Request your input on my design

Discussion in 'Boat Design' started by richardf, Jul 15, 2013.

  1. HakimKlunker
    Joined: Aug 2009
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    HakimKlunker Andreas der Juengere

    Wired brains are ok as long as the fuse is not blown ;)
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    That is a sailing dinghy ? I was thinking more of planing powerboats, certainly if not needing to go directly upwind there is less of an issue.
     
  3. Wayne Grabow
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    Wayne Grabow Senior Member

    The paper by Rabl, conveniently posted by JSL in post #94, is a great article. Imagine having people refer to your article over fifty years later! Quite a compliment. However, he was limited by the absence of computer power, and graphic solutions have their constraints. You needed accurate drafting skills; you are trying to represent a 3-D object in a 2-D medium; and unless you have a very large piece of paper, the result is subject to scaling effects.

    Still a well-written article by Rabl. The diagrams are important, even essential, to understanding what is being said. I have not done a very good job of providing examples to clarify what I am proposing. I am going to be busy for the next few days, but will try to then organize some sample calculations to clarify the simplicity (and some limitations) of a mathematical approach. I only design and build a boat every few years, so a simple calculator suffices, but I am sure that a program could be written to streamline the sequence of steps and calculations involved.

    DCockney asked for the limitations of my method; the main one is my use of a simple pocket calculator. It forces me to keep the computations as easy as possible. Choosing certain combinations of numbers creates a harmony which results in exact dimensions and quick computations. I haven't designed a boat with more than two chines per side; it could be done, but the complexity level increases. I am always thinking about the effective bending radius of the materials involved. But the method will handle multi-conic surfaces, sharp curves (if desired), and has no scaling (you compute in abstract, full-size space) and no accuracy limitations (compute to as many significant digits as you desire).

    Leo, thank you for your comment. It means a lot to me coming from an authority like yourself.
     
  4. JSL
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    JSL Senior Member

    One method to help get accurate offsets & fairing (if you use the old fashioned way-manually) is to foreshorten the 'Lines'. IE: on the length (X axis) you could use 1:24 and on the width/height ( Y, Z) use 1:12. or for more accurate 1:8, 1:4
    The big plus is the curves are 'exaggerated', a big help in fairing.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That's very nice of you to say, Wayne, but I'm not qualified to judge how good
    the boats are as actual conveyances.
    However, I am certainly an authority on nut bowls and finger-food dishes!
     
  6. Wayne Grabow
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    Wayne Grabow Senior Member

    Hey, I didn't say these are great boats; I only said that they are "boat-like shapes". Just a tool; not artistry. But, last night one of my bowls sold for $150 at a charity auction.
     
  7. HakimKlunker
    Joined: Aug 2009
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    HakimKlunker Andreas der Juengere

    Yes. But it moves in water same as the power boat does. I rather had the general idea in mind.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Point I was making is sailboats don't have to punch straight into chop, and most don't plane to any great degree. The typical planing hull of developable shape rides not as well as a similar proportioned boat with straight vee sections, imo.
     
  9. C-mack
    Joined: Jan 2010
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    C-mack Boat Dreamer

    Did you start this build? What about a bare hull like the Carolina skiff 24' or 27' dlx... then just use your time in building the cabin.... put on a 25hp bigfoot mercury and maybe a kicker. electric start/ gen
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Most sailboats that can plane, do so on a different portion of the hull than a powerboat does. It's not so much a function of developed versus undeveloped panels, as it's the shapes selected and where they are located.

    With cylindrical or conically developed surfaces, you naturally have to accept some limitations, but if you're creative, you don't have to accept a performance knock. The same would be true of comfort underway, but the design decisions along the spiral, would force you to make some hard judgments about where the design is going.

    Speed, as Mr. Efficiency points out, has a substantial part to play. Most sailboats, up on plane, are really in semi plane mode. Those few that can actually get into the 2.5 to 3 S/L range, still aren't moving very fast. Take a 15' LWL sailboat screaming along at 3.0 S/L, doing what 12 - 13 MPH (19 - 21 KPH). This isn't a lot of punishment, but a 15' LWL powerboat at a modest 5.0 S/L is cooking along at 22 MPH (35 KPH) and it wouldn't be unreasonable to see this same powerboat at 7 S/L, which is 31 MPH (nearly 50 KPH). These speeds, though still relatively modest, can cause considerable discomfort.
     

