making a scale cardboard model

Discussion in 'Boat Design' started by john mac, Oct 30, 2012.

  1. johneck
    Joined: Nov 2011
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    johneck Senior Member

    In my experience it is a good way of determiningpanel shapes for a small boat. We are not talking about building a ship, we are talking about building a rowboat, dory, skiff... While it may not be the best way, it will certainly work and allow you to see what you are creating. Thin pressed cardboard behaves very much like thin plywood in that it can be twisted and stretched (tortured) slightly to get to the shape you want. If you are finding that you you have to tear or squash the cardboard, you won't be able to make it in plywood.
    As several others have noted, this gives you no measure of how nice a boat you have, or if it float properly or carry the weight you need; only if you can build it or not.
     
  2. john mac
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    john mac Junior Member

    good point Frosty, scaling from a model would be inacurate for building the whole boat, but i'd only be using the model to get the hull right, then let the hull dictate the rest of the measurments. I know that isn't the way to create a beautiful boat, but this is a work boat that will get a lot of abuse and very little love. The guys on this forum create stunning boats, absolute works of art, whereas round here boat building is crude to say the least. I'm hoping to find some middle ground that will allow me to create a solid safe work boat without the flair and refinement of a leisure craft.
     
  3. john mac
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    john mac Junior Member

    at the end of the day this boat will be nothing more than a floating work platform, there are no two boats in the local harbour similar in size or style. Many of them started off as just a pile of timber on the quay side, screwed together with no real design input, and the finished boat looked how it looked and handled how it handled! crude but effective. Believe it or not there is a very good safety record in the local fleets! I'm thinking if I take a tried and tested design like the Tolman, as long as I keep the design to a similar length, beam, weight etc, I should be able to tweek it to my own tastes without jeopardising the safety of the original design? I'm not talking about extreme alterations, maybe slightly less deadrise which will shorten the LOA by two or three inches at the bow, and squaring the transom to remove the camber on the sides at the stern. I've seen people perform much more drastic alterations and still create a safe boat!
     
  4. tom28571
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    tom28571 Senior Member

    10:1 is not very difficult to do accurately, Frosty. I have used 24:1 on lift models with success in transferring to full scale. There needs to be care and lofting of some kind but it is not all that hard. Many use a scale of 3/4" to the foot for much larger boats. I much prefer thin plywood in 1/32" and 1/16" thickness to cardboard though. Available from aircraft supply sources like Aircraft and Spruce.
     
  5. tom28571
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    tom28571 Senior Member

    I actually used 16:1 but the forum lost the edit function for some reason.
     
  6. gonzo
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    gonzo Senior Member

    Plans are a relatively new way of designing. Half hull models were used for centuries to build boats and ships. They would scale them up much more than 10:1. However, they were based on experience and historical background. Builders made small changes to existing shapes and kept them if they were beneficial. For a plywood boat, the panels don't need to be too accurate. They are usually cut slightly larger and then trimmed. Also, the lines should get faired with a batten after scaling up and that will take care of most of the inaccuracies.
     
  7. Frosty

    Frosty Previous Member

    I thought the whole Idea of a hard chine boat was to build the ribs then place the ply on the hull and scribe a line in the chine then cut to size.

    Is'nt that the beauty of hard chine.

    With hard chine ribs I would think a good guy would knock one out in a day
     
  8. tom28571
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    tom28571 Senior Member

    That is the traditional way of building plywood boats but, stitch and glue typically needs whole side and bottom panels that are formed up with no interior structure at first. Interior stuff is added later after it already looks like a boat. You knew that, right?
     
  9. Earl Boebert
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    Earl Boebert Senior Member

    Er, except for Nathanael Herreshoff, for whom the model *was* the design. He had a special 3D measuring machine made to take the offsets from the half hull. All this can be seen at the Herreshoff Museum in Bristol, RI USA. He even designed the model yachts he sailed toward the end of his life by carving a half hull first.

    Cheers,

    Earl
     
  10. Red Dwarf
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    Red Dwarf Senior Member

    I find it hard to believe the naysayers saying models are not useful for testing.

    If that was true there would be no such thing as tank testing and millions of dollars a year is spent on tank testing.

    Here is an example of a small model being used for testing. http://www.youtube.com/watch?v=DBsZEq7hc9A

    The problem isn't the size of the model as much as what you do with it. If you just putt putt around a lake for grins then, yes, it is an absolute waste of time. But today we have dirt cheap gyros and accelerometers from the RC model industry that can collect a tremendous amount of data. We also have cheap high speed cameras.

