CAD vs. Reality

Discussion in 'Boat Design' started by gonzo, Nov 6, 2004.

  1. gonzo
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    gonzo Senior Member

    Has anyone built a half model and then compare it to what software predicts? For example, it would show how accurate plate development calculations are.
     
  2. Andrew Mason
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    Andrew Mason Senior Member

    On the other hand, you could design a boat in a CAD program, then build it full size. This has been done tens of thousands of times from Maxsurf and has shown the plate developments and surfaces created to be extremely accurate.
     
  3. Arrowmarine
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    Arrowmarine Senior Member

    Gonzo,
    My former employer's designer was a Naval Architect and always used half-models to design their boats. I drew several of their boats in autocad full size then made flat templates from the drawings. I found them to be very, very close when compared with the originals. Although I have no half-model experience, from what I have seen they work very well and are very accurate.
    Joey
     
  4. rjmac
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    rjmac Junior Member

    Gonzo,

    I have never built half models...., but I have built several models from autocad and they have been very accurate (taking scaling into account). I get teased everytime I build one, from wife and friends.

    RJMac
     
  5. jehardiman
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    jehardiman Senior Member

    What do you mean by "accurate". If you mean within shipfitting tollerance (about +/- 1/8" in 40') then, yes, most programs do ok. If you mean that you plot the lines full size and then spring a batten down over them and they match to within 0.00001" then, no, most programs suck. If you're making a half model that's 24" long, you most likely wouldn't even notice or be able to measure it. If you scale it up to 240' then you will see some unfairness; it's the nature of the beast (i.e. your accuracy is only a good as your spline model and no spline model is perfect).

    The real question is can you build it that accurate and fair? In most applications the CAD drawing won't matter once the piece is cut. I've drafted many pieces on CAD, but we always loft them on the floor and then fit them on the ship. This is not to say that you couldn't computer cut a hull to be seamed up with no fitting, but in a one-off you would get what you get and most mass produced hulls like that that I know of, the panels have been "tweaked" after the first one for better fit.

    Machine cut full size plugs/molds and propellers are an entirely different matter and are mostly limited by the tooling itself.
     
  6. rjmac
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    rjmac Junior Member

    jehardiman,

    This is lots of good info, especially for me, I have never been involved in anything over 48ft, and that was lofting from prints.

    This tread is taking a small detour for a moment.....

    I have been involved with computers and the output that is produced by computers (CAD/CAE Applications Support Engineer/Designer) for years. What you mentioned in terms of output of the computer for very large vessels is complexed, which I can only imagine with my limited knowledge.

    As an example: What you see on the CAD screen is scaled output and the fun begins with the resolution, such as measure a line on the screen as 10inches, output that to a printer, measure the line on the paper with a machinist scale, and find that it measures 10.015 inches. If I where to create an output of the line vertexes to a file, rounding occurs there also. The output would say 10inches in the file..... What happen....., the accuracy of the output device will determine how accurate an arc/line you get. This pretty much is it in a nutshell, the computer is very accurate, the question is how accurately can you reproduce the information in the machine. The computer software can measure something down to it's maximum resolution (usually defined by the user) and be exact. Can the output device reproduce that....? Sure if you want to spend big bucks.

    This is where the shipwright comes in, he makes things fit, this is what is know a skill and craftsmanship.

    I agree with everything you say, but just a little more information needed to be added...... ;)

    gonzo, does that kinda answer you question....?
     
  7. jehardiman
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    jehardiman Senior Member

    rjmac, don't even start me on using "accurate" laser line setups on large ships in dock or (...shudder...) floating, structures that are so long that the curvature of the earth is measurable and temperature changes effects measurements.
    :D
     
  8. rjmac
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    rjmac Junior Member

    jehardiman, alright cool stuff..... :D
     
  9. MikeJohns
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    MikeJohns Senior Member

    JEHARDIMAN
    I always laugh when I see laser guides come out on the slipway.
    Within the materials elastic range structures tend to move far more than people realise. Bigger GRP boat are particularly prone to distorting from the designed shape when in the water.
    Laser alignment of shafts when on the slip seems to be trendy but they need doing again when launched.

    But I digress

    Gonzo
    I have built several models from Rhinos plate development and ound the 3d view to be a good indicator of the true shape. Also the panels fittted well ( transferred from paper plots )

    As for fairing the curvature analysis and "FAIR" commands used wisely and at the correct stage of design will produce NC cuttable kits without any need for lofting.

    RJMAC
    Always calibrate the plotter when building models. We use a dial vernier on a plotted standard sqaure, plot that square along with each output and check it on each plot.
     
  10. Sean Herron
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    Sean Herron Senior Member

    Thats funny...

    Hello...

