Plating model for round hull

Discussion in 'Boat Design' started by makobuilders, Sep 29, 2014.

  1. makobuilders
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    makobuilders Member

    What are some of the parameters used to define the plates for a metal round hull design? Getting away from the traditional method of bending paper around a scale model, what are some of the calculations used, gap limits, etc.
     
  2. TANSL
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    TANSL Senior Member

    There are several ways depending on whether the plates are developable or not. Can you give us information so that we get a better idea of ​​how is the hull?
     
  3. NavalSArtichoke
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    NavalSArtichoke Senior Member

    If the curvature is really complex, like at the bow and stern, full size templates are usually built and a hot plate is hammered onto the template to form it properly.

    Where the curvature is not as severe, like in the bilge or midbody, a shell expansion drawing is prepared and the individual plates can be laid out to scale. The shell expansion drawing can usually represent plates with single curvature pretty accurately.

    For small vessels, often the framing is constructed first, and a strip of hull plating is attached by welding to one frame and then laid over other frames, so that the vessel framing itself serves as a full-size template for the shell plating. This method is used a lot on vessels which have a single or double chine hull, like OSVs and crewboats, or by yards which don't have access to a lot of sophisticated equipment.

    If your vessel is being designed in one of the vessel CAD programs, you may be able to generate nesting tapes which a steel fabrication facility can use to cut individual shell plates to shape.

    In any event, each vessel is different, and the particular method, or combination of methods, used will depend on the shape you need to build, the capability and experience of the yard, etc. As always, a hammer and a torch are essential tools to getting the final shape right.
     
  4. TANSL
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    TANSL Senior Member

    Although in general I agree with NavalSArtichoke, I would refine two things:
    1. the shell expansion drawing does not work, in many cases, to get the real development of the plates, only the cylindrical body, if it exists. But, in fact, depends on the shapes of the ship.
    2. Bending plates by heat is a method that should be used by very experienced staff as the risk to "burn" the steel (make it brittle) is very high.
     
  5. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Shell expansion drawings were used for many years in traditional ship construction without problem. Plates with single curvature can be accurately represented using a two-dimensional plan. For plates with double, or compound, curvature, the template method was utilized, because it was recognized that a two-dimensional representation of the plate would not be accurate.

    As always, when applying heat to shape a plate, the proper procedure would be to shape the plate first, and then anneal it to release any locked in stress, before it is installed on the ship. Furnaced plates, as these are called, are generally heated as a whole and then shaped; using point heating, like with a torch, is generally not recommended.

    As always, any steel plates which are made of special high-strength alloys or which are heat-treated to obtain proof strength should not be furnaced, at least not without consulting the supplier or class society.
     
  6. TANSL
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    TANSL Senior Member

    NavalSArtichoke, in my opinion, some of the things you say are true and some not. No, I will not make a list of every one to them. I also have worked with many shell expansion drawings in 2D.
    In any case, all this has nothing to do with what the OP asks, who probably does not know what it is or how you draw the shell expansion drawing.
     
  7. makobuilders
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    makobuilders Member

    I am familiar with shell expansion drawings but only from reading and have not created one previously. The application would be a small boat of true round bilge design, with areas that are not developable, like in the turn of the bilge and bow sections. So yes I would like to learn more about creating these drawings but in a slightly-more sophisticated way than laying strips of paper on a model, like pulling them off an AutoCad drawing, etc.
     
  8. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Unfortunately, using strips of paper to measure the girths of a hull using a body plan is exactly how the shell expansion was created traditionally, using a body plan drawn on paper. Some builders would also make special half-models of a vessel and draw the plate seam lines on the surface of the hull.

    Of course, many boatbuilders and shipbuilders also once laid out and faired the lines of a vessel at full size in a mold loft, where the measurement of the girths of a vessel hull could be done more accurately than from a scaled drawing. The loftsmen would also be in charge of creating any furnacing templates used to shape the actual hull plates:

    Here is a picture of the inside of a mold loft:

    [​IMG]

    The loftsmen would use flexible battens (but considerably longer than those used by a draftsman) and batten weights to control them when working in the
    loft.

    A traditional CAD program, like AutoCAD, generally will not come with the specialized tools which allow the automatic generation of a shell expansion plan, but if your body plan can be imported into AutoCAD, you can use the utilities to calculate the length of a faired curve, but it would be a very tedious process of measuring the girths and then transferring these lengths to a shell expansion plan to be developed.

    Some of the dedicated ship design or hull fairing programs might automate the process of generating developed plate shapes, which would be easier to use than just a general CAD program.
     
  9. rwatson
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    rwatson Senior Member

    What is "automatic generation" ? I would think that shell design would be done by an experienced fabricator, as the placement and size of plates is as much a manufacturing issue as simply expanding hull surfaces.

    As far as just dividing hull surfaces, I don't know Autocad, but it would be a doddle in Rhino, the cheaper option.

    I cant believe 3d Autocad couldn't do it.
     

