: or you could let a computer do it... : )

I'd second that! FreeShip is still avilable on the Net and does a nice job of generating plank developments, and some other useful stuff as well.

I don't think it's a question of locating the apex of the cone correctly - you probably don’t have a cone.

When the angle of a boat plank changes along its length it can either behave like a cone, or like a plane’s propeller. The conic does not involve significant stresses within the material and is easily predicted; the material is not being twisted, it is merely being bent with the bend axis changing along the length. The “propeller” style is a true twist and involves stretching along the edges and compression along the axis. If the plank development is asymmetrical to begin with, the resultant shape is hard to predict without a computer. When placing such a plank one a hull it requires more force to get it to fit than a conical plank.

As an example, when I adapted the lines of J. Henry Rushton’s lapstrake “Wee Lassie” to chine ply construction, the two bottom planks seemed to form a single plank. I assumed Rushton had problems getting wide cedar planks, not a problem with ply, so I merged the two planks into one, thinking to save myself some work. When I formed the resulting double-width plank several inches of extra rocker appeared from nowhere *. The bottom edge of the plank had stretched. It took me some time to figure out what had happened but when I went back to using two narrower planks the plank formed as expected. Rushton knew a thing or two!

* FreeShip had provided such an odd plank development that I assumed it had a bug and smoothed out the wonky-looking bottom edge: big mistake!