Spiling, by the way, is the process of copying a plank shape from a curvy boat surface onto a flat plank surface. A plank may look like it only curves on way when you see it on the boat, but the flat plank off the boat can end up having a strange, undulating shape. Spiling allows you to copy the shape from your boat to the flat planking stock very precisely.
The first and last sentences above describe spiling as a process that allows one to copy the boats shape to a flat plank. This description leaves out the fact that there is a very important step between “copying the boats shape” and “to a flat plank”.
A Spiling Batten is bent and twisted into position on the hull, then after allowing the batten to Sny, it is tacked to the hull adjacent to the plank you plan to fit against. Then at each frame, an arc is scribed from the plank edge to be copied, onto the “Spiling Batten”. Upon completion of scribing at all frames as a minimum, the batten’s tacks are removed and it is allowed to relax (un-sny if you will) and lay flat on the board from which the plank is to be gotten out of. The spiling batten is tacked to this board, and then by reversing the process of swinging arcs, points are located on the plank that are then connected and drawn fair, using a fairing batten. The tacks and batten are removed so the board can be spoiled by cutting away the unnecessary portions, allowing the plank to be gotten out.
Many of us suspect that the word “spile” is a corruption of the word spoil, and its origin is unknown. Sny deserves an explanation. Sny is the internal stress that occurs in a plank as it is simultaneously bent and twisted against the hull as the plank’s butt end reaches for the vertical plane as it approaches the stem. Sny causes the end of the plank to rise. The interesting aspect of this sny phenomenon is that our forefathers recognized that a spiling batten would anticipate the sny a newly gotten out plank would experience upon being hung, allowing it to fit versus not fitting. All questions answered as best I can.