Lines plan from measurements

Discussion in 'Boat Design' started by Rom, Nov 2, 2020.

  1. Rom
    Joined: Oct 2020
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    Rom Junior Member

    Hi All,

    Looking at drawing up a lines plan of a 44' vessel from external measurements. In theory I have an idea how to do this but has anyone done it successfully? How many stations, waterlines, etc... do you use to get something as accurate as possible. Also were the measurements taken using simple, plumb-line and ruler/tape measurer or something more precise?

    Thanks,
    Rom
     
  2. TANSL
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    TANSL Senior Member

    Welcome to the forum.
    This has been done countless times, but success depends on how rigorous you are in taking the measures. Define as best you can the longitudinal profile and the deck line and I would say that with 7 cross sections, equally spaced, it would be sufficient, additionally, if the bow or stern are complicated, take some intermediate section in this area. If the boat has chine it would be advisable to also take the necessary points to define it.
    Very important: materialize the best possible reference planes to take measurements.
     
  3. Steve W
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    Steve W Senior Member

    I have taken the lines off a 40ft wood motorsailer. A customer had bought the boat out of storage and at some point all the ballast in the form of internal lead ingots had been stolen and the only way to figure out how much we needed was to measure the volume below the waterline. I only measured from the waterline down. I measured stations, waterlines and buttocks. I had the boat set up in a cradle and leveled to the existing waterline on a concrete floor and used plumb bobs to layout the stations and made up a stand with a laser to mark the stations and waterlines on to the hull, I don't remember how many stations I used as it was probably a decade ago. It did turn out very well as we ended up pouring an external keel of nearly 10000 lbs and the boat floated on its lines.
     
  4. DCockey
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    DCockey Senior Member

  5. Rom
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    Rom Junior Member

    Thank you all for the advise. My only concern was the accuracy of just using a plomb line and tape measurer/ruler. What other tools have you guys used which would provide more accuracy? A laser?
     
  6. Steve W
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    Steve W Senior Member

    Before computers what exactly do you think we used to loft out a boat full sized on the lofting floor. Of course its accurate enough. Iv'e lofted many boats old school, you can work to an accuracy of at least 1/16". We have a digitizer at work (that I would have no idea how to even turn on) that we use to pattern for foam decking that should be great for taking off lines.
     
  7. DCockey
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    DCockey Senior Member

    How accurate do you need the lines need to match the existing hull? What will the lines be used for? A major factor in accuracy is the care and precision of the person taking the measurements.

    I've heard of laser levels, cross levels and 3 line levels being used in various ways. Other methods for measurng boat hulls include using a laser total station, scanning with a laser system and photogrammetry.
     
  8. Rom
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    Rom Junior Member

    Hi All,

    Bit of a delay but going back to this now. I've attached a picture and my proceedure would be:

    1. Level the waterline with the ground and use the lowest part of the vessel as the baseline ( i'd have to guess the water line as best i can if I don't see the vessel in the water).
    2. Split the vessel into stations ... probably 9 of them using the LOA as the total and splitting the sections evenly.
    3. I'll also split into waterlines based on features such as chines, deck, etc...
    4. Starting at midships, I'll drop a plumb line from the widest section of the vessel then measure in at each waterline perpendicular to the plumb line ( This is where I was thinking a laser with an integrated level to get it as perpendicular to the the plumb line as possible, I imagine it being a bit fiddly with a meter) . This would give me a midship section.... great.
    5. Now the tricky part the other stations. Obviously i'm trying to get my offset table using the centre line of the vessel as a reference but this is impractical as all measurements have to be taken from the outside. So you'd want to draw a virtual line perpendicular to the centre line but touching the widest part of the vessel ( see below)
    upload_2021-2-9_16-13-17.png

    How do you do that? Is there a more practical way of doing this if you consider that taking measurements from the centre line is obvioulsy impractical.

    If anyone has any comments it would be greatly appreciated.

    Thanks all
     
  9. Heimfried
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    Heimfried Senior Member

    Dave Gerr is describing a method of "measuring the hull" in his "Propeller Handbook", Appendix A (pages 118 - 124).

    He recommends to build a simple but practical tool from scrap wood and tells how to use it.
    I didn't use this method but it sounds reasonable to me. So if you have acces to this book, look at it.
     
    Last edited: Feb 10, 2021
  10. gonzo
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    gonzo Senior Member

    I like to make a grid on the ground. All your waterlines will be referenced to it. The grid can be built with strings like they do for archeological digs. It doesn't matter what you choose for reference as long as you are consistent.
     
  11. TANSL
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    TANSL Senior Member

    Very good idea (although not original) for an archaeologist but not very practical for the case at hand.
     
  12. fallguy
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    fallguy Senior Member

    CCBAA302-C6F3-401D-A3C2-6A390F4E25D9.png Two guys and a quality self leveling laser would be the only way I'd attempt it. The laser will make a line of height on the boat and then you can use a story stick.

    You still need to measure square off a laser reference, but some version of Gonzo's grid system to get squares and a laser sounds very easy to me.

    But are you splashing? That's not cool. Many of those plans are probably somewhere.

    I don't think you ought to worry about the centerline. It will be inferred. Draw a centerline on the ground on the ends. Square off the line on the ends using 3/4/5 or 6/8/10 or some such. Draw lines, measure at various laser heights. Masking tapes might be needed.

    see finger sketch
     
  13. TANSL
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    TANSL Senior Member

    Quite agree with what @fallguy says. I just want to clarify that the sections do not need to be taken with a certain uniform separation (a grid on the ground is not necessary) but only where it is seen that the forms lose their "continuity" and, it is important, take some measure that allow to draw the longitudinal profile and the sheer line. If you want to be more precise, find out where the maximum breadth occurs and take points from that section.
     
    fallguy likes this.
  14. fallguy
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    fallguy Senior Member

    <~ good point
     

  15. Rom
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    Rom Junior Member

    This makes the most sense. The only issue with the Grid is that it is 2- Dimensional, what I would really need is a grid which can be brought up to each waterline. Boat is already in the water and floats but will of course dry docked for this. So I'm still unsure how to get a reference line perpendicular to the centre line on the widest part of the vessel at the different waterline heights.
     
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