I’ve talked about stitches, but it occurred to me that I’ve left out some details. Yesterday I talked about plywood twist and said I’d talk more about it later. So in this post, I’ll cover both of those topics.
The “stitches” in “stitch-n-glue” boat construction are short pieces of copper wire. In a previous post, I explained how to cut the wire into appropriate lengths. Why copper? It seems to have the right combination of availability, twist-ability, and strength. You can get suitable gauge aluminum wire from a farm store (it’s used for electric fences), but it seems to break more easily when twisting. You could also get steel wire from either a farm store (galvanized, another electric fence wire) or hardware store (for holding rebar together), but I find it harder to twist. The steel rebar wire could rust, but, in theory, once the hull is done, it shouldn’t get wet, and it will be sandwiched between layers of fiberglass, so that shouldn’t be a problem.
So, if you want to use a different wire, feel free to do so.
Next up, how do you get the wire to go through the plywood? The answer should be obvious, but somethings that are obvious to one person are a mystery to another. The answer? Drill holes. What size? Just large enough to easily slide the wires through. You don’t want a tight fit – there is no benefit, and it just makes stitching harder. But you don’t want giant holes either. The small holes will get filled as part of the fiberglassing process, but you don’t want large holes that you would need to deliberately fill.
Why are the stitches inserted from the inside? The “glue” in “stitch-n-glue” is fiberglass tape epoxied on the inside of the seam. You want the stitches to lie as close to the plywood as possible so that wen you can smoothly tape over them. Once the inside is taped, the twists on the outside will be cut flush to the plywood in preparation for glassing the outside.
I use pliers for twisting the stitches. When twisting, you want the twist to grow down towards the plywood (to make it tighter), not up toward the end of the wires (growing this direction won’t make the stitch tighter. If you grab to stitch on the end of the wires, it will grow up. If you grab it right at the top of the twist, the pliers can still apply torque to the wire but the pliers will prevent the twist from growing up. In pictures:
Enough about stitches, let’s talk about plywood.
Why plywood? It is relatively inexpensive (ok, it used to be, but plywood, like everything else, has gone up in price), relatively light, and it has the strength properties that we want in a hull material.
One of the things that gives plywood the right kind of strength is the way it bends (or more appropriately, the way it does not bend). You can bend plywood in any direction, but only in one direction at a time. If you take a piece of paper, you can bend it in any direction (gentle bends, not folds). But while it is bent in one direction, you can’t simultaneously bend it in a different direction without crumpling it. Plywood behaves the same way. The curves in the hull prevent the plywood from bending in a way that causes the hull to crumple.
To put it another way, you can’t create compound curves with plywood. An example of a compound curve is a saddle:
This property of plywood is what gives the hull its strength, but it also limits what shapes can be made with plywood. Plywood hulls are known as “hard chine” hulls – where two sheets of plywood come together, you have a noticeable corner – a chine.
Surfaces that can be made with plywood (or other material with similar properties) are known as “developable surfaces”. So how do you know if your hull design is developable? The reason AVS Hull defines hull shapes using bulkheads is that the results are developable. When AVS Hull computes the shapes of the panels, it does so in a way that the resulting shape, when stitched to the other panels, will bend to the developed hull shape.
Imagine that you put dots along the top and bottom edge of a panel, and then connected all the dots to make triangles (for those who can’t imagine that, here’s a picture).
Now imagine that the panel was made of paper, and that you made a tiny crease alone each edge of the triangles (the red lines and the blue lines). The result would be a developed surface. If you only creased the red lines (or only the blue ones), you would have a gentle curve without any twist. But if you creatively creased both the red and blue lines, you could add twist to your curve.
This is the actual process that AVS Hull uses to compute the shapes of the panels. The curves that connect the bulkhead points become the tops and bottoms of the triangles. AVS Hull places hundreds of points along the curves and connects them with straight lines. It then “flattens” all the triangles (uncreasing the paper, if you will) to create the flat shape that can be bent into your hull.
There are two things that make stitching your hull together difficult: curves that are too tight (plywood is stiff, so it will only bend so far), and too much twist. Tight curves are usually not a problem for reasonable hull shapes and thin sheets of plywood. But twist can be a problem – often at the bow and stern.
These pictures show untwisted joints at the bow and stern:
The amount of twist varies based on the hull shape and which panel you are working with. I plan on adding a feature to AVS Hull that will compute the twist at each point along a panel. That value will indicate how hard it will be to bend a panel into its proper shape. For this particular hull, I was able to use hand pressure to hold the panel in position while I stitched it. The copper stitches easily held the panels one I removed my hand pressure (so far, anyway).
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