Panel Development

Discussion in 'Boat Design' started by smjmitchell, Oct 19, 2016.

  1. smjmitchell
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    smjmitchell Junior Member

    Hello,

    I have been researching various approaches for lofting flat panel developments for plywood and metal boats. I am trying to teach myself how to do this using traditional drafting / lofting methods.

    Whilst I have a few papers and books that discuss the issue none of these really provides a sufficiently complete discussion of the methods used by practising naval architects. Two references keep popping up in my reading, that are long out of print, but which seem quite practical. I have not been able to locate a copy of at a reasonable price. These are:

    Aluminium Boats (Kaiser Aluminium Co) 1978 - Appendix D
    The Plywood Boat Builder (Vol 41 of Motorboatings Ideal Series)

    A previous poster back in 2003 mentioned that there is only about half a dozen pages in each book that are relevant.

    Does anyone have a copy of either book that would be willing to scan or photograph the relevant pages and email them to me ?

    Thanks,

    Steve
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

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  3. messabout
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    messabout Senior Member

    SMJ I cannot know how much you want to do with this concept. I will suggest a couple of methods that I have used with some success.

    If you have a large scale drawing or a full size loft of the basic lines of he subject boat, then you can determine the panel shapes and dimensions pretty closely.

    Project the flat panel side skin of a skiff for example. The skiff may have different angles of flare at different longitudinal stations. First let us determine the actual length of the bent panel and the locations of the stations. The panel will be longer than the length of the sides because of the curve as seen in plan view.

    Bend a batten very carefully about the plan view of the panel. Mark the battens carefully in the locations of the sections. Now the batten marks know where the section intersections with the skin are located. You also know the correct length of the panel to the nearest millimeter or so if you are very careful with the measurements and markings.

    In an ordinary set of drawings you will have a body plan. Now you can take the measurements from the body plan to find the widths of the panel at the various stations. You can also determine the curvature of the top and bottom of the panel by using the dimensions above or below the base lines of the body plan.

    In some cases you will use what we Yanks call an auxiliary view. That is a projected view that lies out side the rudimentary three plane views. Diagonal or double bevel views you might say.

    It is perfectly clear that CAD programs are quicker to develop panel shapes. One or two key strokes and it is done. On the other hand it is far more gratifying to be able to do it the old fashioned draftsmans' way. .....some will argue vehemently about that satisfaction of being able to do it on a deserted island if need be.. That is a matter of "whatever floats your boat". Fun comes in different packages.

    What I have tried to describe above is appropriate for boats or other structures with flat panels as in plywood or other flat stock. All those snazzy boats with continuously rounded sections are a whole 'nother ball game. The same general rules of graphics apply but just a lot more intricacy in the layout of the planking...not sheet panels.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm not sure if the OP is concerned about arriving at the shape of the flat panels that are then bent into place for a neat fit, or something else. It might be instructive to consult a sheet metal working manual to see what methods are used, as much as boat building texts.
     
  5. smjmitchell
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    smjmitchell Junior Member

    Thanks for the replies.

    I have seen the paper by Chris Barry. It is a good paper but does leave some questions unanswered.

    What I would like to do is learn to design flat developable hulls from scratch on a drawing board (or in AutoCAD) using traditional methods. Once the chines and lines have been established this includes developing the transverse sections or frame shapes (which will have curved lines between chines) and also generating the flat panel shapes. It also includes modifying the lines as necessary to ensure that the hull is developable since not all arbitrary sets of lines are developable. The simplest approach is a conic development but that introduces limitations on the shape of the lines. The multiconic method (Rabl) is the most flexible approach but it is not easy to understand exactly how this is applied in practise. Rabl's two page paper is quite cryptic and further explanation of the method would be useful (some examples of it being applied to a real hull would be great !). The Barry paper linked to above is a simplified version of the Rabl method but as I said leaves a few questions unanswered.

    The two references I have mentioned above sound like they could be practical and contain examples that might help understand how to apply lofting methods in practise.

    Software such as Freeship can calculate developments at the click of the mouse but there is no guarantee that hull shapes are actually developable - none the less, the software flattens the panel anyway with some stretching or shrinking. Sure the software can plot Gaussian curvatures and panel stretch/shrinkage etc and you can massage the shape and try to get rid of the areas in the panel that have non zero Gaussian curvature but this is not as easy as it seems in some cases. I would prefer to learn to design developable hulls by the traditional methods that have been used for decades.

