Predicting plywood bending behavior for stitch and glue design

Discussion in 'Software' started by CET, Oct 10, 2008.

  1. alangluyas
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    alangluyas Junior Member

    Conic development drawing

    I forgot to attach the drawing of how the conic development looks. I hope this makes things clearer.



    Attached Files:

  2. michael pierzga
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    michael pierzga Senior Member

    Hi alan

    If I were to construct the small boat in your drawing and the thickness of the chosen plywood was causing bending difficulties...would I cut kerf lines along the conical projections shown on your drawing ? Mike
  3. alangluyas
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    alangluyas Junior Member

    bending plywood

    Hi Mike

    No, I think that is the last thing you would do as the stress risers would likely cause the plywood to fail at the kerfs.

    You seem to have the idea about the projection lines ( in boat design they are usually called ruling lines but mathmeticians have other names for them), in that they represent the straight lines along the axis of the cylider or cone. If a ruler is laid along these lines it will sit flat on the surface. If the ruler is skewed then the surface below will be parabolic to some degreee. If you skew the ruler until it is at 90 degrees the the long axis of the hull, then you see the parabolic shape that we are talking about.

    If the sheet material (this plan is actually a 50 foot semidisplacement power boat designed for alloy) won't bend to the designed curve, then the options are to use a thinner material or in metal, preroll the plate. In plywood, it is quite practical to use thinner ply and then laminate more on top when the hull is assembled until the specified thickness is achieved. Sam Devlin uses this technique extensive.

    In my experience ( my ply boat building and design is limited to about 20 feet), 10mm to 12mm ply is about all you would want to try and bend around most hulls. My design and contruction above 20 feet has been in steel and alloy, not because I think they are necessarily better for bigger boats (I don't) but for the pragmatic reason that in Australia ( especially Western Australia) if you build a plywood boat and a metal one to identical spec, the day you finish it the timber boat will be lucky to be worth 33% of the value of the alloy one.


  4. daiquiri
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    daiquiri Engineering and Design

    Alangluyas, just two questions, if you don't mind:
    1) how do you assure tangency between surfaces when you have to split, say, a hull bottom in two pannels?
    2) what are the Autocad commands you use to create and unfold the conical surfaces?
    Thanks. :)
  5. lewisboats
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    lewisboats Obsessed Member

    In practical experience...the panels will take the slightly convex curvature when stitched together. I am talking 6mm plywood here but all thicknesses will probably exhibit this phenom to some degree. If the program can accurately portray this then that is probably what the end result will be and should be taken into account during the design/build process. If you want perfectly flat panels then you must supply framing to stress the panels into shape.
  6. alangluyas
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    alangluyas Junior Member

    More than you wanted to know on conic delelopment with Autcad.


    I think we are talking at cross purposes here. I have been describing the behaviour of developable hull plates in chine hull forms. I don't see the relevancy of tangency in that application, unless I have missed something?

    I do not use Autocad to create surfaces at all - it is not really a surface modeller, although I do use some of its mesh surface applications in some engineering applications not connected with my nautical interests.

    The Autocad process that I use is just to test whether two curves (the centreline profile and the chine in this case) can be related conically or cylindrically.

    All the conic projection lines (which will be the ruling lines if the surface is developable, but at the early are just theoretical conic projection lines) are manually drawn form an arbitrary source which is forward of the full and to one side. Because we are dealing with two views, there are two graphic representations of the same source (on the plan and the profile drawings), obviously both will lie in the same y,z plane at the same x value. The source in the profile view can be adjusted for z, the source in the plan view can be adjusted for y. In autocad, this can be done by stretching the source point and the projection lines, using an “open” selection box, with "ortho" set to "on".

    So, how so I actually check the plate for being developable?

    With the hull lines set up as depicted in my drawing, I draw a series of vertical gridlines, which define sections through the hull. These are normally at the same location as the design stations in my hull, although I may add some others forward if there are not enough.

    I then create an arbitrary focus point / cone apex / source point forward and below the stem on the elevation drawing. I draw another source point on the plan drawing at the same x location as the other but on the other side of the plan centreline, so that the projection lines cross the centreline.

    Then connect each source point to each intersection of the vertical gridlines and the chine line, in both plan and elevation.

    I use different colours on a rotating basis is to avoid confusion while drawing. You should wind up with a drawing similar to the one I have posted.

    Where each projection line crosses the centreline profile on the plan drawing, a line is drawn up to the equivalent projection line on the elevation drawing. Where this connecting line meets the projection line on the elevation drawing is where the centreline profile must be located. This is done for every projection line and the points are connected with a polyline. The polyline may be “curve fitted” but not “splined”, as in autocad, control points for splined lines do not lie on the polyline.

    If your new polyline sits close to your keel centreline on the elevation drawing, then the hull plate is developable. If it does not, then you have some options:

    1. Move the apexes of the conical lines ( either both forward or back, together) or sideways or vertically, independently. Don’t forget that they must both be at the same x location.
    2. Move the chine line either in/out or up/down and start again.
    3. Accept that the plate is not developable and plate the hull in strips.

    Once you have got your head around this, think about the fact that that the plate under consideration does not have to have a single conical focus, it can have as many as you can cope with, as the governing cone can be of changing radius. What this means is that the new cone focus has to share a ruling line with the previous cone focus to ensure that the cones are congruent. This is simpler than it may sound but it can make for a very messy drawing, so I have not shown it.

    It is quite possible to develop a very shapely hull with only one focal point.

    As you get toward the stern, you can be less fussy, because you can take liberties with the plates as they approach the transom, and you can induce some curve of your own, which can make the panels stiffer.

