Creating a body plans offset table

Discussion in 'Boat Design' started by klowe34, Oct 21, 2012.

  1. klowe34
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    klowe34 Junior Member

    I'm a sophomore in Naval Architecture and Marine Engineering. We were given a project about a month ago that involves designing a marine vessel (a boat) with a given set of parameters. The rest was up to the student.

    I used an excel spread sheet to record the length between perpendiculars (140m) the beam (37.37m) the depth (12.45m) the draft (9.95m) and the displacement (1003.12 LT). From this information I am to set up a table of offsets.

    This table of offsets (broken up into 10 stations) includes heights of keels above baseline, half-breadths (second row) and heights (third row) of chine from centerline and baseline and profile of forecastle, heights from baseline (there are only stations 0-3 for this one?). There is one more chart in the body plan offset: it has Sta. WL in the far left column then followed by 1, 2, 3, 4, Deck, Foscal in columns 2-7 respectively

    I have no knowledge of how to complete this table of offsets with the information I am given. I know I need a ruler, but other than that I am totally lost. A methodical answer is what I am looking for.
     
  2. gonzo
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    gonzo Senior Member

    The table of offsets is the last step in the design process. It is the set of coordinates from several points that will later be lofted to build the vessel. What kind of vessel is it-sailing, barge, high speed powerboat, etc.?
     
  3. klowe34
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    klowe34 Junior Member

    How do I make lines drawing without a table of offsets? I was under the impression that I need to complete a table of offsets in order to even begin my lines drawing. The lines drawing is the ultimate goal of the project, with the profile plan, the half breadth plan and the body plan all shown on one piece of paper using AutoCAD. The vessel is an offshore supply vessel, so I think that would fall under the category of barge?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    The key thing is the target displacement.

    Thus draw lines that "look" ok..or similar to other vessels of this type. Then do a hydrostatic check at the given draft of 9.95m..is it the same displacement or roughly there abouts? If so....job done. You're not after production faired lines. If the hydro's don't match, jiggle to suit until it does.

    That is basically it.
     
  5. klowe34
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    klowe34 Junior Member

    So, is there no way to form the body plan, or at least a rough start of it from completing a table of off sets?
    We have not learned how to perform a hydrostatic check yet. We perform the hydrostatic analysis after completing and turning in our body plan and table of off sets.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    I appreciate what it is you're trying to do. But it seems strange that there is no hydro check...since this is what drives the hull design.

    Thus, if you have "some" offsets, plot them, as they are. Then fill in the gaps manually...see what you end up with. There will be some shape up fwd and aft, which may or may not be given, so try and find similar hull forms and "gap fill" using these data.

    The objective is to produce a shape that satisfies the requirements of its draft with a given displacement. If you focus too much on the offsets or the table, you're missing the bigger picture. A boat floats because of its displaced volume. ERGO, you need to know what that volume is, otherwise why not draw a box shape hull...is does the same job? See what I mean?
     
  7. messabout
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    messabout Senior Member

    Figure out what displacement you want or need. Do some preliminary arithmetic to guess what area the center section needs to have. Draw the center section of the boat tentatively. Use the center section dimensions in plan and elevation and draw the longitudinal lines. This will be the first draft only. After the lines are down, you can lay off the station positions and extract the offset dimensions from the line/section intersections. You will have laid out a tentative waterline. Construct a set of section drawings from the offset tables. Now you can do some number jiggleing to find the areas of the immersed sections. Now you can calculate the various coefficients and probably find that they are not exactly what you had in mind so you can start over. Keep doing that until you get it right or what you believe to be right.

    If your hull is to have some roundness you mght need to include some diagonals in your lines drawings along with buttock lines and waterlines. Your version of Autocad may or may not have all those capabilities. I'm sufficiently hardnosed and old fashioned to believe that you will learn a lot more by using splines and spline weights along with a pencil and several erasers.

    I strongly suggest that you find a copy of Skenes: Elements of Yacht Design. That'll walk you through the whole process. There are numerous other bolts and nuts books that do the same thing. Your library at Ann Arbor will be a gold mine of information on this subject. Get moving in that direction pronto.
     
  8. klowe34
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    klowe34 Junior Member

    messabout:

    The hull is in fact to have some roundness. We are to include buttocks lines, water lines and chine in the lines drawing. Our professor spoke to us about using splines and spline weights, we even did a tutorial in class, but we're to use autoCAD (2013, so it does have those capabilities you spoke of) to complete the body plan.

    When you mentioned the coefficient, I said in my first post that I used an excel spread sheet. This spread sheet, after putting in the length, beam, draft, foscl, depth, required speed, and cargo deck area, calculate our block coefficient, prismatic coefficient, and calculated GM amongst other things.

    Fortunately, I do have an online copy of Skenes book.
     

  9. klowe34
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    klowe34 Junior Member

    There is a hydro check, its just after the lines drawing is completed. The goal of the class is take all the step to designing a boat, just very very slowly. We start with the design sheet to get or parameters, then the lines drawing and table of offsets, then in groups, we come together and pick the best design and perform a hydro check on the boat
     
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