How would I calculate and draw Displacement Curves?

Discussion in 'Stability' started by Howlandwoodworks, Dec 28, 2020.

  1. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    I have been looking for information on Hull, Deadwood, and Lead Displacement Curves for line drawing for some time now without success.
    What are they used for?
    How to calculate them?
    It's the girth that is confusing.
    I have my suspicions that they are only a % of the calculated girth for center lines of Disp. (so they will fit on the drawing) Sometimes they are labeled as 1/2 D. Curve and there can also be a an accompanying LWL, Lead and or Steel Curves.

  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Girth and displacement are not directly connected. Two separate calculations.

    By "Lead Displacement Curves" do you mean the displacement/weight of lead ballast, or lead ("leed") of center of lateral resistance ahead of center of sail area?

    Are you designing a boat or analyzing an existing set of lines?

    Working on paper, CAD with lines/curves or CAD with surfaces/volumes?
  3. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    First I would like to thanks for your interest and comments.

    I should have used the term width not girth. Yes they are two different things. Width is only a single axis, Displacement is 3 Dimensional object “moving through the space time continuum or at rest.” The Displacement Curve in a Line Drawing are a 2 Dimensional objects representation of a 3 Dimensional object at rest. That would be a posteriori judgement. But the width still doesn't add up for me. Is there a ratio or percentage for the width in a displacement curve on Line Drawings?

    I meant Displacement/weight.

    Both, I have been studying and designing. I have no problem with using an Inspired by credit for a design as long as it has less than three individuals in the credits. The Copyrights Terms Act and the Sonny Bono and Mickey Mouse Terms Extension Act are a must read for anyone designing anything these days.

    Working on vellum with the 3D software program that I am running on my Neck Top Computer and Excel spreadsheets as an assistant. I am on my 4th version of line drawings and offsets now. Having done line drawings w/ Offsets for each version and and upgraded Construction Plans Specifications, Spreadsheets, Construction Section line drawings in 1:4 scale, etc... with each version.
    I am just a beginner but after this winter season. I will cut a large white oaks here on the property for ribs floors and deadwood, also have been collecting long clear western red cedar and Osage Orange lumber for a while but if I can get some good honduran mahogany I would prefer that. The mahogany would add some width to my displacement curve over cedar.

    Well, I can hear some wood calling my name now and rattled on long enough.
    Second Draft
    Last edited: Dec 29, 2020
  4. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    For curves of displacement, the x axis is length (ft), and the y axis is the area of the section (in ft^2) at that at ordinate. By finding the area (x times y) under the displacement curve, what you are actually calculating is a volume (i.e. ft times ft^2 is ft^3). Additionally, the curve of displacement lends itself to Simpsons Rule, which also makes it easy to calculate LCB and other factors. The issue with this is that they are called "curves" of displacement, meaning for each draft and/or trim you will need a new curve. For this reason, Bonjean curves are more typically used during the old pen & ink design process once the lines are finished.

    If you are talking about a lead "displacement" curve, it is the same as the curve of displacement above, except just the lead. Again, this curve is useful to calculate the volume and therefore the weight (and center) of the lead casting that conforms to the drawn lines or shape. However, there is often another "ballast curve" for use during detailed design, usually on larger vessels. This curve is more difficult to construct as it shows the center and weight of the possible ballast over the length of the vessel. The x axis shows any required center of ballast with the weight (or volume) shown on the y axis. This curve is very necessary when working up arrangements and the weight report, if the center and weight of required ballast does not fall under this curve, you do not have a viable vessel.

    As to how to draw the curves, what texts (if any) are you using for you design? There are many texts, such as PNA, BST, Skene's, half of Chapelle's works, etc. that cover this in detail better than I could in a text forum. If you need me to point you to one I can.
  5. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    Actual your explanation are more than adequate.
    I have always valued your post. Have you ever thought about publishing a book and
    I am working out of Skene’s mostly but have been using the web/global brain more than textbooks lately.
    Thank again,
    Last edited: Dec 30, 2020

  6. Howlandwoodworks
    Joined: Sep 2018
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    Location: USA MO

    Howlandwoodworks Member

    I woke this morning with that "DOH" moment. Displacement Curve are elementary, just making it hard.
    "Never Mind" (Emily Litella/Gilda Radner) LOL
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