Table of offsets

Discussion in 'Boat Design' started by prabs, Jun 14, 2021.

  1. prabs
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    prabs Junior Member

    Do I need to add buttock lines(profile view) and water lines(half breadth view) to generate table of offsets in the scale down drawing.(1/8th)
    Do I have to scale up to obtain table of offsets for the full size lofting.
    For example, assume a 24 foot trimaran hull (round bilge) drawn in 1/8th scale. Assume I have added buttock and waterlines. Can I make the table of offsets from this drawing and multiply 8 times for full size lofting.
    Please explain me where I can find a tutorial of a sample .txt file containing a table of offsets for a canoe to play with.
     
  2. wet feet
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    wet feet Senior Member

    Buttocks are extremely useful where the waterlines intersect the hull surface at very shallow angles.Additionally,you might add diagonals to really define the shape.It may only be a canoe and not a huge ship,but learning the correct way to do things will pay big dividends at some point.You are correct in understanding that the offsets can be derived by using a scale rule to measure the actual drawing-or measuring and multiplying by the scale of the drawing.That way when you come to loft the lines you will have the actual dimensions that you need for building and the process of lofting is a big aid in finding those places where minor errors have crept in.A fair curve is the most important thing to aim for and when all views have fair curves that pass through the same locations you my feel very satisfied and build an accurate boat.
     
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  3. TANSL
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    TANSL Senior Member

    Of course it can be done and, on large ships, it was done with 1/10 scale models. But keep in mind that an error of 1 mm (negligible) in a measurement of your model reduced to 1/8 scale, will represent an error of 8 mm (perhaps not negligible) in the real ship.
     
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  4. bajansailor
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    bajansailor Marine Surveyor

    Prabs, do you know how to draw this lines plan by hand, on paper, rather than on a computer?

    If not, then I would politely suggest that you try doing it this way - I think it would help a lot to understanding what is going on.
     
  5. prabs
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    prabs Junior Member

    Thank you bajansailor. I have been watching tutorials on how to draw body plan view, profile view, and half breadth view in scaled view.

    I have to put it in practice.

    However, the demos I have watched are based on just measurements like length, beam, etc. and nothing from offsets. I think table of offsets makes sense only when lofting full size plans.

    It will not be possible for me to draw the full size plan of a boat in profile view or half breadth view when the length is 40 foot on a huge piece of paper or plywood. How would I obtain the values for those columns in table of offsets??!!!!

    When they sell a boat plan, I think they may not give the table of offsets, rather the full size patterns only.
     
  6. TANSL
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    TANSL Senior Member

    It looks like you can work with Delfyship. If so, or you have any other CAD system, you don't need to draw your plans at 1/8 scale or similar. You can, and should, draw the ship to 1/1 scale and in millimeters, if possible.
    Don't get obsessed with the offsets table. If you work directly with surfaces, you shouldn't need that table at all. It is a matter of creating the surfaces of the hull based on some points, curves, initial surfaces, or calcining them from a paper. For example, for many years, fortunately, I have not used an offset table at all.
     
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  7. bajansailor
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    bajansailor Marine Surveyor

    How far have you got so far with the design of the 24' trimaran hull?
    Can you post any photos of diagrams here showing what you have drawn so far?

    If you have a side elevation view, with the stations (straight vertical lines) and waterlines (straight horizontal lines) and the buttocks (nice smooth curves defining the shape of the hull longitudinally at specified distances from the centreline), then you can take some offsets off this drawing, such as the heights of the buttock lines at each station.

    In similar fashion, with your plan view, you will have the deck plan and the waterlines (all nice curves probably) along with the stations (straight vertical lines) and the buttocks (straight horizontal lines).

    And also with your body plan - here you will have the shape of each section along the length of the boat, and in this view they should be nice curves. The buttocks will be vertical straight lines, and the waterlines will be horizontal straight lines.

    Once you have all three views fair (I am ignoring diagonals for now for simplicity) then you can start to measure the offsets off the drawing.
    If it is a computer print out you could measure the dimensions with a ruler, to check what dimensions the computer might give you.

    And I have just seen TANSL's post above - I would agree that there should be no need for the offsets table, but if you have never drawn a lines plan by hand, or tabulated an offsets table manually, it would be a useful exercise to do, and doing this will no doubt help you to understand how all the various views relate to each other.
     
  8. Howlandwoodworks
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    Howlandwoodworks Member

    prabs,
    I use a chart in a spreadsheet to smooth any high or low spots in the waterlines buttocks and diagonals Tables of Offsets. It can show any high or low spots of +- 1/16 of an inch without any problem. You can smooth out anything less than that with a batten and plane or sander on the molds before planking.
    I would recommend that you do not calculate your hourly wages and just do a dollars per fun ratio.
     

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    Last edited: Jun 15, 2021
  9. TANSL
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    TANSL Senior Member

    Any tool is good if it is used correctly and you are aware of its limitations. If Excel charts work for you, great. But let me doubt that the precision of these graphics is sufficient for a correct smoothing of the shapes of a boat of more than 6 m (to give a figure). That goes totally against the precision that every loftman has always wanted to give to his lines. In the same picture you can see areas that do not seem very smooth. As I say, if they are worth it to you, there is nothing more to say.
    What a correct smoothing attempts is precisely to avoid "... a batten and plane or sander on the molds before planking" which is a much more expensive job than doing it on lines drawn on a board or with a CAD / CAM program.
     
  10. Howlandwoodworks
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    Howlandwoodworks Member

    TANSL,
    I should have written:
    in the Line Drawings of the Waterlines, Buttocks, and Diagonals for the Tables of Offsets.
    I just use the SS to make sure that I am not to far off in the Table of Offsets.
    There are some other errors in the spreadsheets, tables of offsets, and line drawings... that I haven't corrected. Maybe the yard could correct these errors and send them to my office in a Change Order to the original bid.
    Thanks for setting the record straight.

    prabs,
    Make a set of line drawings and table of offsets then do them again and again improving them a little each time.
    Doing the tables of offsets in a Spreadsheet will save you from redoing all of the inputs, like a savable scratch pad, if you are not using a CAD / CAM program.
    I had calculating 3 to 5 years to do a set of planes for a 26.5 Lead Sled. I think I have under estimated the timeline and am asking for an extinction from the client, oh thank goodness that is me.
    Below each page of posts is a "Similar Threads" list.
    Table of offsets measurement https://www.boatdesign.net/threads/table-of-offsets-measurement.61918/
    Problem with Tables of Offsets https://www.boatdesign.net/threads/problem-with-tables-of-offsets.56991/
    It took me a while before I starting using it to its full potential. Well, I hope to use it to its full potential.
    best of luck
     

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  11. TANSL
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    TANSL Senior Member


  12. Howlandwoodworks
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    Howlandwoodworks Member

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    Last edited: Jun 15, 2021
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