Apparent Discrepancy in Displacement (Free, Imperial)

DELFTship forum Hydrostatics and stability Apparent Discrepancy in Displacement (Free, Imperial)

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    • #38732
      Grieg Pedersen
      Participant

      I’ve been working on a boat for some time in some FreeShip versions/derivitaves. I started re-creating it in DelftShip and noticed that the displacement was very much less despite the hull being very similar. I ran the report (see parameters screenshot) to show every inch right up to the first leak point. That ensured I wasn’t misreading the units. The thing is, because I ran the range as 1 inch increments (.083 foot), I’d expect the TpCm (ton/inch) column to come very close to simply accumulating in the Displ FW column, and it doesn’t. I end up with a 38′ (11.5m) catamaran with little more than a ton and a half of displacement and nearly 2′ of draft.

      Am I misreading something or is there a problem (possibly with imperial units)?

      Thanks!

    • #38733
      Robert Holme
      Participant

      Where is your model relative to the baseline?
      Maybe the problem if it’s not line up properly.

      Regards
      Maryak

    • #38734
      Grieg Pedersen
      Participant

      The lowest point of the hull is at 0.

    • #38735
      Icare
      Participant

      It’s the first time I hear from such a problem… :dry:

      Is the water line correct?
      Just a glance at the water line level may tell you there’s a problem with it or not. Check for le color change location.
      If the water line is correct, then…

      What’s the model’s volume?
      Since its displacement is 1.5 tons, does the volume fits such a datum?
      In the metric system, a displacement of 1.5 tons means a volume of nearly 1.5 cube meter. Check it broadly in the imperial system.
      If you find an inconsistancy, then…

      Did’t you change the water density datum?
      Broadly, in the metric system, fresh water has a density of 1 Kilogram/liter, ocean water 1.015 and the mediterrannean 1.033 (as far as I remember :whistle: ). Check for your water density, convert it in metric system and make sure you get a consistent number.

      I grew up with the metric system, and trust me: Imperial system %§¤# ! :sick:

    • #38737
      Grieg Pedersen
      Participant

      Look at the attachments. The far right column is displacement for the next inch in tons. The left column is displacement in tons. Look at the parameters. You’ll see that the report is inch by inch, yet the displacements don’t accumulate accordingly. That’s my question: why don’t they?

    • #38738
      Grieg Pedersen
      Participant

      1. The water line is correct.
      2. The report doesn’t show volume, but each of the two hulls is 40′ long and the beam is over 4′. At 1.75′ draft, I’ve got a lot more than the needed 50 cubic feet of volume.
      3. Why would I do that? Broadly, in the English system, “a pint’s a pound the world around” in distilled fresh water (a gallon’s 8# and a cubic foot is 8 gallons) and seawater is about 65# per cubic foot.

      I grew up with the Imperial system, and I have to order parts and materials in it. DelftShip does everything in decimal feet, so what’s the issue?

      As per my original post, it’s an apparent discrepancy ON THE REPORT that I’m asking about, not the volume of my model.

      Look at the attachments. The far right column is displacement for the next inch in tons. The left column is displacement in tons. Look at the parameters. You’ll see that the report is inch by inch, yet the displacements don’t accumulate accordingly. That’s my question: why don’t they?

    • #38740
      Robert Holme
      Participant

      What does the Design Hydrostatics report show?
      Regards
      Maryak

    • #38741
      Grieg Pedersen
      Participant

      It’s very similar to the left column, but that still doesn’t answer my question: why do the leftmost (Displ FW) and rightmost (TpCm) columns on the hydrostatics report not seem to be consistent with each other? With the draft increments set to one inch (.083 ft), I would expect that displacement would fairly closely be an accumulation of the TpCm weight to displace in tons per inch. It’s nowhere close.

    • #38742
      Grieg Pedersen
      Participant

      The really amazing thing is this: when I run the design hydrostatics in Imperial units, I get a CP of .0947 (I can’t imagine how I could make such a thing), but on the same hull, run in metric, the CP is .3731. Now we all know that CP is dimensionsless, so the units of measure shouldn’t matter.

      Here’s the model (slightly modified since initial numbers were given, but still showing discrepancies). Try it yourself.

      Attachments:
    • #38743
      Robert Holme
      Participant

      The design appears to be in unstable equilibrium which may account for all the weird numbers in the hydrostatics report.

      From your fbm
      G= 1.877m
      M= 0.718m
      B= 0.252m

      I don’t know why there is no baseline or perpendiculars in your fbm so z=0 relative to?

      HTH
      Regards
      Maryak

    • #38744
      Grieg Pedersen
      Participant

      The hull is only one hull of a catamaran. I removed the other one to simplify things. One hull or both, the problem persists.

      I don’t add a baseline. It’s just the plane at Z=0.

      Directly from the manual:

      Reference, perpendiculars and baseline
      Positive values for heeling angles are also to port side, so for negative values the ship heels to starboard side.
      Illustration below shows the key reference lines. The most important are:
      ·The aft perpendicular which is located at the origin (x=0.0).
      ·The forward perpendicular. The location is specified by the user in the project settings and must be a a value greater than zero since the aft perpendicular is located at the origin. The length of the ship is defined as the distance between the two perpendiculars.
      ·The base line. The base line corresponds with the horizontal plane through the origin (z=0.0). All molded drafts are measured with respect to this baseline.

      As for the forward perpendicular, I can’t find anywhere in project settings to add that. The longitudinal reference poit is the aft perpendicular. I don’t work with the coordinate axes displayed, and I don’t know if that’s what you think seems to be missing.

    • #38745
      Grieg Pedersen
      Participant

      Let me put forth another discovery.

      1. Run the hydrostatics in Imperial units. Note the displacement as the penultimate line (right before the first leak point). 0.671 long tons.
      2. Change to Metric. Run again. 2.593 metric tons! One long ton differs from one metric ton by only 1.6%. The calculations differ by nearly a factor of four!

      Note that during this process the units do convert, and the draft intervals are fine.

      Conclusion: there is a displacement problem when working in imperial units.

    • #38747
      Marven
      Keymaster

      We have investigated this, and we indeed found a bug in scaling the layer thickness when switching project units. We wll fix this as soon as possible.

      You are using the actual layer thickness in your hydrostatic settings, so as the layer thickness is not scaled correctly your hydrostatic values will change.
      I also noticed that your appendage coefficient is set to a very small value (0.125) and that the layer thickness of the hull is set to 0.80 meters. So you have a very, very thick outer shell (80cm) but then multiply your displacement with a value of 0.125, thus accounting for only 12.5% of the calculated displacement. Generally your appendage coefficient should be close to 1.0

    • #38748
      Grieg Pedersen
      Participant

      Thanks! Just to let you know I’m not crazy 🙂 , when building the model (in imperial units) I set the thickness to a very small number (3/4 inch). I would also add that I never changed the appendage coefficient that I’m aware of, but I’ll be sure to set it back to 1.

      Thanks again!

    • #38764
      Grieg Pedersen
      Participant

      I should also add that the appendage coefficient resolved the displacement discrepancy. Thanks again!

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