Meditation on spiling

Spiling is the process of determining the shape of an object by using an intermediate object as a partial template. It is most commonly encountered when talking about the planks of a boat.

A flexible batten (known unsurprisingly as a spiling batten), often jointed so that it more or less follows the shape to be spiled, is placed on the boat adjacent to the line that is to be spiled. A series of marks are made on the batten so that the distance and direction of points on the spiled line can be re-constructed later. The spiling batten is then placed on the planking stock to be used and points on the spiled line are then recorded on the planking stock by ‘reversing’ the marking process. A fairing batten is then used to construct a fair line through the points marked on the planking stock.

In the case of Seapod the spiled line for each plank is the lower (closest to the keel) edge of the plank.

Spiling starts with Plank 2 - as the Garboard (Plank 1) is laid up against the keel and plywood templates are made for the ends. Anyway - the spile didn’t work - and in fact spiling, for us, didn’t really work until we got to plank 4. We went through three separate spiling battens trying…

Thin and floppy to start with…

Then stiff and wide (perhaps too wide)…

Then stiff and just a bit narrower than a typical plank…

Anyway - whatever the batten - for the early planks the spiling process was just not accurate enough. We ended up spiling as best we could - leaving loads of waste north of the line and then scribing the rough cut plank once it was bent and twisted to shape.

I ended up concluding that when you have both shape - ( we had quite a lot ) and twist in the same region of a plank the material of which the spiling batten is made has to have the same torsional characteristics as the plank in order to get an accurate spile. The easiest way to spile the plank is to use the plank as a spiling batten (err… wish I could do that) . Anyway, once we got to plank 4 the level of twist reduced and spiling started to work as expected.

From plank 2 to plank 8 all the planks were a uniform (sort of) ‘C’ curve with the curvature reducing as we turned the bilge. Here is a curvy plank, plank 4 - the first plank that spiled correctly.

With plank 9 (spiled late this afternoon) everything suddenly looked different. Here is the lower edge of the new plank as laid out on the planking stock.  The blue batten shows the line….

The line was so straight I could have (more or less) lined it in using a straight edge. How could this be right - a straight line? Look at the top edge of Plank 8 - surely it must be curved…

What happened - and it must happen fairly often - it that a plank line happens to hit a ‘great circle’ round the boat and has no curvature. I decide to trust the spiling batten - line in the upper plank edge from our table of calculated plank widths and get another different - but of course curved - plank line for the top edge of plank 9. (For the first time since the garboard strake I don’t need to scarf planking stock to get the required curvature - !!!!).

This time our familiar ‘C’ curve has transformed into a double ‘S’ curve. I was expecting this at some point - so feeling a bit more confident I line in the plank and cut it out. Once planed a bit to take off the rough edges I offer it up for a quick sanity check.

Pheww!! That looks like it will fit just fine. Time to go home before I start making mistakes.

In a few planks I have gone from a total distrust in any spiled line (back at plank 3) to having enough confidence in the spiling batten to follow it even when what it shows seems counter-intuitive (how can an obviously curved line be straight ???). That’s what I call a learning curve.

Of course - when I start my next build project the same issues will recur. Hopefully I’ll be a little better prepared.