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.
It is worth noting that this generator creates a hull with a bow-to-stern 'rocker' (which differs from the side-to-side 'rocker' on the first two 2xSat designs). This revised orientation will enhance both the stability and manoeuvrability of the hull (it's also a more traditional form).
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 chines (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). The smallest bulkhead/rib has no cut-out - the space it encloses at each end should be sealed/watertight, and thus keep the hull afloat if it ever becomes inundated. Most of the bars across the top of each bulkhead are cut out after the hull has been assembled, although I usually keep one of the centre ones to use as a back-rest.
You'll need to cut 4 copies of the 'Half-Side' shape (i.e. a pair for each side).
You'll need to cut 2 copies of the 'Half-Bottom' shape.
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). As a minimum, consider using a pair of the smallest topsides to enclose the bow/stern floatation cells.
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 (unless you have a really big monitor) 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 use 'Edit -> Clone -> Unlink Clones recursively' to separate all the shapes.
When laying out the shapes for cutting, I applied a few tweaks:
- The shapes provided here make no allowance for plywood thickness, so it's worth upscaling the bottom (and optional topside) panels to be slightly oversized, with the intention of planing them back once they're fastened on the hull. In Inkscape, use the 'lock' button to ensure the width/height ratio is conserved, and add 25mm [1"], or so, to the width.
- If you zoom right in on the topmost (widest) bulkhead/rib, you'll see a horizontal red line just above the bottom rib. This line matches the height of the inside of its bulkhead/rib neighbours. If you'd like a horizontal plank to span the middle three bulkheads/ribs (e.g. to sit on), adjust the topmost bulkhead/rib accordingly.
- Because I built my hull in two halves, I manually added sacrificial 'tail flaps' to my 'Half-Side' shapes (see the last step in this section for an explanation). If you're building your hull in one piece, you may want to think about how your panels are going to be joined (e.g. scarfing allowances).
- When setting up the CNC job (using, Vcarve, for example), I specified no special treatment (e.g. 'dog bones') for the internal cuts on the bulkheads/ribs - they get a nice fillet for free as a result.
- I manually fabricated 50mm [2"] gunwale reinforcing strips (using the 'Half-Side' shape as a guide). You may want to create these for CNC cutting instead. The easiest way to do this in Inkscape is to make two copies of the 'Half-Side' shape, offset them horizontally by 50mm [2"], and 'subtract' one from the other. You'll probably need to lengthen them a little as well, so you have a bit of overlap to work with. Or just stick with doing them by hand?
- I've not included any notches for chine logs at the base of each bulkhead/rib - if you want to include chine logs, notch each bulkhead/rib template to meet your requirements.
And so, without further ado...
Dory Hull Calculator
Using the default parameters above delivers a hull where: 100mm [3.9"] of emersion displaces 122.8kg [271 lbs] of (fresh) water; 120mm [4.7"] = 153.5kg [338 lbs]; 140mm [5.5"] = 184.9kg [408 lbs]. Bear in mind that the displacement includes the weight of the empty hull - my build weighs in at 24.5kg.
What's So Great About A Dory Hull Form?
The Dory is a traditional hull form for one major reason - they're easy to build. This 'ease of build', in turn, concerns one major building constraint: plywood doesn't like to be bent in more than one axis. A Dory is made from three (or more) 'patches' cut from the side of a cone (conic sections). Such patches guarantee a single bend axis, no matter the orientation of the slice (it is worth noting that a cylinder is just an infinitely long cone, so it can also be used to extract hull parts from). The strength (and beauty!) of the Dory comes from joining patches with a mixture of bend axes. In order to visualize how a simple Dory hull is put together, please refer to the following sequence:
In brief: a pair of matching cones is offset and intersected (the desired hull parameters determine the dimensions and offset of the cones) - the outline of the intersection is a hyperbola; this intersection is itself intersected with a transverse cylinder to create the bottom of the hull (note that many traditional Dory's also cut the top of the hull with a transverse cylinder to create sheer); the resulting body is sliced at regular intervals along its length to locate/describe the bulkheads/ribs.
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