Reverse engineering scale models

Discussion in 'Boat Design' started by boataid, Feb 25, 2013.

  1. boataid
    Joined: Sep 2009
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    boataid Junior Member

    Hello all,
    I have shaped a 1/10th scale outrigger canoe model and am looking at ways of creating a parametric CAD model in Solidworks.
    I was quoted $1400 to have it scanned and the model created. This seems outrageous to me and I was wondering if anyone has tried photogammetry or any other process that may be cheaper.
    The CMM process would yield fantastic accuracy but to be honest its a greater accuracy than is necessary.
    Any thoughts or ideas on how to cheaply pull data from a scale model?
     
  2. DCockey
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    DCockey Senior Member

    I've used photogrammetry on full size boats, and created CAD models using Rhino. Quite do-able but not always a simple process.

    You might want to consider using traditional methods. Have a look at Chapter 7 of Boats: A Manual For Their Documentation which is available for free download at http://www.museumsmallcraft.org/publications.html While the descriptions in the book are for full size boats the methods will work for models.
     
  3. philSweet
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    philSweet Senior Member

    Okay, you made a scale model. How did you do that? What did the source data look like?

    What is it you want in the end? A parametric model just needs a few paramenters. Like maybe three parameters and a length and a beam and a draft. Or it could mean 20 parameters and a length and a beam and a draft. Or maybe length and displacement and five parameters.

    It sounds like you want a surface mesh, not a parametric model. Does it have chines? Can it be made out of sheet goods?
    Or is it a carved log?

    What purpose does this digital model serve? Tell us more.
     
  4. boataid
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    boataid Junior Member

    Thank you for the link DCockey.
    philSweet i hand sculpted the scale model. The source data for the model was intuition and personal experience in and around canoes. Ultimately I will be cutting a full sized canoe with a cnc router.
    I suspect there will be some discrepancy in the cad model that I will want to adjust before I push go on the router.
    I want to be able to make adjustments to a model, whether its a surface mesh or parametric. (I dont know which one I actually need as I will not be doing the cad work.)
    In reality I just want to build a full scale version of what Ive sculpted at 1/10th scale and Im not sure how to best recreate that shape in full scale.
     
  5. philSweet
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    philSweet Senior Member

    If you are not overly fond of the model, you can cut it into slices and trace them on paper. Some models are intentionally built using the bread-and-butter method so they can be taken apart. But I just noticed you want to work with solidworks. I don't use that so I should probably step aside.
     
  6. Petros
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    Petros Senior Member

    You are really doing this the hard way; this is what I would do. take manual measurements off the model and than draw them full scale at perhaps 5 or 6 stations, than build a full sized skin-on-frame test model using plywood frames and cheap wood stringers, wrapped with plastic and duck tape, than go out and paddle it around. If it is a shape that behaves the way you want than input the information to carve your full sized plug. It should not take you more than perhaps 2-3 hours to create something large enough to go try out full sized on real water.

    Usually designers start with the properties they are looking for, and than try and create a shape to achieves that. It seems entirely backwards to create a nice model, and than use a very expensive process to created a full sized one, and than find out it how it behaves. On small water craft like a canoe, there are simple means to build the approximate shape and try it out, on large water craft is is too costly to build a test model so we have developed all of the analytical tools for determining how the hull will behave BEFORE we build it.

    but on something small, it is far faster (and more fun) to make a full sized mock-up and get it on the water. All of the messing with fancy software and computerized machinery are supposed to make it faster and easier to build a hull, but if that hull is a bad design, you are just wasting your effort. One of my engineering professors used to say "one simple test is worth a thousand expert opinions". Throwing together ad quick and dirty test hull using low cost materials will teach more more about hull shape than all the computer models in the world.

    Good luck.
     
  7. Alex.A
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    Alex.A Senior Member

    Pictures of your model would really help......
     
  8. DCockey
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    DCockey Senior Member

    Good advice.

    Carving a model with very few target parameters was the traditional method used for lobster boat design, and carried over into design of some fiberglass lobster boats. Also, Nate Herreshoff designed his hulls by carving half models and then taking offsets from the half model. What is not clear to me is how he ensured that the hull designs of racing boats met the racing rules. http://www.herreshoff.org/hmm/hmm_05_02_virtual_4.htm http://www.herreshoff.org/hmm/hmm_05_02_virtual_3.htm

     
  9. messabout
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    messabout Senior Member

    Phil and Petros have the simplest solutions. Carve up another model whose original block is made of horizontal lifts. That is to say, rip and plane some planks about 1/8 or 3/16 thick. Plank should be half the width of the model or full width if you must. Stack them up and fasten together. Now carve. When you are finished take the lifts apart. Now you have perfectly traceable waterlines and stations that you can scale up as you please. This method was good enough for some of histories finest designers who had no computer programs. Caressing a nicely carved model is much more etherially pleasing than pounding a cold and inanimate keyboard.

    After that, build a cheap and dirty, full sized hull, and try it to see whether your carved inspiration had the hoped for merit.

    P.S. Use a larger scale than one tenth. For a canoe sized boat one sixth or one fourth scale makes for better accuracy. Use an architects scale to do your offsets. They are available in three inch and two inch, and one and a half inch scales.
     
  10. boataid
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    boataid Junior Member

    canoe.png

    Alex.A If I attached correctly you should be able to see the model. It is 1/10th scale.

    Petros thats really is a great idea but not applicable in this case. The canoe is 45 feet long and needs to hold 6 grown men in rough seas. I very much doubt that slapping something together would not hold up under the kinds of conditions and use needed to test it.

    philSweet that looks like the direction I am headed but I may pull templates off it first at specific stations. I just got another quote today for $1900 so at that price I have no problem cutting up the model.
     
  11. Submarine Tom

    Submarine Tom Previous Member

    That's a pretty cool looking hull.

    Can we see it from each end?

    I'm designing a three man (double bladed paddle) hull.
     
  12. boataid
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    boataid Junior Member

    Submarine Tom The bow and stern pics dont show up well but Ill post pics when I begin the full size build if you'd like.

    Im building for a race in september so i will hopefully start building next month
     
  13. DCockey
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    DCockey Senior Member

  14. Earl Boebert
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    Earl Boebert Senior Member

    Another way to take sections off a model:

    Get a digital camera with manual exposure control and a laser level. Line the camera up on the longitudinal CL of the model. Set the laser level so it throws a vertical line at the desired station. Darken the room and take a picture. With a little fiddling you can get a good set of sections, verified by printing on cardboard and testing for fit.

    Cheers,

    Earl
     

  15. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Nice idea. :idea:

    Photographers call this technique "light painting". :)
     
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