making a scale cardboard model

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

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    - reminds me of an experiment I tried years ago, which impressed me until I realized I had reinvented the Bruce Foil . . . I carved a wood hull, weighted it for the correct W/L and CoG, rigged it then floated it on a long tray and tested it with a fan. The model was only 6" long but it showed me what the issues were. I fixed it, scaled it up and the real thing worked first time. I was only 40 years too late . . . pity!
     
    Last edited: Nov 2, 2012
  2. ancient kayaker
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    ancient kayaker aka Terry Haines

    - I use the very similar FreeShip quite a lot, and the plank developments have been fine. It's rather counter-intuitive so it's a bit of a pain to learn, and you cannot get a precise boat shape, but if you are prepared to keeping fiddling you can get close.

    I usually use it together with Carene2008, another freebie, simpler but more limited. Carene allows you to enter the shape and size of the hull you want - more or less, and will generate a file that FreeShip can import as markers. That gets you to what you want quicker. However carene handles stems poorly so that part usually needs revision, and it only does the basic hull not superstructure.

    Carene has some hydrostatics but FreeShip is far more powerful.
     
  3. rwatson
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    rwatson Senior Member

    All that trouble to inaccurately gauge displacement, when you could read the waterlines off the table in Delftship in a matter of minutes. How do you measure the displacement of a boat in an 18" model ? .. measure the rise in water in the test tank after you try and measure the draft of the model ? What a waste of time for approximate results Just key in the chine heights from your 2d lines drawings (about 20 minutes) , and you can get a complete analysis like the one attached.

    In 'old times', the tradesmen spent more time correcting the drawings and lofting marks than the designer spent drawing them. They also spent a lot of time adjusting the weights and ballast to get an acceptable waterline. What visual guys got from models was about 10% of what they really need. Dont give me 'old times' !

    Its not the models that do this - its the laser measuring equipment, in calibrated strain gauge arms, and digital photography. Without all this gear, the model is what I said - an interesting visual aid.

    You ought to investigate your software a bit more. If you can do a plan and side view in 2d paper or cad, it takes minutes to create the table of offsets stored in a text file, to import into either DS or FS to get a complete hydrostatic analysis. You can also unroll the panels with diemensions, and either read the offset values for manual lofting, or import them into cad for a CNC mm accurate cut.

    See the attached text file of chines of a small 16ft kayak (12 lines of a text file) , which generates the 3d image in seconds when imported, calculates the expanded panels and provides full hydrostatic analysis, and then tell me again how hard it all is.
     

    Attached Files:

  4. pdwiley
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    pdwiley Senior Member

    Yeah, but does it actually float when you put it in the water?

    PDW
     
  5. Ilan Voyager
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    Ilan Voyager Senior Member

    I do know all that. I began to work (in fact all the team) with CAD in 1979 at the DTCAN and I was one of the first users of the betas of Catia as I had a very good friend at Dassault Aviation...Like all guys of my generation, by necessity, I was able to program in several languages, to cope with a Unix computer and to create hundreds of spread sheets.

    Computers and softwares are good for repetitive and complex works; excellent for optimizing a race boat or a warship, solve complex equations, make finite elements analysis etc...

    But...what I mean it's that for drawing -when you know your job- a small power boat in hard chine plywood, to use a "big" software, with a long land step earning curve, is like taking a 120 tons press for bending at 45° an steel flat 1.5 wide by 1/8 thick...Even if the soft is good, powerful and more than that, the investment in time is not worthy for drawing ONE SMALL boat. I'm more interested to build it and go fishing.

    There is no need of a big software to find the LCG or calculate the metacenter on a small lobster boat. A piece of paper, a pencil HB, and a small scientific calculator do the job for a guy like me. If I feel lazy, a simple soft like Hull Design is largely precise enough and a model will validate the thing and show if the bow won't be too full (a common defect in plywood boats), or make an "oil can" just in the topside.

    The shape is known since decades, the engine probably an outboard will be behind at the stern and the skipper hopefully in front of the engine inside the boat. The cooler with the white wine won't change significantly the center of gravity. The width will be about 1/3 of the length, and it's known by the trials by Saint Peter, fisherman at the Sea of Galilee and later pope at Rome, that a 15 degrees angle is a good compromise for a planing boat, and that monohedrons are good boats and indecently easy to build, even by a guy with 2 left hands.

    The rest is affair of a slight compound of the plywood in the topsides and a nice sheer. No soft is able to predict the amount of compound of the plywood you can introduce. It's not made for that.

    I'm engineer and used to calculations but sincerely I do not see the need of sheets of ciphers for a 18-20 feet lobster or fishing boat. Even the offsets will take at most the quarter part of an A4 paper. And abou the CNC, that depends how far is the shop making that. A CNC cut will save days of work on a 40 feet boat with a complex lay out...for a 20 feet boat with 2 chines, 1 floor and 3 bulkheads....ok good for a guy without knowledge of boatbuilding. For an old monkey like me...doubtful.

