Author: Phil Howard

I finally finished stitching the hull together after taking about a week’s break. Several things on the setup before showing pictures of the process.

The hull is currently held up by the stern and the center bulkhead. These in turn, are clamped to the strong back. They can easily hold the weight, but I was concerned about the whole thing tipping over. My solution was to clamp a pair a “legs” to the forward part of the hull. These legs became more important because I decided to remove the center bulkhead while stitching the final panels in place. If the panels were off (in particular, too narrow), the center bulkhead would interfere with my ability to stitch the final panels into place. The bulkhead isn’t required to maintain shape at this point (the shapes of the panels are doing that), so I didn’t see any harm in removing it.

Here’s a picture of the legs. You can see the center bulkhead.

Next, I stitched the final two panels along the center line. I was then able to lay the final panels into their approximate position. The scrap sticking out on either side near the front of the hull is to prevent the panels from falling into the hull. The center bulkhead would have served that purpose, but I removed it so I needed another way to keep the panels in place while I started stitching.

The stern has a fair amount of twist in it. The previous picture shows this: Near the center of the hull, the panels are in proper position, but at the stern, the corners “stick out”. I drilled holes in the appropriate places, placed the stitch in place, and was then able to use hand pressure to twist the corner into place. A few extra hands would have been handy (two to pressure the panel into place and another to start the twist in the stitch), but I eventually managed.

Here’s a picture of the stern with the panels stitched down. From here, I started working my way toward the bow a few stitches at a time on each side.

Once I’d stitched about half way along the hull, I was able to pull the thin support out of the way. The stitches were preventing the panels from falling into the hull. I eventually had the reverse problem. The required twist at the bow was keeping the panels apart. A ratchet strap solved the problem. It didn’t take much pressure – the weight of the strap was almost enough to pull the panels into place.

The most difficult stitching was in finishing the bow. It was cramped inside the hull when trying to feed the stitches through the holes, and the last bit of twist made it difficult to get the panels in position for the stitches to hold. A second set of hands would have been helpful.

But eventually, the final stitches were in place. The shape of the hull is now fully defined by the panels. However, to give the hull a bit more strength and stability while turning it over, I re-inserted the center bulkhead. It’s only held in place by two stitches – one on each side – but I think it will help when flipping.

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).

My last post ended with this picture showing the first panel attached to the bulkheads. I have two stitches attaching each panel to the stern (top and bottom of the panel), one attaching the panel to the center bulkhead (which will later be removed), and three attaching the bow.

To hold this panel in place, I attached “L”-shaped scraps to the bulkheads. Now that I have one panel in place, it’s easier to hold the next one. I clamped a couple of scraps to the outside of the first panel (one near the bow and one near the stern). These held the spring tension of the second panel as I stitched it into place.

You can see the stitch at the top corner of the second panel. Once that is in place, the clamps are no longer necessary. Next I stitched the two panels together at the bow (without attaching them to the first panel). At this point, the other clamps can be removed. The stitch at the stern, the outward pressure of the center bulkhead, and the bow stitches hold the panels loosely in place while they are stitched to the first panels.

I started stitching at the stern and worked my way up to the bow. As I worked my way forward, the gap seen in the picture above closed up.

You might be wondering, “How do you form the stitches?” and “How many do you need?”. In terms of “how”, I cross the wires over and take a couple of twists by hand. Then I grab the crossed over wires with a pair of pliers and then twist away while pulling the stitch away from the hull. The pull creates neater twists. I keep twisting until the hull panels are pulled together.

As for “How many?” the answer is “As few as necessary to pull the panels together.” Along some sections of the seam few are necessary because the panels naturally align. In these regions I do one stitch every 8-12 inches. When the panels have twist in them, it takes more stitches to handle the strain of the twist (more on this in a later post). In these regions, I do a stitch about every 4 inches. But I don’t measure the distance between stitches. I eyeball it with “one about here should do it.”

Here are pictures of a completed stitch and the completed second panels.

The third panel on this design lays flat enough, I didn’t need any special clamps to hold it in place. I put one stitch in the stern, then joined the bow, then started at the stern and worked my way forward stitching the third panel to the second. In the picture of the bow you can see that one end of the bow seam is tight and the other has a considerable gap. This is because of the twist I mentioned earlier. I was able to tighten the stitch to get the gap to close up.

