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Re: [AtkinBoats] Re: Matty with leeboards?

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  • jkohnen@boat-links.com
    The fold should be a little below the waterline, and parallel to it. The cutout is just of the _underwater_ portion of the profile. The purpose of the fold is
    Message 1 of 4 , Mar 16, 2004
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      The fold should be a little below the waterline, and parallel to it. The
      cutout is just of the _underwater_ portion of the profile. The purpose of
      the fold is to stiffen the cutout and make a handy place to put the point of
      the pin, in the gutter. I should have said a _big_ pin. I've only tried the
      process on small cutouts from lines reproduced in books, and I used a rather
      large safety pin those times. While doing some research for this reply I
      found designers saying to use one point of a set of drafting dividers or a
      compass, which makes sense. I thought for sure I had a drawing somewhere of
      the process, but couldn't find it... The cutouts should show the centerboard
      or the leeboard lowered.

      Here's what Howard Chapelle had to say, in Yacht Designing and Planning. The
      multiple folds he mentions are a good idea with large cutouts, a single fold
      works fine for small ones:

      "Finding Center of Lateral Plane

      "The center of lateral plane may be found first. The quickest and easiest,
      as well as most accurate, method of finding this center is to trace the
      underbody profile, omitting the rudder, on a scrap of tracing paper; cutting
      out the profile thus obtained with the scissors and then folding it
      lengthwise a number of times, giving it a pleated effect. This will stiffen
      it so that it will not bend when supported in the middle. Make sure that the
      ends do not droop; fold the figure so that the ends are stiff. Then open
      the dividers and stick one leg in the drawing-board; bring the other leg up
      until it is vertical and balance the folded tracing on it, shifting the
      tracing on the point of the leg until it rests in a level position. This
      point can be marked by merely pushing it down on the divider point, making a
      small hole or series of holes in the tracing. Unfold the tracing, lay it
      over the sketch so that it coincides with the underbody profile and prick
      the point of balance into the sketch. It will help somewhat if the underbody
      profile and tracing are marked with coinciding vertical lines, say at the
      position of the mid-section; in this way the point may be either directly
      transferred by pricking or scaled off from the tracing to the sketch.

      "Some designers include one-third or even the whole area of the rudder in
      the lateral plane; there is much divergence of opinion in this matter. It
      seems best to omit the rudder in most hull forms, however. In catboats
      having large outboard rudders of the "barndoor" variety it is practice to
      include the forward third of the rudder in the lateral plane because of its
      proportionately great area compared to the lateral plane of the hull.

      "This method of finding the center of lateral plane may appear less
      scientific than the method usually recommended in textbooks; measuring
      ordinates and their moments from a chosen point. The latter method is
      uselessly laborious and unless the ordinates are closely spaced and located
      with judgment the re- sult will be erroneous. The ordinates must be located
      so that moments can be taken at such points as the heel of the sternpost and
      at the angle of the forefoot to properly calculate the center of lateral
      plane by this method. In view of the fact that the center of lateral plane
      is only an approximation, practically speaking, it seems best to find it in
      the quickest and least troublesome manner. It is hardly necessary to pursue
      this subject further here; those desiring to employ mathematics are referred
      to the standard text- books of naval architecture. It will be observed that
      only the longitudinal position of the center of lateral plane need be found
      by either balancing the underbody profile or calculation; the vertical
      position is of no importance whatsoever."

      On Mon, 15 Mar 2004 00:18:35 -0000, Steve wrote:
      > ...
      > then you said:
      > put a fold in it parallel to the waterline,
      > (do you mean ON the waterline directly, or at
      > some point above or below it?)
      > then you said:
      > then balance it on a pin. (OK, on what kind of pin will a
      > folded piece of paper balance? Does the paper rest on the pin
      > head in the "gutter" of the fold, or on one of the sides
      > above or below the fold? Or should I just do it and
      > figure it out from the doing...like I did the boat I built?)
      > You also said:
      > Make drawings with your leeboard instead of the centerboard
      > and do the same thing. (where do I make the fold in this
      > drawing? Along the waterline when the leeboard is in the down
      > position? And do I also fold this drawing? See what a
      > complete neophyte you're dealing with here?)
      > ...

      John <jkohnen@...>
      All the troubles of man come from his not knowing how to sit still.
      <Blaise Pascal>
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