  11. Wayne Grabow
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    Wayne Grabow Senior Member

    Developable design using simple math, part 2

    Sample computations for Developable Design

    To illustrate the generation of many exact points along the chine curve, the following is a list of some coordinates of the chine at 7” intervals for the anterior chine of the hull I am now building: (0, 0, 24.48), 7, 3.2, 22.56), (14, 6.3, 20.7), (21 ,9.2, 18.96), (28, 11.9, 17.34), (35, 14.4, 15.84), (42, 16.7, 14.46) ,.…. until (119, 28.8, 7.2). This series is generated from the parabolic curve Y = 28.8- (119- X)squared / 490 and also Z = 0.6Y + 7.2 and is valid for values of X between 7 and 119.

    Where did these equations come from? I want a boat about 6’ by 18’, so I picked a length of about half that which I could evenly divide into 16 segments [more segments = more accuracy]; 16x7 or 112” will suffice, next add a short straight 7” segment onto that. Have you ever noticed that when you bend a batten, the curve does not extend all the way to the end of the board? At the batten end there is no fulcrum to apply torque. Thus, last few inches have no bend unless confined along its entire length. So I always add a straight segment at the end of every curve. A 6’ beam gives a 36” half beam; minus some width for flare of the topsides (4”) and an allowance for a chine flat (3.2” at maximum point) and you end up with 28.8” chine beam which is 25.6” of camber in the 112” length and 3.2” of offset along the 7” straight end segment.

    To create the forward keel projection, I most frequently use a parallel projection. A conic projection, with the apex of the cone forward, will create sharp curvatures in that area. It may be what you want, but it will also be hard to plank. A conic apex amidships will give a mild curve at the keel, if that is what is desired. The parallel projection has a controllable curvature and is easy to calculate. We already have defined a 28.8” chine beam, and picked a 7.2” deadrise to go with it [based on estimated displacement]. So deciding on the slope for this parallel projection from chine to keel only involves selecting an X intercept from the maximum chine point (119, 28.8, 7.2). The further forward we select this point, the more pronounced will be the forefoot of the keel with sharper curvature. I chose a point 52.5” forward of maximum chine beam. 52.5/7.5 = 7; 28.8/7.5 = 3.84; 7.2/7.5 = 0.96; thus we have our slope, X: Y: Z=7: 3.84: 0.96 which coincides with the 7” segment intervals.

    Now all anterior keel intercepts can be calculated to define the keel. At the keel, Y=0, and we solve for X and Z. X@keel= X@chine- (7/3.84)Y@chine and Z@keel= Z@chine– (0.96/3.84)Y@chine. Aft of the point X=119”, the keel is straight and Y=0 & Z=0 for the keel. After finding the shape of the keel, we next move on to the shape of the transverse frames below the chine. To calculate transverse frame shapes below the chine, the same slope or ratio is used. We set X = 14, 28, 42, etc., or whatever other frame locations desired, and solve for Y and Z. For every 7” forward we project a chine coordinate, the Y dimension will decrease by 3.84” and the Z dimension will decrease by 0.96”. Simple math creates the entire shape of each frame. Just connect the dots.

    Creating topsides with some flare to the bow, transitioning into a tumblehome stern, seems to be best accomplished with a conic projection forward, linked to another conic apex further aft to create the transition, then a parallel projection extending aft to finish the tumblehome contour. Conic projections involve finding a third point on a line give two defined points. Rather than discuss an entire design, I will list selected apices and show sample calculations.

    The apex of the first cone was selected at X, Y, Z = (56, 39, 70.2). This will give a bow angle which matches the forefoot of the keel and provide moderate flare to the topsides not to exceed a 6’ total beam. A sample calculation would be to calculate the Y and Z intercepts at the frame location X = 98 for a line between the apex and the chine coordinates (119, 31.9, 7.2). The calculation is Y = 39– (98- 56) (39– 31.9)/ (119– 56) = 34.27 and Z = 70.2– (98– 56) (70.2– 7.2)/ (119- 56)= 28.2. The relation is that the change in any one coordinate of a point on a line is proportional to the change in any other coordinate. Since we choose our X intervals, we can then find Y and Z.

    When calculations are complete for X between 0 and 126, we select a second apex (91, 35.5, 38.7) which lies on the ruling line, halfway between the first apex and the point of maximum chine beam (to the outside of the chine flat) which is (126, 32, 7.2). This new apex is then used to calculate points aft to the chine location (189, 32, 7.2). From there a parallel projection is used with the slope 7: 0.25: 2.25 and defined points every 7” along an extended chine equation to X= 294. Although the actual chine ends at X= 213.5, the extended portion will determine the shape of the tumblehome at the stern when projected forward. As more curvature is included in this extended curve, the tumblehome will also increase.

    The entire shape is not yet designed, but sequence and type of calculations needed should be understandable. The results, when finished, are full-size measurements in three dimensions with fair curves and excellent accuracy using simple tools. Enough offsets are generated that all you have to do is connect the conveniently-spaced dots.
     
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