Correct Hull Shape from Existing Lines Plan

DELFTship forum Hull modeling Correct Hull Shape from Existing Lines Plan

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    • #44692
      patmat41959
      Participant

      Many of us want to create accurate CAD models of existing ships from their original plans. Delftship makes this easy with its tools… except that the results are not so accurate! When the control points are defined from the original lines, the resultant surface is influenced by the control mesh, but does not pass through the control points (except at corners).
      I cannot see what the work flow should be to pull the DS surface closer to the intended shape.
      Maybe I should:
      1. Create a marker curve on the original control points;
      2. Then move the control points until the new section comes close to the marker curve?

      I tried this on one section, but didn’t succeed in moving the generated section.

      In the image:
      White = Control points and mesh
      Green = generated section… far from the intended location!
      Purple = the control curve

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    • #44704
      patmat41959
      Participant

      So here’s the second most important thing for DS users to know:
      Lines imported from a lines drawing DO NOT create a hull true to those lines!

      First most important? It can be fixed.

      The developed hull shape does NOT go through the control points, which are input from the original lines drawing… they just INFLUENCE the shape. Fine except it’s not, because the developed form no longer matches the original. There must be a work flow that gets the generated shape closer to the desired form.

      Indeed the DS developed hull cuts corners, it’s always well to the inside (concave side) of input curves, except at the “corners”.

      Imagine you have a magnetic rubbery sheet pegged by its 4 corners to the floor, exactly where you want the corners.

      Then you hang a bunch of magnets from the ceiling, and each magnet is hanging exactly where you want the sheet to stretch and rise to, defining some sort of hill (I’m picturing¬†Richard Dreyfuss building Devil’s Tower in his living room).

      But while the sheet is stretchy, it’s not THAT stretchy, and it hovers over the floor but doesn’t quite reach the magnets. Oops. That’s what we have in DS, with the developed hull not reaching the input points.

       

      How to fix it? Just keep raising the magnets until the sheet rises to the proper place.

      In DS, I did this, though there may be more efficient ways:

      1. Import the control points once to define the control mesh and its resultant malformed hull.

      2. Import the control points a second time as “marker curves”. Markers are static curves, immutable, and they serve as references of where the original control points and desired hull sections are.

      3. In an end view, compare the developed body sections (which are wrong) to the desired shape shown by the marker curves. Now start moving control points sideways or vertically to slowly pull the body sections towards their markers. Yes, this modifies the original control points and mesh!

      4. Note that this is iterative… moving one point affects the adjacent surface faces, so you’ll need to sweep back and forth over all the points to get everything close.

      5. Also note that the four edges of your rubber sheet between the corner pegs were also pulled off the floor when the magnets started lifting the center. Same thing in DS- the edges at deck edge and keel also have moved out of place. So the adjusting exercise needs to address those too.

       

      I did this for my tugboat hull, and while it still needs some adjustment, it’s much closer to the desired shape now (below).

       

      More: While you can get close by doing this, you can also turn on curvature displays to better smooth (fair) your new shapes. I haven’t done this yet, may not need to for my present purposes.

      More #2: The Pro version of DS has an “automatic fairing” tool. I’m not sure how this works, but it could be of help.

      20221027b-1

       

      Here’s an end view showing some of these features:

      White: Original control points and the control mesh with its straight Tinkertoy connectors. The control points lay on the proper body sections from the lines drawing.

      Green: Body sections cut through the DS developed hull surface… they miss by a lot. Even at the deck edges.

      Pink: Nice curvy marker curves that DS draws for the second import of the original control points. These markers are the target, I need to move the green lines to match them.

      To move any one point: Click on it, and you can enter new numbers or use the little arrow to move the point by set increments, very handy. Move each and every one this way to drag the developed sections around.

      20221027c-1

       

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