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

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-- John

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