16 October 2009

Lofting - Take 2

Out of ignorance comes innovation. Or at least the chance to re-invent the wheel.

Back at my lofting board, I had banished my cheap plastic spirit level to an out-of-favor position on a lower shelf in my 'work shop'. However, I still hadn't lost my faith in gravity. If my spirit level couldn't handle the simple job of drawing a vertical line, I had an even simpler tool that could.

I found my old fishing tackle box, stored away with the camping gear. I'd had that box since my college days, when I lived on Burden Lake in upstate New York. That lake was home to some mighty feisty Blue Gills and an expert angler, named Dewey, who'd taught me the trick of catching them (make sure your hook is in the water, not the trees.)

With a 1/2 oz. sinker and some fishing line, plus the dim memory of how to tie a fisherman's knot, I 'built' a plumb-bob.

When I'd done that, I hammered a small nail near the top-middle of my lofting board, and hung my new plumb-bob on the nail. Instant vertical line.

Then I used a trick found in every boat building book. Under the fishing line, I made a small mark with my pencil every 2 inches. Then I removed the plumb-bob and used a straight edge to connect the dots. That was my Station 3.

Next, I needed to draw the Load Waterline (L.W.L.). This is where my old High School Geometry came in handy:



If you have a line AB, and draw two arcs (red) AC & BD (AC=BD) and then draw a line (blue) at the points of intersection E & F, then you have a perfect perpendicular.

To draw such arcs, you need a big compass -- either a monster version of the two-legged dividers used in school, or something called a beam compass. I had everything needed to build a beam compass, so that's what I built...


 
Home Made Plumb-bob and Beam Compass
on my adjustable work bench (Helena's ironing board)


All you need is a bit of wood (I got fancy and used oak). Drill a few pencil-sized holes along one end, and bang a nail into the other. Then you can cut off the end of a pencil and press-fit it into any of the holes. That's all there is to it.

So, to draw the L.W.L., here's what I did:



  1. mark on Station 3 where you want the LWL to cross
  2. press the pencil into the #1 hole on the beam compass (shown above). This makes the shortest arc possible.
  3. put the nail of the beam compass on the LWL mark
  4. swing the compass and draw two small arcs that cross the Station 3 line -- one above the LWL mark (A), and one below the LWL mark (B).
  5. move the pencil in the beam compas to the #5 hole -- the one farthest out
  6. put the nail at A and draw two arcs, where you think the LWL line will fall -- one near point E and one near point F
  7. move the nail end to B and draw two more arcs near points E and F
  8. E and F are where the two arcs cross
  9. connect E and F with a straight edge, an you have a perfectly perpendicular L.W.L. line.
Again, easy and very accurate.

Once you have these two perpendiculars, it's a simple matter to draw in the rest of the grid. As they say in High School, that exercise is left to the reader.

And that's how I drew my super accurate, totally level-to-the-face-of-the-earth lofting grids.

Lofting Lessons Learned

Hopefully, readers of this blog will realize by now that this is not a How-To... it's more a How-Not-To. Nevertheless, I feel compelled to pass on some lessons learned. They say you can't teach experience, but...

1. Don't mount your lofting board on the wall. I didn't have enough room on the floor for both the lofting board and the boat, so I thought I needed to mount the lofting board on the wall. But as future posts will illustrate, space is not a real problem. Loft on floor. By the time you start to put your boat together, you won't need the lofting board so much, and you can get it out of the way by leaning it against the wall.

2. Don't worry about making your lofting grid level with the surface of the earth... i.e., perfectly vertical stations, and perfectly horizontal L.W.L. Having a vertical lofting board planted this idea in my head, and I did manage to draw a perfectly level grid using the above methods, but as I later learned, this didn't help me in any way!  The lines do need to be perpendicular to each other, but it doesn't matter a bit if they are level, gravity wise. In retrospect, that effort was a total waste of time.

3. Do use a variation of my method, which is the 'trammel point' method mentioned in many boat building books. Since I didn't know what trammel points were, I had to re-invent the wheel, but you don't need to. After laying your lofting board on the floor:
  1. use fishing line -- stretched twanging-tight between two nails -- to draw a perfectly straight Base Line (connecting the dots under the line, which is more accurate than snapping a chalk line)
  2. use a home-made beam compass to erect your Station Lines
  3. measure up the station lines to plot the L.W.L. and Floor Line
    I personally think a beam compass will help you draw more accurate perpendiculars than a carpenter's square, so I'm happy I didn't spend the money for a square. But, fortunately for my poor readers, I don't have time to prove that assertion...

    The build must go on!

    Next episode: Building the Bones
    I hope you're enjoying "The Unlikely Boat Builder" as much as I enjoy writing it. Some people have asked for a way to be notified automatically when I post new episodes. I've just figured out how to do this, so if you'd like to be notified, please click on the link below. I promise I'll never spam you (and Google will have my head if I do.)

    Thanks for your interest!

    -- John

    6 comments:

    1. It might be easier to look to carpentry for help making perpendicular lines.
      Create a straight baseline, then create a perpendicular line using 3-4-5 rule (Pythagoras). You can then measure off that line, to create your parallel line (stations).

      ReplyDelete
    2. John,

      You're a great writer and I am really enjoying your blog. Good luck with the build!

      Cheers,

      Murray

      ReplyDelete
    3. dear john,
      I am loving your blog.
      I still think this is too complicated. I have seen magnificent boats constructed by eye. All this accuracy is (in my humble opinion) overkill. I think boat building is more art than science and often close enough is good enough. But I am learning heaps.
      Shane

      ReplyDelete
    4. Hi Shane, good to hear from you. A master builder might be able to build something by eye, but I'd probably end up with a big upside-down birdhouse :-)

      Where are you sailing these days? Last I heard, you were up in Alaska. You're the one with the interesting stories, I bet. Why don't you start a blog and share your experiences with us? I'd love to hear what you're doing.

      -- John

      ReplyDelete
    5. It is allways good to hear what other builders do to make it happen after all the pyriamids where built with a plumb bob and there still standing they used a rock and a string
      its nice to see that others think for themselves and come up with different ideas of creativity to acomplish a specific goal
      good luck keep it up
      bob here not a plumber

      ReplyDelete
    6. Wow! I made the blog. Amazing!
      Cool that your new boat will have as history the reuse of Burden Lake bluegill tackle.

      I have been using the tackle box my Dad gave me when I graduated from eighth grade in 1959. It is just more comfortable than those plastic boxes.
      It sits on a newly designed section of the aluminum boat. Well, "designed" is stretching it a bit. What I did was leave it upside down under the hundred year old pine during a huge December ice storm and large limbs redesigned it. I don't think the pine thought about spirit levels and certainly not plumb bobs or squares. But one of the enormous dents raises the floor of that part of the boat about six inches and so I can better reach the tackle in my tacklebox.
      Enjoying all your work and efforts and learning and the fine writing of it. Thanks for the blog.

      ReplyDelete

    I'd love to hear from you. Please comment!

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