Sailboat Modelling

Discussion in 'Boat Design' started by VVV, May 27, 2018.

  1. VVV
    Joined: May 2018
    Posts: 1
    Likes: 0, Points: 1
    Location: US

    VVV New Member

    Hi, all
    New to this site.
    I've been wondering for some time now which comes first - the half-breadth (floor plan), sheer profile or body plan?

    I gave it a shot (on paper and CAD) to see if i'd manage to figure it out after doing some reading but in vain.
    I'd draw the half-breadth, sheet profile and the body plan (at it's widest beam). The next step alludes me - do i continue drawing the rest of the body plan section curves (splines, or tangent curves?). Or is it that i need to go back to the half-breadth and add the rest of the water lines till i reach the keel? I found a book in the library but it doesn't specify how the rest of the body lines drawn (are they a product or a starting point?). Tried looking online as well but nothing specific came up.

    I printed out a copy of the J-class Rainbow boat (as an example) to have a look at it and see if I can figure out my next step. See image below:

    I'd really appreciate it, if anyone of you takes me through, or links to document that does that, the overall process of drawing the half-breadth, sheet and body plan!

    Thank you!

  2. Richard Woods
    Joined: Jun 2006
    Posts: 2,117
    Likes: 128, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

    Drawing a lines plan is a spiral, you go round and round. Throw it away and start again, and again. Drawing by hand is much harder as the hydrostatics need to be calculated by hand at each iteration as well. So you would start by guessing a displacement, Cp, CofB plus the desired LWL, BWL draft etc. Then use those to draw a sectional area curve. Then a midship cross section. The use the SAC to get approx. areas for station 2 and 8, say. draw those and then fair up. The draw in some more stations. Do a hydrostatics calc and modify/repeat as necessary

    But before you draw a lines plan you must have a pretty good idea of the CofG and weight estimate. Otherwise the boat might have the correct displ, but not float level, or float level but not on its marks

    And no point in drawing an accurate linesplan until you have a good idea of the interior layout, the deck layout etc. So surprisingly the lines plan is usually the last thing to draw, not the first

    CAD simplifies it a lot and you can cut out several stages but you still draw the lines near the end of the process

    Richard Woods of Woods Designs
  3. messabout
    Joined: Jan 2006
    Posts: 2,796
    Likes: 142, Points: 63, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    Richard has given you the straight dope about design. The title of the thread is Sailboat Modeling and I am tempted to assume that you want to build model boats. If your intention is to design your own boat then I will not encourage you. In fact if that is your aim then I am outta' here.

    If you only want to copy dimensions so as to build a model then here are some fundamentals of draftsmanship.

    Observe the vertical lines in the side view of the boat. That is actually called the elevation view. Those vertical lines describe the location of the bread slices that you have used to chop the boat up. The bread slices will have a specific location. You can see the numbers on the side view. We will use that bread slice to establish the size and shape of the particular section of the boat that it describes. In the side view you can also see a bunch of evenly spaced horizontal lines. Those lines are also imaginary bread slices. Those horizontal lines are called waterlines even though some of them will not be in the water when the boat is afloat.

    Now look at the top view of the boat. That is technically the plan view if using old time draftsmans parlance. You can see that there are curvy lines labeled WL number whatever. Lets pick a section, say number six....we are back in the top view now. We would like to know how wide and how high that section is and be able to determine the shape of that section. In fact what would that particular bread slice look like? We can measure the is a scale drawing of course.... to see how wide the section is at the deck, we can see how wide it is at each successive waterline on the way down the various waterlines. We are measuring the distance from the centerline of the boat to the edge of the waterline of whatever waterline we are measuring. You have made some dots on a piece of paper that are spaced sideways according to the waterline dimensions. You know how high the waterlines are above a base line. The waterlines will usually be spaced the same vertical distance apart so you know where to put the dots in terms of the height location of the dot. When you have located the dots correctly you can then connect the dots by using a spline or Copenhagen curve template. Now you have drawn half of section six. Keep doing this with all the other sections and overlay them on a drawing like the one you see at the right on the Rainbow drawing. That is the body plan. It is customary to draw about half the section views on the right side of the centerline and the other half on the left side of the centerline of the body plan. It is most essentially useful because it lets you calculate the immersed area of the immersed section so that you can put all the immersed areas into an equation that allows you to determine how much buoyancy the whole boat has. Of course you can calculate those areas when taken at different depths of immersion to get different amounts of buoyancy, or as we might call it; displacement.

    This is just a small part of the whole process. If you understand what I have written (I confess that sometimes I do not understand what I have written) then you have a general idea about the way a boat is built by using the lines on the drawing. The shipwright is not burdened with the task of measuring the drawing. The designer will have constructed a table of offsets that tells the builder what all the dimensions are to be. The shipwright knows how to interpret that table of offsets. It is pretty much as I have tried to describe.

    Don't sweat the lines on the drawings that are labeled Buttocks and Diagonals. That is for the next informational that I may or may not try to do.

    May the Force be with you.
  4. Dolfiman
    Joined: Aug 2017
    Posts: 366
    Likes: 170, Points: 43
    Location: NICE (France)

    Dolfiman Senior Member

    You can try the spreadsheet application that I have developed for that kind of early stage design, Gene-Hull 2,2 version, quote #5 of this thread :
    Gene-Hull : hull generation for sailing yacht early stage design
  5. TANSL
    Joined: Sep 2011
    Posts: 5,108
    Likes: 116, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

  6. tspeer
    Joined: Feb 2002
    Posts: 2,151
    Likes: 141, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I approached the problem backwards. First I defined the hydrostatics I wanted, starting with the displacement and center of buoyancy, then generated the cross sectional area distribution. I then described each section with a few geometric parameters, and varied those parameters along the length using low-order curves so as to create a fair blending of the shape. That gave me a provisional hull shape between the forward and aft perpendiculars. The sheer and the crown of the deck came from the choice of curves for the parameters.