    Aparté; A lot of good measures have been done in towing tanks before lasers and tutti quanti. Modern tools are useful and precise but are not the panacea.

    Software do not replace knowledge, theoretic and empiric, and that's the big delicate point.

    And it's far better and simpler to buy the knowledge and experience in a precise domain; for example on 1998 a client asked me a 20 feet sport fishing boat. Instead of designing it, I bought the plans of a very, but very excellent one to a NA I know in France, adapted it for "placoplastic" GRP without mold method, added my own salt grain in the details, and made the boat in 3 weeks in my shop. Done well and fast. Happy client, so happy that 2 years later he asked me a trailer because he was going back to Canada with his beloved boat.

    This French NA had designed dozens of small sport fishing boats for 30 years. He knew all the tricks and details that make an outstanding small boat. Why bother to re-invent the wheel and take a risk?
    A fortiori, when the person is a total beginner in design and building boats, better to buy plans and follow the manual line by line.
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    Well, you don't really need to go to all the trouble of building a boat if you just want it to float: just toss the wood into the water and watch it drift away . . .

    Seriously though; er I forgot what I was going to get serious about!
     
  7. Frosty

    Frosty Previous Member

    Oh ,--thats interesting --can you get none floating boats.

    Is it still a boat? If it did'nt float what do you call it.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Oh yeah . . . it's a wonderful world, you can use pencil and paper, carve a model, knock yourself out on a drawing board, sharpen a computer or just build the blasted thing. Each to his own I say. It's all good if a boat comes out of it . . .

    Hey Frosty: doesn't matter what you call it, like the dog with no legs, it ain't gonna come when you call. Hmmm - I think I'll finish the bottle now.
     
  9. rwatson
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    rwatson Senior Member

    I cant figure your point out - you start out telling me that scale models are the way to go - and I know from experience that they take more than a day to build.

    I show you how to create a computer generated hull with 12 lines of measurements, that produces all the info and more you would need to decide if it floats ( thanks PDW ), what the displacement is with different loads etc (your requirement done with tedious scale measurements) - that takes 30 minutes - and you say I am using a 12 tonne press !

    "Computers and softwares are good for repetitive and complex works" is complete nonsense. It makes the creation of simple hulls, rough measurements and creative ideas done in seconds. I and thousands of others do it all the time.


    Some people stick to a line of opinion to save face - whats your reason ?
     
  10. michael pierzga
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    michael pierzga Senior Member

    Well..many people..like me when I was a kid, need a new crab skiff and have no computer, nor computer skills. Going down to the waterfront...observing other skiffs, then making crude Half models and messing around with shapes ,rocker, sheerlines, for simple boats is a good skill. I had dozens of them piled up under my bed
     
  11. john mac
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    john mac Junior Member

    this style of boat building is more my thing. seeing as the debate on cardboard models is still out, and i'm useless on computer programs, then rattling up a couple of sides, a bottom, transom using cheap chip board, stitching them together with a few frames to create the right beam, hey presto. A bit of trimming to get everything exactly right, then use the chipboard as templets for the marine ply. whats the worst that can happen?:)
     
  12. michael pierzga
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    michael pierzga Senior Member

    Try this. Get your camera and head down to the waterfront.

    Once at the shipyard find a suitable...SIMPLE boat that is of interest and take pictures. Photos of Side on PROFILE and BOW ON , STERN ON stations and shape. Now take the photos and photocopy them to a suitable jumbo size. Take the photocopy and trace it on to a blank sheet of paper.

    You now have the PROFILE lines plans of the boat. Next Take the Bow On and stern on photocopy , trace the lines onto your blank white paper and you now have the transome station and beam max station. Approximation !

    Transfer to a block of wood and start sawing and chopping....................... go slow and soon you will have a pretty decent shape .Bring it down to the shipyard...compare to the original, make some notes,measurements..bring homeand refine.

    Once it starts looking good...bring it down to the waterfront pub and pass it around for the guys to look at and comment on.

    Remember...pick a very simple boat.
     

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  13. Frosty

    Frosty Previous Member

    You gotta be kidding --right!!
     
  14. michael pierzga
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    michael pierzga Senior Member

    No...this is a great way to concieve a concept half model.

    Of course Its much easier to go to the library and photocopy real plans to bring home and make your half model , but in some instances...local type fishing boats of special interest...a camera and a photocopy is perfect.

    Try it...it works
     

  15. gonzo
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    gonzo Senior Member

    The Carolina Sportfishing boats where usually designed as Michael says. They were up to 55 feet or so. I have worked with the old timers and they didn't have more than a length, beam and a rough profile; usually on a scrap of plywood or paper. I don't know how long it takes to generate the lines for a skiff on a computer, but for a workboat finish I can build one, including paint, in a day.
     
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