Calculated Weight of Volumetric Shapes

DELFTship forum Hull modeling Calculated Weight of Volumetric Shapes

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    • #37065
      Kenneth Weir
      Participant

      Context:
      I’m working on a sailboat design, a 45′ strip-planked schooner.. I have the hull shape, keel, stem, sternpost & transom all modeled. I’m currently working on the frames & floors. As far as the model is concerned, the internal parts do nothing. They’re just there to help me see how much headroom I’ll have in various parts of the boat. i thought it’d be very convenient to have a running tally of component weights as I’m modeling it to give an idea how much ballast it’ll need on completion, +/- 5-10%.

      In the program units are set to feet. Density is being entered as specific gravity.

      The problem:
      The weights it’s giving me for various parts are extremely high, sometimes 4x what they should be. The hull appears to be accurate as it’s 1 surface only, the ID of the hull. It’s giving me a weight of 2,842 lbs (1.292t) when I’d done a loose estimate beforehand of 3,000 lbs.

      The keel (including stem) or any part that I modeled with all sides is very high, however. Is there a sort of rule of thumb for entering thickness on a fully-sided shape to get the correct weight? As an example one of the frames, at 0.176′ x 0.176′ x 10′ at 39 lb/ft^3 (SG=0.609) should weigh about 12 pounds. The program, when using a half-thickness of .088 feet, gives a weight in the neighborhood of 0.016t (35 pounds), almost 3x heavier than it should be. Using 1/6 thickness doesn’t quite seem to work out either as some parts can have drastically different dimension ratios.

      Now I realize a difference of 27 pounds won’t mean much of anything, but that’s just one frame member and a very small one at that. There’s still 22 more frames, the floors, clamp, shelf, bilge stringers, deck beams, engine beds, mast steps & several other parts to consider & that could easily add up to a couple of tons or more of difference when it’s all said & done.

      Any ideas on how to get a correct weight on parts that are modeled on all sides? Or maybe I’m just doing it wrong?

    • #37067
      Kenneth Weir
      Participant

      After having slept on it for a night, I came back with a different approach today.

      The keel, measuring .35′ x .6′ x 45.3′ @ 39 lb/ft^3 should weight 371 lbs, or 0.168t
      After dropping all but 2 opposing sides into a layer with 0 density delfship is giving the remaining 2 sides of the keel at half-thickness a weight of 0.167t, which is pretty close. That’s within 0.6% & I probably didn’t keep the thickness quite uniform where it curves upward into the stem.

      This seems to be the easiest solution, although it means adding a second layer with 0 density for everything.

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