Step one of my 2xSat Transparent Canoe v2 project involved designing a dory hull form. On that project I used OpenSCAD to do the 3D stuff, and then did a lot of post-processing to create the cutting patterns ('flattening' the skin pattern was especially difficult).
For 2xSat v3 I decided to build an online 'calculator' to turn hull dimensions into a set of flat patterns. The only post-processing required now will be laying out the patterns for use on your favourite CNC machine. The hull build process is identical to 2xSat v2, and it's worth referring to that page if anything below requires further explanation.
My calculator outputs a single SVG file. The important parts are: the grey shapes (the bits to get cut); and their red lines (helper markers 'engraved' on the cut parts).
The leftmost shape on the output is a set of curves representing a plan view of your new hull - this is for reference only (i.e. not for cutting). Green lines are the gunwales (the line where the side joins the topside/deck). Blue lines are the garboards (the line where the side joins the bottom). The lines perpendicular to the keel line detail where each bulkhead/rib is located.
The sides of the hull are wrapped around a set of regularly spaced bulkheads/ribs (1/10th of the hull length between each). You'll need to cut a single copy of the largest bulkhead/rib, and a pair of each of the others (for a total of nine bulkheads/ribs).
You'll need to cut 4 copies of the 'Half-Side' shape (i.e. a pair for each side). The engraved lines will help keep everything in a line during assembly.
You'll need to cut 4 copies of the 'Half-Gunwale' strip shape (i.e. a pair for each side). These are glued to the top edge of the sides to reinforce the gunwale.
You'll need to cut 2 copies of the 'Half-Bottom' shape. The engraved lines will help keep everything in a line during assembly.
The 'Half-Topsides' stack is an optional component. If you want to enclose some portion of the topsides of either end of your hull, cut a copy of the relevant part(s).
You can choose the viewBox scaling for the SVG output. The 1:20 scaling is for in-browser viewing (e.g. for testing out your ideas). The 1:1 scale is for 'real' use. Whilst it's technically possible to use either scale for further processing, I found that this confused many post-processing tools (mostly when it came to converting pixels to millimetres, e.g. Inkscape imports at 90dpi, but the software with my friend's laser-cutter imports at 96dpi). Keeping everything 1:1 helps prevent import fails. Bear in mind that the 1:1-scaled file you produce here won't make a lot of sense until you 'Right-click + "Save Page As..."' it, and import it into something like Inkscape.
Incidentally, if you're using Inkscape to do your post-process layout, the first thing you'll want to do is select everything and cycle though a few rounds of 'unlink clones' and 'ungroup' (Shift+Alt+D then Shift+Ctrl+G, repeat at least 3 times) to separate all the shapes.
More On Bulkheads/Ribs
One piece of post-processing you might want to consider involves customising some of the bulkheads/ribs to accommodate your seat. Here I've squared off the bottom of the middle bulkhead/rib and added a notch to take my seat plank. The easiest way to achieve this in Inkscape involves using the 'Edit paths by nodes (F2)' tool; breaking the inside path at the nodes at either end of the arc; and then deleting the arc. This will leave you with a straight line across the inside-bottom of the bulkhead/rib.
A final point worth noting is that an especially flared hull design can push the smallest bulkhead/rib beyond the 'chin' (where both sides meet each other and the bottom). The shape for that bulkhead/rib will look a bit screwy, and can be safely ignored/discarded from your cutting set.
And so, without further ado...
Dory Hull Calculator
So, is it possible to specify numbers that generate a junk hull (or worse!)? Most definitely! Caveat emptor! ;-)
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