    In my experience, plywood is stiffer than steel plate, which is stiffer than alloy. This makes plywood is very close to a “perfect” plate material, in that it will not readily conform to any noticeable degree of compound curvature, unlike alloy, which can be coaxed into a surprising amount of compound.


  7. alangluyas
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    alangluyas Junior Member

    Drawing the sections.

    I forgot to mention the following in the previous post.

    You use the elevation drawing to draw the curvature of the sections by running horizontal lines forward from the intersection of each projection line with the gridline (between centreline and chine). This will give Z values for some control points on the section lines. If you then draw some buttock lines on the plan drawing, you can obtain y values where these buttocks intersect the projection lines. When you fair these lines in section view, you should see the parabolic section shapes.

  8. Chris Ostlind

    Chris Ostlind Previous Member


    You are seeing the result of the bow profile curvature being pushed through the panel until it is stress equalized along the length of the panel. Notice that the same panel a bit further aft, does not exhibit the kind of curvature that you are illustrating in the photo. I use Rhino for my design work and when you have a curve as one of the component elements of the lofted, or swept surface, you will get a degree of this.

    I have never seen any notion of a program out there that can "predict" the amount of this curved panel function. Perhaps the best thing to do, is to build models of the boats you wish to build full size and examine the behavior of the panels when building. Sometimes the curved panel surface can be advantageous and other times it totally screws-up the look and the utility of the boat.

    In general, flat panel designs are far easier to stitch and glue than are those that have integral curves at one end or the other. A panel such as the one you show, is probably not going to be difficult unless you are traying to make the bow curves out of a thickness of plywood that is not going to take to the bend very well.
  9. sholt85
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    sholt85 Junior Member

    At the school I went to in Maine, the Landing School, they built a planing boat that was cold molded top sides. The bottom was a developable surface that they faired using a straight edge in the direction of the apex. They used thinner veneers at the forward part of the bottom where there was more cupping. Glass the whole thing up after that and its all good.
  10. daiquiri
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    daiquiri Engineering and Design

    Thanks, Alangluyas. That's an interesting method. I'll give it a try.
  11. CET
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    CET Senior Member

    Wow! Thanks for all the great info guys. I love this forum. :cool:
  12. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!


    I know this thread is long dead. I was reserching bending ply and think I have something to contribute. FreeShip and DelftShip will also do it. Maybe for the professionals, but ACAD certainly isn't in my hobby's budget.

    Just to try it out, I plugged in Dave Gerr's Nester Dingy into DelftShip, made sure they were developable and here are the views and panels flattened out, ready to trace.

    I hope it helps someone in the future.

    Attached Files:

  13. ThomD
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    ThomD Senior Member

    "Phil Bolger is, by a long shot, the master of plywood prediction. His stitch-and-glue plans fit so well that the wire stitching is completely unnecessary, hence why they're called "tack-and-tape"."

    I wouldn't want to shortchange his mastery, but all the plans of his I have built had missing pieces the builder had to develop by filling in the spaces left by the pieces he did provide the design for, sorry about the laboured description. So he would provide the side and bottom of a 5 piece hull, and one would develop the 2 missing pieces from the full size project. So in a sense how could he miss since there were gaps in the designs.

    As far as I know his designs are tack and tape because Payson wanted boats that did not require epoxy, had many extra frames and nails as a result. That said, they are very fast to build because there is little down time, one just keeps nailing them together.
  14. petereng
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    petereng Senior Member

    Hi All - I know this item is old but I'd like to answer CET's question. 1) Yes there are programs that can predict the cupping, but these are FEA programs not CAD programs. FEA stands for finite element analysis and these are structural or stressing programs not CAD programs. 2) CAD programs like Rhino or other NURBS programs model surfaces in a particular way. NURBS surfaces are effectively surfaces of best fit to the construction lines and do not take any of the material properties into account when you build them. Effectively you can build any shape you want, its really like clay. 3) so to build surfaces that behave like plywood or metal requires some rules to be applied to the construction lines so the resulting surface can be developed afterwards. Conics and cyclinders being mentioned before are the simplest. If the surface is to be bent in two planes slightly a hyperbolic would work, but it does have the cup you are describing. This is because a hyperbolic is a "ruled" surface eg you can put a ruler on it and there are areas that are straight in both directions. It is called a "saddle shape" by some. Metals can be stretched so can be forced into shapes that timber can't be as timber does not stretch. Obviuosly we can bend it more if we heat it. This is becuase the cellulose in the timber becomes plastic when hot so it bends easier. But to bend it significantly you need to do a compression bend not a tension bend as when hot the timber is less strong. 4) Getting back to cupping. If you used an FEA program you can bend the plywood into the required shape just like you would in the real world and see if its possible. If possible it would show you the cupping or not. This is because in FE we take the sheet which has stiffness and try to bend it into the shape we want vs creating the shape we want (which has no stiffness) then unfold or develop it (which is always geometrically possible given various options) then try to reverse this using the plywood which has stiffness... ie we don't get what we want! 5) to summarise if you are using Rhino read up on developable surfaces (use the least "degree" surface to get what you want) and only use these to create your boat surfaces. These will be conics, cylinders, hyperboles (twisted surfaces). These will always work as the surface does not have to stretch or shrink to work. Plywood will do a little more than this but once you have some experince you will be able to understand the limits. Hope this helps. Peter S

    Hi CET - the cupping you describe is called "camber" by boat builders and if its inward its called "hollow" cheers peter s
    Last edited: Oct 3, 2013

  15. ludesign
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    ludesign Senior Member

    I disagree. TouchCAD can do it, as has a dynamic link between the 3D model and the unfolded parts. You therefore don't get a message that it can't be unfolded, you get indications where it goes wrong and what to do about it interactively. It is simply an integrated part of the hull general fairing instead of something you do as a last step.
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