    So if you do your homework I think you can do some valid testing with a reasonable size model and a good test plan.
     
  11. Harry Josey
    Joined: Jan 2012
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    Harry Josey Junior Member

    Half models were a useful if not essential part of the design process. Any inaccuracies in the scale up could be corrected on the loft floor. With chine and multichine designs I feel it is better to make full models. The duplication of the panels helps to correct twist and misalignment when assembled. As Gonzo points out the lofting corrections are made as the scaled up panels are installed.
     
  12. Perm. Stress
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    Perm. Stress Junior Member

    Accurate to the mm detail drawings from a design office is VERY recent luxury in ship and boat building.
    Before this, models, full size lofting in the shop and large margins to cut-off on-site were used.
    Also, even older methods exist , that allow creation of frames on-site, without any drawings or models at all, using a single "master frame" and set of cut-offs (or waht the proper name is ...).
    Half model is certainly good enough to make a table of offsets, from which full size lines drawing is lofted. In process of lofting (I did catch some of this work ~20 years ago) NOT A SINGLE NUMBER in that table is trusted, except those points that determine LOA, hull depth and Bmax (and some special features, like keels, skegs, rudder shafts... ). All the rest is used as a guide, to start from.
    Every single curve is corrected numerous times, until all of them are considered fair enough and all the intersections are perfectly aligned in all three views.
    In old shipbuilding books they write, that every single lofting shop will create a different set of full-size lines from the same table of offsets.

    Cardboard models are usable for checking your plywood ideas, but cardboard is really too flexible, if you want to investigate something dependent on stiffness of material.
    However, I even did some sailing models out of cardboard, but that was a wrong idea to begin with :). They ended up too heavy, too soft, and too easily damaged by water.
    I could see the use of cardboard model to check if your design of intersecting bulkheads/girders/webs is easy enough to assemble (or at all build-able :) ).
     
  13. rwatson
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    rwatson Senior Member

    Scale models and Tank Testing are not the same animal.

    Scale models dont sail the same as full size boats on open water, and when they are put into tanks they are specially supported and oriented.

    This is the reason for the huge costs in tank testing, that is the preparation and setup of the model and the tank
    http://www.sailboat-technology.com/links/HPYD-Conference-2002.pdf
     
  14. yipster
    Joined: Oct 2002
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    yipster designer

    Most of us made model boats I think, ivory board available in various weights / stifnes can be bought at art supply shops and works nicer as cardboard. Do I understand it was common practise to bend board around a half model to check and find devellopable lines? Did not know that. Old yachdesign offiches had often lots of halfmodels on the wall, very impresive but now outdared by computer. In the past i did trial and error hard chine paperboats, never a halfmodel but now with freeship boatdesign developing is a pleasure but should still see this printshop that prints bigger than my 100 or 120 grams A4 paper as for boat in question see some plans alter what you like and just build it •in a day?• I doubt models be very usefull than
     

  15. messabout
    Joined: Jan 2006
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    messabout Senior Member

    I have been doing this a whole other way around for quite sometime now. First I do a drawing in as large a scale as my facilities will allow. A scale of 1/4, or 3inch to the foot, is not a problem if you have a good sized drawing board. A 20 foot boat would need only five feet of paper. Sometimes I use two inch scale or even 1.5 inch scale but bigger is better. Depends on the size of the proposed boat. Three inch scale allows measurement transfers as small as one sixteenth inch (or about 1.5 mm) An architects scale is a blessing for this kind of work.

    Having had all the fun of drawing my raving beauty and using the eraser a lot, I do the basic math to see if it will float where I hoped it would. When I am satisfied with the brilliance of my inspiration, I build a model that I play with in the pond. Sometimes they are radio controlled. OK so the models do not give me definitive information but it does give me some great fun and satisfaction and a general idea of the appearance of the real boat.. Every now and then I actually build the full sized boat. The model told me that it was possible (or not) to bend and twist materials in the way that my drawings demanded.

    It is much easier and more accurate to build the model at a large scale. A five foot model of a 20 foot boat is not the least out of the question. My attic is full of them in various sizes. Balsa sheets are available at model hobby shops. Easy to work with and moderately priced. You can get them in many different thicknesses.
     
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