    I never knew anyone to calibrate their plotters with a square and what I call a 'very near' - I thought I was crazy - I am not alone...

    SH.
     
  11. gonzo
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    gonzo Senior Member

    There are probably thousands of CAD programs. A few are supposed to be quite good. My question was to see if designers have compared the data a program produces from a set of points to a half model. Usually it is left to the builder to fix problems. In lofting, finding 1.5" error on a 50' hull is not unusual. Some designers produce very accurate sets of lines. For example, offsets by Herreshoff, who didn't use CAD, require minimal tweeking. I believe that computer generated lines can be fair and easy to build. However, I want to know what the difference is between actual lines and what the program produces from a set of points. This thread is becoming interesting.
     
  12. jehardiman
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    jehardiman Senior Member

    Gonzo, the real problem is not the offset points, but the curves between the offsets.

    Example; I was involved with a ship sized hull that was drafted on CAD and lofted on the floor when built. the CAD offsets were about as perfect as you could get...+/- 1/32 in just a couple of places against the battens. Several years later someone wanted to fit something into the shape against the shell between stations where there was a fair amount of curve. They used a different CAD program and drew up the hull section and added structure to fit. Again the CAD layout showed it was perfect. It was built, but when we tried to install it the shell curature was all off, more than you could bridge with weld. There was a difference between how the CAD splines worked and the real steel. Yet everyone's offsets were correct.
     
  13. MikeJohns
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    MikeJohns Senior Member

    Dont get too hung up over the accuracy of production.

    I wonder if measured after completion, how many boats would be overall within 40mm of the designers specs.

    As for distortions from the designed shape, you have to accept a tolerance in any construction. In specifying the tolerance you have to be realistic. No yard will produce a 50 foot boat to a 10mm tolerance. With a steel vessel the framing strength can give better accuracy of the final product but even if the frames are within 10mm, the stringers plating and welding will have their impact on the overall shape.

    Bilateral symmetry is more important but even that has a tolerance. I have seen keels a few degrees out of line with the hull with no detriment to performance. Yet obvious to the eye

    Smaller moulded GRP vessels can be pretty accurately reproduced but then as mentioned before they change shape when subject to bouyancy forces.


    If you scan a curve into a nurbs type CAD package the line can be matched very closely and can be faired better than the original drawing.

    Measuring off a lines plan is more accurate than measuring off a half model yet the measuring and scaling produces a very low accuracy and this system has been used for years. I often see two supposedly identical traditional wooden yachts out of proffessional yards that when measured have major differences.

    I think the modern CAD outputs make the builders more answerable, and that accuracy is vastly improved.

    I use polynomials to fair lines from older offset tables.
    Generate a best fit through the points, discard the obvious errors use a decent curve fitting program (not excel) then you can regenerate offsets accurately at any point. Many of these trad plan offsets show significant unfairness which is the limitations of paper pencil and subsequent measurement.
     
  14. MikeJohns
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    MikeJohns Senior Member

    Sean Herron

    I like the verynear its suprising how often that 100mm square has saved the day when our plotting agency has got their scale wrong in one dimension.
    When making test models that square can be ploted in the corners and the centre (as a faint line) and checked before cutting out the plates. You could plot rules but squares work well. ( you see we are not printers). We engineers are all pretty square!

    Happy Printing
     

  15. Sean Herron
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    Sean Herron Senior Member

    Reality...

    Hello...

    I used to plot squares back in dot matrix on a C64 - I am old - I am fragile...

    I think that we need to add to this thread a bit...

    Take into consideration the day that a plot is laid to material - Mondays are very bad and nothing can be expected - or inspected - until late in the day...

    By mid week the floor monkeys have shaken off the weekend and things start to look promising...

    On a Thursday you can usually expect things to be 'worked out'...

    Forget about Friday - just think about the next Thursday for production...

    Now - lets factor in a few bits to this 'equation' - first - the semi retired guy who does not know how to turn a computer on - but has been building boats for 30 years - but leaves on Thursday at 2 PM...

    Now we have the street kid 'who just wants to build boats' who tacks a 15 foot panel to the previous shifts perfect work - on a Friday afternoon...

    The following Monday the guy who you trust - but he likes to show you his guns - sees same - and tears into you for letting this happen...

    And the old man who just wants to retire and golf goes into a funk and does no work that day from same...

    Then you have to introduce a new welder from some third world country that the management hired on a minimum wage contract to the crew - that is always a good time...

    Then the shift changes - and the government sponsored and wage matched crew comes in - things just get better...

    Then you go home a check the chalk board that your wife has hanging in the kitchen of 'things to do' ...

    CAD versus 'reality' - WHAT REALITY...

    Cheers...

    SH.
     
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