The lines drawing

Discussion in 'Boat Design' started by elcapitan, Mar 20, 2007.

  1. elcapitan
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    elcapitan Junior Member

    Im attempting my first serious design (im a fifteen year old high school student) and want to know exactly how im supposed to show the curvature of the hull.
     
  2. duluthboats
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    duluthboats Senior Dreamer

  3. alan white
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    alan white Senior Member

    It's hard to do without drafting equipment such as a collection of bendy strips of wood and weights with downward-pointing hooks to hold the strips down so they stay in place right where you want to follow the curves you're drawing.
    You could also build a model or half a model (called a half-hull). It all depends on who you are showing the design to, and why.
    Boat designers use simple techniques to establish points measured from a straight line alongside the drawing of the body of the boat. The straight line might be marked every inch along its length, for instance, which would represent 12 inches on the real boat.
    Visit your library and see if there is a book that explains how the lines drawing is done on a single piece of paper and how that drawing produces a table of offsets, or measurements from three seperate baselines (one next to each view).
    The boatbuilder doesn't need the drawing so much as the table of offsets. From the tables alone, a full-scale drawing just like the original can be drawn on a big surface like a barn floor, in a process called lofting.
    Most traditional boatbuilding books such as were written by Chapelle explain this all in detail. Then you'll need a drafting table (or square-sided table), some drafting supplies, some little bendy sticks (found at craft/model-making stores, and those weights I mentioned, which are called ducks, which are better made by you than bought (or you could nail tiny brads through into the drafting table, which could be no more than a piece of 2 by 4 foot 1/4" birch plywood (available off the shelf at Home Depot).
    Have fun. If you can produce a good table of offsets, you can design a boat. It may not be a great design, but send the offsets to a hundred good builders, and they will all produce an identical hull.

    Alan
     
  4. Tim B
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    Tim B Senior Member

    There are two methods (but lots of ways to do either)...

    Consider the hull to be a single surface, symmetrical about it's centreline.

    1) define the surface by a series of points (using offsets)
    2) using a mathematical function on which discrete points exist.

    It used to be the case that lines were drawn (by hand) offset from the centreline, keel and midship, and these lines then give the shape of the boat (method 1). However, it is now more common to use a mathematical function (usually a spline) to create a surface in mathematical space (method 2).

    The simplest form of hull defined by a mathematical function is the Wigley Hull which is simply a parabola fore and aft, and a parabolic section at the midship.

    Using the second method is often found to be most convenient (with modern computers etc.) as all the required volumetric properties of the boat can be found very simply and accurately.

    Lines-plans are still used to check that the surface is smooth (if it joins other surfaces), and to get an idea of the boat's shape. However, the lines plan is now an output from the design process, rather than an input.

    Go and question the maths department about surfaces if you want more detail on the modern methods. There is plenty of info (and a few examples) of both old and new methods on this forum.

    Tim B.
     
  5. elcapitan
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    elcapitan Junior Member

    what mathematical formulas can be used to create the table of offsets?
     
  6. CGN
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    CGN Senior Member

    elcapitan, have a look at this information, it has good references, and shows a lines plan and what should they include, now if you have no idea of where to start at all, look for "How to Design a Boat" by John Teale.

    http://www.usna.edu/naoe/courses/en200/ch02.pdf

    cheers
     
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  7. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Try to imagine a small boat, draw it in plan (seen from the top), then in profile (from the side) and the body view (front/back).
    You can do this with a pencil on a normal paper.
    Now, if you sketch a station (frame) in the body view and decide where that station is located longitudinally, then you have one point for each waterline you want in the plan view. And the other way around, if you sketch a waterline in the plan view, then you have one point for each station you want to draw in the body view...
    I agree that John Teals book is good, and easy to read.
    In the example below a flat bottomed boat is drawn in plan and profile. Then you have all you need to draw a station wherever you want, for example the transom.
    A lines plan for a round bottom boat or one with many chines can be made in a simmilar way, but with more work.
     

    Attached Files:

  8. Tim B
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    Tim B Senior Member

    I think (couldn't find the book so I've derived it) the Wigley Hull is defined as follows:

    y = B*(1-(x/(2L))^2) * ((T-z)/T)^2

    where B is beam, T is draft, L is the length. And the hull is defined with limits of +B to -B in y, L/2 to -L/2 in x and 0 to -T in Z.

