This brilliant insight almost tripped me up, of course.
But before I explain myself, let me make up two definitions. There may be words for these processes already, but I don't know what they are. (If you happen to know the correct terminology, please pass them along using the comments form below.)
In this blog, I will use the following definitions:
To Spile a Point: to draw an arc from a point on the form onto a spiling batten
To Unspile a Point: to draw two arcs from an arc on a spiling batten, onto pattern or planking stock, thus recreating the original point.
With those definitions in mind, I can now easily say that you can use any divider settings you want for any point, as long as you use the same setting for spiling and unspiling that one particular point.
For example, I used a pretty wide divider setting up in the bow, where the distances from the points to the spiling batten were several inches, and a smaller setting in the stern, where the distances where much smaller.
To enable me to reset the dividers correctly when unspiling the points, I marked off both divider settings on the batten itself, by marking a starting point on the batten, drawing an arc, and labeling it. Once this setting was recorded on the batten (it could have been recorded anywhere, but the batten was handy), it would always be possible to reset the dividers to the proper size.
Of course, you also need to record which set of points use which settings... which I will definitely do the next time I use this marvelous trick...
Actually, as Bob Easton helped me see, you don't need to use this trick at all. It is always possible to use the same divider setting for all points, which is much less error prone. However, even if you are using the same setting for all points, I still think it's a good idea to record that setting on the batten, just in case...
Resuming our tragic story...
Once the spiling batten is tacked onto the form, the next step is to spile all the points you need onto the batten. This is very easy to do. Just put the pointy end of your dividers onto the point to be spiled, and draw as long an arc as possible. Keep the dividers perpendicular to the spiling batten for maximum accuracy.
By the way, this blog post is a classic UnlikelyBoatBuilder how-not-to guide. Practically everything I show you in this post is a mistake. Builder beware! Read the next post before you run out and try spiling!
In the photo below, I'm spiling from the rabbit line. Notice that I haven't cut the rabbet, yet.
Drawing an arc from rabbet line to spiling batten
Once all the points you need are spiled onto the batten, remove the batten from the forms and lay it flat on your pattern stock.
If you have more nerve than me, you could spile directly onto your planking stock, but my expensive sheet of marine plywood would have been the size of a matchbook by the time I finished learning how to spile. Much cheaper to practice by making patterns. When the pattern fits, then trace it onto your valuable planking stock.
Spiling batten removed from forms and clamped to pattern stock
I then clamped the spiling batten onto the pattern stock. I didn't want the batten shifting around as I unspiled the points.
Once secured, it was pretty simple to unspile all the points. You just pick two points on each arc, and draw two arcs onto the pattern stock. Ideally, the two arcs will cross at right angles, but sometimes that's not possible.
Rediscovering original points on pattern (same point as above)
When all the points are unspiled (isn't that a useful word?!?!), just connect with a batten in the usual way.
This is when I discovered that I had used the wrong divider setting for one set of points. No big deal, though. I just bypassed them. The batten supplied the missing points.
Connecting the spiled point with a batten
Here's another view of the unspiled line of the garboard plank... Looks great, right? Of course, there are two lines to draw.
The same line from another angle
Once the perimeter of the plank has been transfered to the pattern stock, it's time to cut it out. I used my bandsaw to cut out this pattern, but I must say it is not easy to guide a floppy, 8' piece of plywood accurately through my machine. After cutting out this one piece, I was looking for a different way to cut out these long patterns.
Cutting out the pattern on the bandsaw
Now that I had my pattern cut out, it was time to try it out on the forms. Using the BBB-approved method, I clamped the pattern to the chine log in the middle of the boat, then clamped down the bow end. This lined up pretty well. So far, so good!
But then it was time to clamp down the stern end. As you can see in the photo below, I had two problems:
First, the pattern did not follow the line of the chine log... at all! It wasn't just a little bit off, it was several inches off. Not even close!
As if the pattern was disgusted by my spiling failure, it cracked as it bent around Station 3, right in the middle of the boat, where the angle is smallest.
Or perhaps the pattern was just anticipating it's fate, since it shortly suffered a few more 'cracks'. In fact, it ended up as kindling for that night's fire.
Whoops... how the finished pattern should *not* look!
>>> Next Episode: Mountain Climbing
But before reducing my hard-won pattern to firewood, I sat in my 'thinking chair' and stared long and hard at it. What had I done wrong?
It wasn't my rebellious use of two divider settings. That did cause an error, but the error was so large and so obvious that I just discarded those points.
Could it be my spiling or unspiling technique? It was the first time I'd done it, but the laws of Geometry seemed to rule out any significant error. My points might be off the thickness of a pencil point or so, but not much further than that.
There was only one mistake that could have generated such a large error, I reasoned. I must have edge set the spiling batten when tacking it onto the forms. I had no proof, but the more I thought about it, the more I was sure that must be the error.
Source of the problem... edge set spiling batten
When I sat down to write this blog, this was the first photo I looked at. Remember that the two-part spiling batten is made from two straight strips of plywood, spliced together with a gusset.
So how did the aft strip of plywood curve so sweetly along the long edge of the chine log?
I didn't think I edge set it, but I must have. That must be the cause of my problem, I thought...
Such a small mistake... so much work wasted... That must be the mistake, mustn't it?
There was only one way to find out...