    Steve
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I recall a little book called Small Boat Design for Beginners by a bloke called Bailey had a section about Rabl's method, not a great amount though. I think he designed the De Havilland aluminium boats back in the day. What kind of hulls are you interested in, sail, power, planing etc ?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    The relevant sections of the book ( click to enlarge to readable size)
     

    Attached Files:

  8. smjmitchell
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    smjmitchell Junior Member

    Thank you for the scans. I am not familiar with that book. It looks quite useful. I will have a read later today and see if I can understand what is going on. I am keen to understand not only how to do the loft but why the various lines are drawn the way they are.

    I assume that the author is Frank Bailey who was the manager and an engineer/designer at DeHavilland marine. He was also well know here in Sydney for his ultralight aircraft designs. I believe I met him once many years ago at an airshow where he was exhibiting one of his designs.

    My immediate interest is the lofting of a plywood planing hull that I intend to build. I am also working on the design of a multi chine plywood sailing boat.
     
  9. jehardiman
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    jehardiman Senior Member

    Most modern CAD expansions are by triangulation because this supports meshing. The development of a hull from the first lines by conic development is a little more tricky. FWIW, the method shown above is only approximate, not true development. There is way too much to try to describe it over a BBS. To put it simply, the limiting curves of the panel must be the intersection of cut planes on the surface of a conic where the two edge ruling lines are radials from the apex which is at some point in 3-space.


    Try to get your hands on Ship and Aircraft Fairing and Development by S.S. Rabl and/or Lindsay Lord's papers on polyconic development.
     
  10. smjmitchell
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    smjmitchell Junior Member

    Yes the Rabl and Bailey method (as posted above) are approximate methods and are similar. The Kilgore method which (now I think about it some more) is similar to the method in the Barry paper (see link above) seems to be more exact. However here is the crux of the problem ....

    I have Rabl's book "Ship and Aircraft Fairing and Development". I also have his "Multiconic Development of Hull Surfaces Paper". I have struggled with this for a while because it is clearly an approximate method and thus it is impossible to know what sort of errors are implicit in it without some practical experience. Kilgore's method is complex and I wonder if it is practical for everyday use ? So the real question is what method have naval architects used traditionally in practise ? Is the Rabl method sufficiently accurate for practical purposes or am I better off accepting that I need to adopt the additional complexity of the Kilgore method ? Or is there perhaps some other method out there that I have missed ?

    The Bailey book is good because it appears to confirm that this method works in a practical setting (any Australian knows tens of thousands of DeHavilland boats were built). That is also why I was interested in the first two references I posted because I wondered if they might provide some sort of confirmation of what works in practise.

    Do you have any specific references to the work of Lindsay Lord ? Are you referring to his text "Naval Architecture of Planing Hulls" or did he publish some other papers ?

    My thoughts at this stage are that I need to do a loft using the Rabl/Bailey method and a second using the Kilgore method and see what sort of differences I get in the shape of the developed frames at a number of stations. My other thought is that I probably should build a large scale model of my hull in 3mm MDF say 6-7 ft long as a way to confirm the accuracy of the loft.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    This is something that you have designed yourself ?
     
  12. TANSL
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    TANSL Senior Member

    If you build the hull, or some of its parts, by conical surfaces, the development of the plates can be completely accurate. But I guess everyone already knows that.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    The key element in a developed hull shape is the chine. Have that fair and defined in three dimensions, and the rest is straight-forward.
     
  14. smjmitchell
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    smjmitchell Junior Member

    Yes it is a design I am drawing. Just a plywood speedboat design for about 10 hp. More of a learning exercise than anything.
     

  15. TANSL
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    TANSL Senior Member

    I do not know if I understood you correctly but the hull surfaces may be non-developable, even if they are supported by a beautiful three dimensions defined chine.
    On the contrary it may also occur: the intersection of several developable surfaces results in a chine, with very bad appearance.
    That is, in my opinion, the appearance of the chine has little to do with the developability (I donĀ“t know if this word is cotrect) of adjacent surfaces.
     
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