    Virtually any function can be used to define a hull. The simplest form are Quadratic Splines and Surfaces (see the equation above that's a simplified version).

    Of course, modern 3D CAD packages will do all this without you needing to understand the maths. You can tell a CAD drawing done by someone who understands what's going on though!

    Tim B.
     
  9. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think it's important to start with pencil on paper first, then try the cad and "yacht design programs".
     
  10. Tim B
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    Tim B Senior Member

    I think it's understanding what the program is doing that is really important. Yes, you need to be able to present a drawing, but as I pointed out earlier, there is a change of thinking between defining the hull-shape by the lines and defining the lines by the hull-shape.

    Tim B.
     
  11. alan white
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    alan white Senior Member

    In the end, there are practical considerations. This is supposed to be a learning experience, and not necessarily to the end of efficiently producing a set of plans with the least effort. I particularly enjoy pencil-on-paper from an aesthetic perspective. It's an organic thing. Completely personal and subject to abilities and inclinations.
    If you have access to a CAD program, which is possible, then learning to use it will take some time. You will end up with two skills; using a CAD program and designing a boat with it.
    The question is, what is your priority? Slocum's Spray (which was the first to circumnavigate the globe single-handed) is still considered by many to be one of the best cruising sailboat designs ever conceived, and yet that particular design was first built in the 1700s. Slocum's boat may well have been originally built by eye for all we know.
    One can enter into a realm of knowledge by various means, and it shouldn't be supposed that there is a best way to do so. You might use a CAD program to design a reasonably good boat because such a program measures instantly every "what if?" against known variables. Those known variables are still basic rules of thumb as they always were, and are based on both laws of physics and the combined experience of thousands of past sailors and builders.
    Or you might prefer another means to arrive at your objective, such as modelling or hand-drawing, or a combination of several disciplines. I've done patent drawings for a client without any CAD program because I prefer the old style of drafting. However, I cannot (in general) compete with good CAD users in patent drawing because my way is far slower.
    I also enjoy using spokeshaves and planes rather than much faster methods when I work on wood boats. I like the sound and the feel, just as i like the sound of a pencil on paper and the feel of a real table in front of me.
    My advice to any youngster is to always consider what particular method suits your personal likes and dislikes. The best way is the one you are the most motivated to carry out. The worst way is the one that isn't pleasant enough to justify the intended result.

    Alan
     
  12. elcapitan
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    elcapitan Junior Member

    I love doing manual drafting because I can take the visualation in my head and produce it onto the paper more accurately than with cad. When working in cad its alot harder to get the right line shape and look than it is by manually drawing. The only downside is manual drawing takes A LOT longer. Ill post some pictures later today of the boat that I have been working on by hand. I am currently taking an architecture and engineering course at my school (2 periods back to back) and "attempt" to draw in auto cad, but i find myself veering back towards the more traditional methods of manual drawing. Does anyone know of a software that is great for 3d hull design or boat design in general, or have any tips on manual drafting methods?
     
  13. alan white
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    alan white Senior Member

    I don't know what equipment you possess, but insofar as manual drafting goes, a drafting machine is really easy to afford these days since virtually everyone has switched to CAD. I got my brand new K&E through Ebay for a fraction of the cost of a new one. The table should be equally cheap to find.
    Beyond those items, drafting ducks and some assorted bendable battens (hobby shops have a great assortment of plastic and wood sticks of every length to at least 36", or you can make your own on a table saw out of whatever wood that is straight-grained.
    A standard drafting tool kit is obviously needed, including pencils, pens, triangles and french curve templates.
    A good light is essential, and a good chair.
    Drafting supplies can be bought through Charrette online, or other suppliers.

    A.
     

  14. Tim B
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    Tim B Senior Member

    AutoCAD is not the easiest CAD package to use. QCad is better for 2D work. For 3D work use the Rhino evaluation or one of the free/low-cost 3D Cad programs around.

    You'll find if you become an engineer that most drawings are sketched by hand and then drawn fully on computer.

    With 3D CAD you still have to visualise what you're drawing, but it's a different skill from visualising a 2D object.

    Tim B.
     
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