    Next, I calculated the hydrostatics and then scaled each section so it would have the desired cross sectional area distribution. That gave me the final shape between the perpendiculars. Finally, I extrapolated the topsides to the centerplane to determine a stem shape that would be fair with the rest of the hull. Had I wanted overhang at the stern, I could have extrapolated the topsides to a transom plane behind the aft perpendicular.

    You can find more details here: For my first version, I did everything with a spreadsheet. Later, I linked the spreadsheet to a CAD program to generate the geometry.

    I like this inverse parametric approach because the low-order shapes are inherently fair and I know I'm going to end up with the correct hydrostatics. The parameters allow me to express what is important to me in the hull shape in a predictable way. And being parametric, it is easily revised as the top level requirements change.
    Dolfiman likes this.
  7. tspeer
    Joined: Feb 2002
    Posts: 2,151
    Likes: 141, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Bezier curves could have been used, and the conic sections are, of course, a special case of cubic splines. My intent was not to produce a general purpose CAD geometry, but to define the hull with a restricted set of parameters that would not allow unintended inflection points.

    I only used parabolic curves to define the parameters in the longitudinal direction, having values at the ends and midship. However, when the sections were rescaled to match the cross sectional area distribution the parameter variation was no longer parabolic and the shape needed to be defined using NURBS in the CAD model.

    I also had a historical interest. The Mustang fighter of WWII was the first plane to be lofted using conic sections, and part of its performance was attributed to the fairness of its lines. The Mustang was lofted using manual drafting techniques and I wanted to explore the same thing using a spreadsheet. The cross sectional area distribution I picked was a generalization of the Sears-Haack body, which has the minimum wave drag in supersonic flow. While I hardly expected my sailboat to go supersonic, the area distribution was similar to that of the cosine wherry, which does have low drag at displacement speeds. I thought it would be fun to incorporate these two aeronautical elements into the design in a way that was suited to the naval architecture of the boat.
  8. Dolfiman
    Joined: Aug 2017
    Posts: 366
    Likes: 170, Points: 43
    Location: NICE (France)

    Dolfiman Senior Member

    Many thanks Tom for your document that I am reading with great interest (being myself in the development of a similar tool on spreadsheet for early stage design, Gene-Hull) and this complement. My apology for my first and quick comments that I eventually deleted, being not satisfied with my text, I will post a better text asap.

  9. Dolfiman
    Joined: Aug 2017
    Posts: 366
    Likes: 170, Points: 43
    Location: NICE (France)

    Dolfiman Senior Member

    Regarding the LCB, you say in your document that ".... the cross sectional area at its maximum... will be located twice as far aft of midships as is the center of buoyancy "
    That sounds like an a priori ? (an analytical consequence of your equation), or it is an objective, a state of the art rule, that you put in the equation ?
    I think it is not the order as such but the sum of various order with polynomials in the form a + b x + c x^2 +... + p x^n which causes problem regarding fair blending, leading to oscillating lines with numerous max of curvature. On the other hand, high degree function but in the simple form of a + b x^n , or up to the degree^degree form a + b x^(c + d x^n), so avoiding the sum of various degrees, can give you all the curves you want within the rectangular box with only one max , or a max and a min, of curvature (1/R), so a priori compatible with what we called fair blending of hull body. I use mostly such polynomials in Gene-Hull, sometimes with degree n as high as 100 or as low as 0,1, and up to now never be in trouble (cross the fingers ;) )

    There is an interesting story to tell about the creation and the development of the Bézier curves, likely that the conics were also developed independently, then all this was grouped in the Berstein-Bezier curves, leading to the NURBS which are the core of the current CAD tools. Pierre Bezier was a mechanical engineer at car maker Renault, and he faced the problem to programme the first machined tools from plans of the drawing office. It was a production issue at first, not a design one ..... There is a letter of Pierre Bezier in French which tell all that and really worth a translation, I will open a thread in the Software forum when I will have time available for.

    I like this reverse parametric approach, although not easy to automate with a spreadsheet tool : are your iterations fully automatised ?

    My spreadsheet approach with Gene-Hull is more in line with the traditional one with drawing lines. I first defined the keel line (>> Zo(x)) and the sheer line (> Y1(x) and Z1(x) ) with polynomials as here above. In particular, the degree^degree function help draw all kind of bow shape. Secondly, sections Y = f(Z) based on Zo, Y1, Z1 , with same kind of polynomials and with parameters themselves function of x and of dimensionless parameters, through regular polynomials from aft to fore without discontinuities inc. for derivates and second derivates. Doing so, the hull body is both fully mathematical and with fair lines, but of course not automatically a good one from the naval architect point of view. So all the hydrostatics parameters are computed (automatically updated for each input modification), and one can converge step by step, by modifying both geometrical and dimensionless parameters, toward a good hull for desired displacement, LCB, Cp, wetted surface, sections areas curve, etc.... To that basic process, I add the keel and the rudder(s) , and the study of the heeled hull : hydrostatics of the heeled hull is not exactly the real life dynamics but gives additional important and more exact information than the traditional upright hull approach : trim and elevation, wetted surface, righting moment RM, shape and obliquity of the floatation surface. RM in particular is so added to the evaluation process at each iteration. And currently for next versions 2.3 in preparation, I am working on the connection with a standard sailplan ( the two mainsheet and jib triangles) to add the lead (X sail -X lateral resistance) to the board of control parameters, and on the connection with a mass spreadsheet to have the connection with the displacement versus mass and LCB versus LCG.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.