Waterline

Discussion in 'Sailboats' started by Mason123, Sep 19, 2015.

  1. Mason123
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    Mason123 New Member

    Hello Boat builder's/ Naval Architects-

    I'm in need of some help,

    I have a half-model built to scale for a sailboat that my Dad had made, unfortunately he wasn't able to built the sailboat ( time run out ). I urgently need some help or direct me to books or videos that can answer my question.

    Waterline- I'm having some difficulties determining the waterline, I need to established where on the half model the waterline should be set.

    I have already determine the length and beam of the sailboat using 3/4 scale to an 1 inch.

    Any help will be appreciated.

    Mason Dyer
     

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  2. Kailani
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    Kailani Senior Member

    Do you have any drawings and calculations that were done for the sailboat or only the half model?
     
  3. gonzo
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    gonzo Senior Member

    You need to know the displacement and center of gravity. The displacement is the submerged volume. The center of gravity will determine the angle of the waterline.
     
  4. DCockey
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    DCockey Senior Member

    What is you objective? Do you want to put a waterline on the half model or do you want to build a full size boat from the model? If you want to build a full size boat why do you need the "waterline"?

    At a scale of 3/4" to the foot it appears to be a small boat, and the waterline in use will depend on the weight and location of passengers and gear in the boat as well as the weight and center of gravity of the boat itself.
     
  5. Mason123
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    Mason123 New Member

    Unfortunately, I have no drawing and calculations of the sailboat or halfmodle, I just a few notes of the lenght, beam and scale used. My father was one of those boat builder that would build the half model to scale, then shortly after start building the boat.

    Currently my overall goal is not to build the sailboat yet, perhaps somtime in the near future.
    However, your question asked above is what I'm trying to accomplish. I would like to create a line drawing of this half model. For me to do that, I do need the waterline correct?

    Mason
     
  6. Mason123
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    Mason123 New Member

    Hi_

    My current goal, is to create a line drawing of this half model, I do need the waterline to do that correct?
    Now as far as building the boat, I would like to do that sometime in the near future. I'm very new to boat building, so I'm trying to get a complete grasp of this trade. So feel free to correct me if my questions are logical.

    Yes, the boat is a bit small overall length 17 feet - 6 inches.
     
  7. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I think with this data it is impossible to correctly determine the water line but I may be forgetting something so, Gonzo, to explain your method would be really interesting.
    Manson123, although it is a very personal opinion, it seems to me your model much nicer, and probably much more authentic, without water line.
    Unless Gonzo give us the solution, I do not think there is a way to determine the water line, except look at pictures or drawings of old ships, the same type as this. So if you want to draw one, draw it where you like.
     
  8. gonzo
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    gonzo Senior Member

    You don't need a waterline to draw the lines of a hull; or any object for that matter. There are several methods to take off the lines. One is to separate the layers of the model and trace them. Other is to use a lead strip to take the shape and then trace them. There are some apparatus that can be used too, but may be overkill to build one for a single job.
     
  9. DCockey
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    DCockey Senior Member

    Remember that the actual waterline of a boat in the water depends on the total weight of the boat, passengers, gear, etc, and the location of the center of gravity.

    The primary purpose a waterline serves when drawing the lines of an existing boat or model is to establish a baseline with the stations perpendicular to the baseline. Analysis of the boat using traditional methods of calculations using Simpson's rule, etc is simpler if the stations are perpendicular to the nominal waterline.

    But the baseline does not have to perpendicular to the baseline. Many boats in the 18th and 19th century were designed with the baseline parallel to the keel. The disadvantage of using the keel as the baseline is that when building a boat from a traditional set of lines and offsets it is simpler to have the bulkheads, frames and any molds used during construction parallel to the stations. For boats with the keel not parallel to the surface of the water this can result in bulkheads which are not vertical.

    Will you be creating the lines using traditional methods, or will you be creating a 3D virtual surface model? If you will be directing drawing and fairing a traditional set of lines then the choice of baseline is important. If you will be creating a 3D virtual surface model the choice of baseline is not as important. After the 3D model is created it is very simple and very quick to "slice" the model in any orientation to create a traditional set of lines and offsets, or to obtain the shapes of bulkheads, frames and molds.
     
  10. TANSL
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    TANSL Senior Member

    I have to say I was wrong completely. I misunderstood the question in the OP and, therefore, the second paragraph of my post # 7 does not make any sense.
    That said, I must also say that I would like to know the method by which, from the displacement and c. of g. you can get a boat line (even if it is just one line).
    I must also say that not quite understand anything of the paragraph:
    How is the device could be built if it were not a single job ?. Of course I'd like to know it as several times I had to get the forms from an existing boat or a model to scale. Maybe I decide to build it and, perhaps, Mason123 may be interested to know it.
     
  11. WhiteDwarf
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    WhiteDwarf White Dwarf

    The model appears to have only three layers. This will restrict the amount of information which can be captured by taking the model apart. Taking a table of offsets would be practical if the half model could be secured to a marked-up backing board and a saddle utilized to identify the 3D positions. The table of offsets would allow the necessary calculations to begin the iterative process which filling out the design would require.

    The OP says the length is about 17'6" this is quite small for a boat with such a raked stern.

    I have a couple of questions. (1) Is there a centerboard, if so where? (2) Is this light displacement, or heavier construction? There does not appear to be a large freeboard to allow the latter.
     
  12. DCockey
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    DCockey Senior Member

    Based on the photos the three layers are glued together so disassembly would not be an option.

    Mason, re-reading your original post it sounds like your father had the half model built, possibly to a drawing. The model has numbered stations so I would use those when creating the set of lines. If it was built from a drawing then the stations probably line up with stations on the drawing. Also the joints between the layers may be parallel to the waterline/baseline on the drawing, and also from the photos appear to be parallel to the keel.

    Any idea about what the design might be, who built the half-model or when it was built? What is the beam and depth amidships? It has a straight keel, a close to but not quite plumb stem, and a raking transom. Also the forefoot has a corner rather than a smooth transition from stem to keel. There is hollow in both the bow and stern quarters. Those characteristics are shared by many traditional boats but with some more information it might be possible to identify what drawing or source of information was used to create it.

    Back to your original question about the waterline. The boat would have been almost certainly trimmed so that the straight keel was immersed deeper at the transom than the stem. However from the photos it looks like the joints between the layers are parallel to the keel, and the stations on the model are perpendicular to the keel. This is a typical practice used for older boats.
     

  13. DCockey
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    DCockey Senior Member

    Photogrammetry would be an alternative method to "measure" the half-model. The result would be a cloud of points which could be imported into software such as Rhino3D to create a surface model and/or set of lines. A good introduction to Photogrammetry for measuring boats is available at https://vesseldoc.wikispaces.com/

    Or the model could be manually measured. For information about traditional methods for measure a boat or model see Boats: A Manual for Their Documentation which is available as a free PDF download at http://www.museumsmallcraft.org/publications.html
     
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