When attaching the second panel to the first, I was able to reach over the top to insert the stitches. Because of both the size and position of the third panel, this was difficult to do. I have a rolley stool that is real handy for this. I sit on the stool and duck my head while I roll under the hull to insert the stitches.

A couple more tricks: I put a few scraps of wood on the bow to help the panels bend down into position. I also clamped “legs” to the hull to prevent it from rocking. Up until this point, rocking was prevented by the bulkheads being clamped to the strong back. As the hull progresses, I wanted a bit more stability. These flimsy legs will get replaced by sturdier ones shortly.

And finally, here’s the completed 3rd panels.

The final panels are a bit more complicated because of their twist, so I will save those for another post.

I’ve cut the panels, I’ve cut the stitches, now it’s time to get set up to assemble the hull. With some boat designs and construction methods, you start by building a “strong back” – a straight and stiff beam that is used to align the frames that determine the shape of the hull. The strong back has to be straight because it is the reference frame for the hull. And it has to be stiff so things won’t move as the hull is constructed.

For this hull, the panels define the shape, not frames. As a result, a strong back is overkill – a 2×4 would have worked. But I had a strong back from a previous boat, so I reused it.

Here’s a picture of the strong back with the stern and one of the bulkheads attached. The stern is necessary because it is part of the resulting hull. The bulkhead I attached could have been omitted – it is not part of the resulting hull. But this bulkhead helps support the panels as I stitch them together.

The bulkheads need to be square to the strong back. The bulkhead in the middle needs to be vertical relative to the strong back. The stern is at a 15 degree angle off of vertical (this is a parameter you can set in AVS Hull when you design the hull). To hold this angle, I cut a 2×4 at a 15 degree angle on my chop saw, then attached a small scrap of 2×6 to the beveled end of the 2×4. When this is clamped to the strong back, it holds the stern at the proper angle.

Since the top of the boat is not flat, the tops of the bulkheads can’t be placed directly on the strong back. When AVS Hull outputs the offsets for the bulkheads, it also shows how high off the strong back the bulkhead needs to be placed. I cut a scrap of plywood the correct width and used that as a spacer between the strong back and the bulkhead. For this particular hull, the bow is the highest point on the hull. As a result, the bow will rest on the strong back.

Here’s a picture of the stern setup (taken after I had attached a couple of panels). The 2×6 is screwed to the 2×4 to hold it in place. The 2×4 is clamped to the strong back, and the stern is clamped to the 2×6.

The panels will be attached to the stern (the stern is stitch-n-glued into the hull). The first panels need to be attached to the middle bulkhead to hold them in place while the next panels are stitched on. The remaining panels will not be attached to the middle bulkhead (it is not part of the hull).

In order to hold the first panels in place while attaching the stitches, I cut “L” shaped pieces from scraps of plywood and clamped them to the bulkheads. I also attached a clamp to the end of the “L”s on the stern to prevent the panel from springing off the end of the “L”. The center bulkhead did not need this because the spring tension held the panel against the bulkhead.

The pictures show spring clamps. After attempting to place the first panels, I replaced the spring clams with real clamps. The spring tension in the panel was enough that the spring clamps could not hold it in place.

Before placing the first panels, I stitched the bow together. This allowed the bow to rest on the strong back while the bulkheads held the rest of the panels in place.

Here’s a picture of the bow stitches (also shown in the previous blog post), and the panels positioned on the frame.

It’s starting to look like a boat!

AVS Hull is primarily for designing “stitch and glue” plywood boats. In this post, I’ll show you where the stitches come from.

Here’s a picture of the first stitches I made in Anna’s Boat. The stitches are made by small pieces of copper wire twisted together (this is the bow of the top panel on the hull, but more on that in a future post). These stitches are intentionally loose to allow the bow to open up when I place the panels on the frame.

But where to get the little copper wires? I used a short piece (about 3 feet) of AVG 6 stranded wire (it was a left-over piece from when I installed my hot tub). I stripped the insulation off (harder than I anticipated), then split the wire into groups of 2-5 strands. The smaller groupings made it easier to do the next step.

I wrapped each bundle of wires around a scrap of wood and then cut the wires leaving pieces equal to one wrap of wire. The result was pieces about 4 inches long.

