Scaling plywood

Discussion in 'Boat Design' started by Anatol, Oct 3, 2015.

  1. Anatol
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    Anatol Senior Member

    I'm currently making 1:4 scale models of some persuaded ply hull designs. But I have a challenge estimating the appropriate thichness of ply to give scale-accurate bend charactistics. ie by simple arithmetic, 1/4" ply would scale up to 1" in the final hull. But would the bending characteristics of the 1/4" in the 1:4 model map correctly to 1" ply in the full scale build? Any guidance gratefully received
    thanks!
     
  2. TANSL
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    TANSL Senior Member

    Do you think an iron plate of 40 mm can be bent with the same ease as other 10 mm ?. There you have the answer to your question.
     
    Last edited: Oct 4, 2015
  3. LP
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    LP Flying Boatman

    You miss the point. He is not talking about ease of bending. He is wanting to know if a 1/4 scale panel will scale linearly to full scale after bending.

    I don't have the intimate knowledge to know, but the books I've read have not talked of any kind of scaling factors. These examples have people do preliminary design iterations with scaled down sheets of cardboard before attempting full size with ply. I get the impression that linear scaling is accurate enough.
     
  4. upchurchmr
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    upchurchmr Senior Member

    This guy has already been told to get the Gougeons's book.
    It has a chapter on torturing plywood, which is what he is talking about, and it has a whole section on scaling plywood for the exact purpose.

    Most people don't have the ability or want to put in the effort to work out the scaling, but it was done 30 years ago and written down.

    He doesn't even have an idea of what he wants scaled.
     
  5. Anatol
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    Anatol Senior Member

    LP, you are right, thankyou for clarifying my question to TANSL
     
  6. Anatol
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    Anatol Senior Member


    Not only has this guy "already been told to get the Gougeons's book.", he had it long before he joined this forum and it is one of his primary reference texts.

    Chapter 25 (p310 in my 5th edition, 2005 copy) does give a nice rule of thumb table. Useful as it is, it only give values for 4-6mm ply in the final project, and only for 12:1 scale and only for aircraft model building spruce.
    "
    .8mm 5⁄32" 3-ply (4mm)
    1.0mm 3⁄16" 3-ply (5mm)
    1.2mm 1⁄4 " 3-ply (6mm)
    1.5mm 1⁄4 " 3-ply (6mm)"

    As I read it, This data simply scales thickness. My question, as LP correctly recognised, is whether the various bending and strength quantities vary linearly with thickness. Gougeon repeatedly comments that there remain many unknowns in this building method. Different kinds of ply have different qualities too. if I build my model out of doorskin ply, will it behavior scale up for okoume ply? So I was looking for rule of thumb wisdom based on experience. If my question is irrelevant or non valid for reasons I do not understand , I'm keen to learn. Thankyou for your patience.
     
  7. Oleboynow

    Oleboynow Previous Member

    if you can solder or silver alloy braze(it is simple) and you live in a country where copper sheep is cheap, do it in copper, your bending characteristics are very similar io ply
    I not use balsa, if you have the patience
    Beauty of copper is that if your have accurate plate developments, in a chined boat, you can just cut em and solder on the cut lines , it will pull the boat into shape
    allow for overlap on deck though,l it other words run the ply or material higher
    make 4 patterns of the sections from ply(even if you use copper) and run two dowels through at a waterline to set the boat up on, if you have no developed plates(sheets), same principal as I set a metal boat up with using two steel beams to hang frames from
     
  8. LP
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    LP Flying Boatman

    I might have been wrong about the cardboard. Here is an excerpt from gougeons book.

    It sounds like they expect linear scaling though it doesn't sound like they necessarily scale the thickness linearly.

    image.jpg

    Compounded 1" panels? Heavy. You may need hydraulic assist. I also attached some hull scantlings from compounded hulls in the book. They may not be what you are looking at though.
     
  9. LP
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    LP Flying Boatman

    Ha! You beat me to it. :D
     
  10. Oleboynow

    Oleboynow Previous Member

    like this
     

    Attached Files:

  11. PAR
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    PAR Yacht Designer/Builder

    In terms of bend properties, no plywood doesn't scale with thickness as much as you'd expect. Simply put, it isn't a true "modulus", mostly because it's a "wood product" and can have many variables. So if you use a relatively flimsy and weak door skin for you model bends, the reality at scale, particularly using quality, higher veneer count marine grades, will be much more effort, at least more so than the scaling exercise would indicate. On the other hand, you could double or triple layer thinner material once up to scale, which would mitigate this effect to a great degree, of course at an increase in cost and build effort.
     
  12. SamSam
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    SamSam Senior Member

    It would seem to relate to exponentials and whether the volume and size exponential relates to the thickness and stiffness exponential.

    If I relate it to a 1' cube, that would have a volume of 1 cubic foot and a surface area of 6 square feet. A 2' cube (1:2 ratio) would have 4 cubic feet volume and 24 square feet of surface. A 4' cube (1:4 ratio) would have 16 cubic feet volume and 96 square feet of surface.

    So if you use a homogeneous material such as aluminum, which would eliminate the variables found in plywood, does the exponential of thickness to stiffness match the one for volume and surface area? Would 1" thick aluminum be 16 times stiffer than 1/4" thick aluminum? If a 1/4" thick by 1' square piece of aluminum supported on the edges deflected 1/16" with 100 lbs of force concentrated in the center, would a 1" thick by 4' square piece deflect 1" with 1600 lbs of force? (The deflection numbers are a hypothetical guess)

    If the ratios matched, that would indicate you could take a square piece of a size (S) and thickness (T) of ply that was used on the 1/4 scale model, support it on the edges (or maybe just the corners), put (F) amount of force in the center and measure the deflection (D).

    To make the full size boat, a (16 x S) size piece of ply identically supported that deflected (16 x D) with (16 x F) of force applied would give you the thickness (T) of ply needed that would create the same shape as the 1/4 scale model.

    With aluminum or something similar you could probably figure the thickness with math, but with a variable like plywood it would be a bit of trial and error.

    Of course this is piffle, twaddle and poppycock if it's all wrong, which is a possibility.
     
  13. Oleboynow

    Oleboynow Previous Member

    talking deform leads to yield
    ply yields little before it breaks
    al al yield a great deal before it splits
    One of my builds piled into a rock at 14 knots dead of night, the bottom between frames push up a long long way before it split, amazed us all actually We cut out this 5x 1 foot section and hung it on the wall for years, was rather like abstract art with the deep gauges and antifoul smeared all over
    One thing leads to another here
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member


  15. SamSam
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    SamSam Senior Member

    I'm not talking about strength or failure, I'm talking about getting an identical shape.

    I think the OP is talking about compound shapes in plywood (persuaded ply hull designs/tortured plywood) and how to figure to get the same shape in a full sized hull as was achieved in a 1/4 size model.
     
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