Junior Member

    I hit on an old thread about building a small 'see through' boat from PC. I was a bit surprised by the dismissals of polycarbonate as a material for building boats. To dismiss a material out of hand is to ignore the engineering process. If one can build a canoe out of concrete one can surely build one out of PC.

    PC is less than 1/2 the density of AL5083. You can build every rib or stringer twice as thick (or deep) and still save mass. A double thick hull is 8 times stiffer in equivalent material, until I do the math I can't say it wouldn't work.

    As for UV there are coatings, additives, or films for that.

    A quick look at Mcmaster also tells me that a 12x12x1/4 inch sheet of AL5083 will run you $56.70. The same size and thickness PC is $14.01. 1/4 the price per unit volume.

    Double your hull thickness, double your ribs and stringers, I think it's an idea that needs further study. If only there were more hours in the day...

    BTW, http://www.makeitfrom.com/compare-materials/?A=5083-H34-Aluminum&B=Polycarbonate-PC
     
  2. beachcraft
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    beachcraft Junior Member

  3. PAR
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    PAR Yacht Designer/Builder

    I think you need to look at some of the other physical attributes of PC before you jump in and start building. Urethane or PVC would be better options, though not without their own difficulties.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    A professional NA/designer would not dismiss any material out of hand, only a someone with little knowledge on the material/subject or a personal 'beef' about the material.

    If you get negative replies about a material by professionals, it is perhaps because they have been through the whole exercise of designing with it, trying to satisfy structural calculations fabrication and through life costs etc. If at the end of the day it appears to be not feasible, that shall then be their stance based upon their calculations and experience of such a material. Does this mean you can't do it...no. Does it mean you would be foolish to do so, probably yes.

    Thus when proposing a new material or an "odd" material, you need to listen to the advice of professionals. You quote ferrocement as one. Fine , can be done. But how many ferrocement vessels around the world are built, as a percentage??....and that shall give you your answer.

    Just because you can, doesn't mean it is a fully working solution to your dreams/goals/desire/whatever....but there are always one or two that continue. And good luck. Just don't expect the rest of the marine world to follow suit.
     
  5. truecougarblue
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    truecougarblue Junior Member

    Where did I say I was going to try to built a boat with PC?
     
  6. PAR
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    PAR Yacht Designer/Builder

    No one suggested such, though the physical qualities of PC are easily understood and several of these attributes, stick out badly, as areas of concern to say the least. A port, a light, sure, a hull shell or substantial portion of one, a different mater. The only successful attempts at this, don't use PC as the hull shell, but in effect as large lights in very strong frames, which addresses many of the issues associated with a "small, see through boat" of PC.

    As a side note, in most instances of "glass bottom" applications, you don't really see much. Viewing down through your own shadow, possably into less than perfectly clear water, can be a visual challenge. I've made repairs of several of the "attraction" boats around central Florida, thus equipped and every single one has lights (big ones), to deal with this issue. So, unless you are in floating with 6" of water under you, in a very clear waterway, you'll need more then some PC hull panels.
     
  7. messabout
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    messabout Senior Member

    You can not say that double the thickness produces eight times the strength. Eight times moment of inertia (I), a function ot the third power of section depth, yes. You must also deal with the elastic modulous (e) of each material. That can change the whole ball game.
     
  8. upchurchmr
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    upchurchmr Senior Member

    This is a bad idea, discussed several time in the short time I have been reading this forum.
    Do a search.
     
  9. PAR
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    PAR Yacht Designer/Builder

    This assumes the original poster has a clue about engineering principles . . . his posts suggest other wise.
     
  10. upchurchmr
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    upchurchmr Senior Member

    PAR,

    Thanks for being here to say what I didn't want to.

    You are now my hero - really. Someone has to give the simple truth occasionally.

    Marc
     
  11. MikeJohns
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    MikeJohns Senior Member

    Structural Stiffness is as important as strength, the problem with materials like polycarbonate is that they have a high modulus of elasticity and are structurally poor materials.
    They don't fail so much through ultimate strength deficiency as the strain increasing at a greater rate than the stress (aka buckling). That's also why some Aluminium alloy structures are actually heavier than the equivalent in steel.
    You also need to look at the shear strength of the material (Polycarbonate).
     
  12. upchurchmr
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    upchurchmr Senior Member

    Mike Johns,

    Name one aluminum structure properly designed that is heavier that the equivilent in steel.
    I assume we are talking statics, not life limited structure.

    There is no doubt that someone has poorly designed an aluminum structure, but if you come up with a real situation it will be so far from normal usage as to be completely misleading to the average person.

    I'll even conceed defeat on two things, bolts and gears, in most situations. But those have nothing to do with buckling.
     
  13. truecougarblue
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    truecougarblue Junior Member

    Wow, tough crowd, I hope I never need to deal with a 'yacht designer' or 'naval architect' on any of my projects.
     
  14. FMS
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    FMS Senior Member

    It's not clear what you are proposing to build from polycarb then.

    Without knowing the requirements, the only response can be polycarb will be an ideal material choice for some things, will be a reasonable compromise for others, won't be a good choice for some things, and won't work at all for other things. Specific requirements are needed before assessing any material's applicability.
     

  15. gonzo
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    gonzo Senior Member

    truecougarblue- you said: "If one can build a canoe out of concrete one can surely build one out of PC." So you are the one saying you are going to try build a boat with PC.
     
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