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  10. NavalSArtichoke
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    NavalSArtichoke Senior Member

    I'm talking about a package geared specifically for ship construction, like ShipConstructor, or one of the large shipyard packages like Tribon or AutoKon. With these installations, you start by fairing a hull, and then once you are satisfied with the lines, you can check the stability of the vessel and then move into more detailed parts of the design process: detailed steel layout and nesting, piping and structural arrangements, weight take-offs, etc. These packages automate a lot of the general scut-work which used to occupy the denizens of design offices.

    I'm not saying that AutoCAD would not be able to perform some of the work necessary to lay out plating, but AutoCAD is a general drafting program designed to work across different engineering and architectural disciplines. As a general program, it does not incorporate many of the features that a package like Rhino or ShipConstructor would include, because these packages are intended primarily for the ship or boat designer. All I'm saying is that using AutoCAD to lay out and develop plating shapes from a body plan would probably be just as tedious (but more accurate) as using a paper plan.
     
  11. TANSL
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    TANSL Senior Member

    I'm not here to defend AutoCAD, I'll just talk about what I know well, but the statement that "AutoCAD is a general drafting program " is incorrect. AutoCAD is a CAD program, very complete. I hold everyone, and I mean everyone, naval architecture calculations with AutoCAD. Many of these calculations are not possible with Rhino. The same goes for many other popular programs on this forum, are not able to calculate everything a boats designer needs.
    Developing plates with AutoCAD is not automatic but allows traditional methods working on a 3D body lines plan therefore much more accurately and quickly than by hand.
    I think makobuilders would appreciate us to focus on answering his queries instead of talking about how nice it is certain software.
    Maybe the attached file is useful for makobuilders
    There are some other system to develop developable plates.
    By the way, I've never seen a loftman using the shell expansion drawing to develop a hull´s plate.
     

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  12. NavalSArtichoke
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    NavalSArtichoke Senior Member

    A 'general drafting program' means that AutoCAD contains the basic tools for creating a drawing, and it is a piece of software which can be used by many different professions right out of the box. However, the basic package does not contain many of the tools which come with more specialized applications, like those designed for naval architects. There are different versions of AutoCAD, some which are intended for civil engineers, some which are intended for mechanical engineers, etc.

    Can you use basic AutoCAD as a aid in calculation for naval architecture, even though it is not tailored specifically for naval architects? Sure. It can be used to calculate areas and volumes, locate centers of gravity, etc. But all of this must be done under the control of the operator, especially if more than one step is required to complete the calculation. You can plot and calculate the properties of a waterplane, for instance, or create a set of bonjean curves for sections in a hull. But it is a tedious procedure.

    That may be, from your personal experience, but I'm sure it has been done at some point, in the two hundred odd years of metal ship construction. Otherwise, why go to the bother of creating this very unusual drawing in the first place?
     
  13. TANSL
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    TANSL Senior Member

    I repeat that I do not intend to defend AutoCAD, everyone is happy with what he has.
    Rhino alone does not perform calculations of naval architecture. Someone has created a plug-ins that do. The same goes for AutoCAD.
    Is it a tedious job to operate AutoCAD ?, analyze the attached video and we´ll talk about it.
    Make all calculations of naval architecture, starting from a 2D body lines plan, may cost me, with AutoCAD, about 5 hours. Is that a tedious process?
    My experience with the shell expansion drawing is what it is and, based on it, I can say that I believe it impossible to obtain the development of any plate (not belonging to the cylindrical body). Now if you, NavalSArtichoke, tell me how this is achieved, I would be really happy to learn something new.
     

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  14. NavalSArtichoke
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    NavalSArtichoke Senior Member

    If you are happy with what you can use AutoCAD for, great. I'm not commenting here to dissuade you from what methods you use in your business.

    I never claimed the shell expansion drawing can be used to develop any plate, regardless of its curvature. All I ever said was that the shell expansion drawing provides reasonably accurate dimensions for plates having single curvature, like those found in parallel midbodies with a rounded bilge.

    What I did say was that for plates with compound or double curvature, the shell expansion drawing cannot, as a matter of mathematical principle, provide an accurate representation of a plate strictly in two dimensions. This is why shipyards would construct 3-dimensional templates, using the lines of the ship as lofted, and then form the actual shell plate using this template, a process which is sometimes referred to as 'furnacing'.

    Compound curvature is why one is unable to create a map of a spherical (or near spherical) surface of the earth, which preserves areas, angles of bearing between two points, etc. simultaneously. The various map projections, e.g., the Mercator projection, have been developed to show certain characteristics about the earth surface in a reasonably accurate fashion, while other features become distorted. In the Mercator projection, for example, the lines of bearing between two cities can be determined with good accuracy from such a map, but the areas of a given region become distorted, especially if these areas are located close to the poles. The purpose of a Mercator map was to allow navigation, and the guys sailing the ships didn't particular care that Greenland appears to have almost the same area as North America. All these guys cared about was in which direction to sail after leaving England or Europe and wind up hitting New York or Santo Domingo.
     

  15. TANSL
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    TANSL Senior Member

    Sorry, I have translated this phrase wrong.

    Not only templates but the whole hull. Here yes you're right.

    I have no idea Mercator procedures used in their work.
    Plate development by the geodesic method is known from long ago. Maybe Mercator used it to measure the shortest distance between two points on the earth's surface.
     
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