After doing all the bundles, I had a couple of hundred pieces ready to stitch my hull together. Three feet of wire yields 10-11 stitches per length, and there were 19 strands in the original wire.

In my last post, I explained transferring the offsets onto the plywood that would become the boat. Today’s post shows the cutting process.

I have a bandsaw that I’ve used to cut panels for previous boats. This picture show the setup. You need a long work area. For an 11 foot long panel, you need at least 22 feet of clear space. (Although on a previous boat I cut half way into a panel, then backed it out and cut from the other end.) And unlike ripping plywood on a table saw, you need an area wider than the panel because the cuts are curves, not straight.

After making the first cut this way, I decided it was easier to have a stationary panel and movable saw instead of a stationary saw and movable panel. So I switched to a jigsaw.

This cut is fairly straight, but with a jigsaw, curves are no problem. I just had to be careful to make sure the narrow pieces were properly supported. When the uncut piece was wide, I used a weight to hold it in place. As it got narrower, I switched to spring clamps.

I intentionally cut the pieces oversized by about 1/16-1/8 inch all the way around. I then used a belt sander to do the final shaping. I found I could be more precise with a belt sander than I could with a jigsaw.

Using a belt sander this way was more work than I had figured on. Belt sanders tend to be heavy because they are designed to sit on top of what you are sanding. The weight provides the force of the sand paper against the work. But I was sanding edges, so I had to hold the weight as well as resist the sideways force caused by the friction of the sand paper against the wood. Why pay for a gym membership when you have sanding to do?

Once I had each panel sanded, I stacked the left and right side matching pieces on top of each other and went over the edges again. This guaranteed that the matching pieces were the same shape, so my boat will be symmetric when it’s all assembled.

Here are the finished pieces. This is everything that goes into making the hull. The only other pieces I’ll need are the seats (cut from leftover pieces from the panels), and stiffeners for the gunwales (the top edges of the boat).

This picture shows the layout of the panels I need to cut. In the last post, I showed how I prepared the long panels: 2 feet wide by 11 feet long (actually, two of them are 9.5 feet long and the other two are 10.5 feet long). Now I need to mark the designed onto the panels so I can cut them.

AVS Hull outputs tables of offsets. You can set how you want the table prepared. I opted to have the offsets printed every 6 inches along their length, and printed to the nearest 1/16 inch. For you metric people, you can choose to print decimals instead of fractions.

Here’s a snippet from the offset table.

The first column is the length along the long edge of the panel (the X coordinate). The second is up from the long edge (the Y coordinate). Notice that all the X coordinates except the first are on 6 inch boundaries. The first coordinate is the coordinate of one of the corners, so it was output at its exact location.

When transferring the offsets to the plywood, I start by drawing lines every 6 inches. I can then measure from the edge of the plywood along the appropriate line to locate the point.

For this to be accurate, you need to make sure the end and edge you use as a reference are both straight and square relative to each other. I made sure to use the factory cut edges of the plywood figuring that they would be more straight and square than the edges I ripped on my table saw.

After marking all the points, I use a straight edge to connect the dots. If I wanted to be more precise, I’d tap small nails into each position marked on the plywood then use a thin piece of word bent along the nails to draw a curve to connect the dots. If I had wanted a more precise shape, but didn’t want to do the nail-and-strip thing, I could have simply printed the offsets at a closer interval (say, every 2 inches). More points mean that the straight lines form a closer approximation to the curve, but at the expense of having to transfer more offsets. For this build, plotting every 6 inches seems to be a reasonable compromise.

Here is a picture showing one of the marked sheets. Next up: Cutting the panels out of the sheets.

This is the design of Anna’s Boat. The design was created using AVS Hull. The boat is a light weight rowboat suitable for two are three people. It is suitable for a car topper, or for tossing in the back of a pickup. I think the design looks good, but it is being built for utility, not as a show piece.

Here are the lines of the boat:

Here is the layout of the panels used to construct the boat. It can be built with 3 sheets of 1/4 inch plywood. For scale, the rectangles on the layout are 2′ x 4′. Each of the side panels are cut twice. Some of the bulkheads are displayed on top of each other because one set is cut from the first set of panels, and the others from the second set. The transom is cut twice and laminated together to create a 1/2 inch transom.

If you are interested in this design, leave me a message, and I can get you the design files.