After too long of a hiatus, progress has resumed on the Mill Creek. The blanks cut some time ago to form the various panels of the kayak’s hull have finally had scarfs cut into them, so they’re ready to be joined together lengthwise. After that, they can be cut to shape, and then stitched together to form the hull.
Scarfs cut into stacked 4 mm Okoume plywood.
Note the 3 bands of plies in each blank.
The process of cutting scarfs is common to the Mill Creek, Sanderling and Dunlin, stitch-and-glue kayaks generally and, indeed, to all plywood.
Okoume (Aucoumea klaineana) is a fast-growing, plantation-grown tropical hardwood from west Africa. Among other uses, Okoume is used to manufacture plywood for use in aircraft and boats. Okoume marine plywood is particularly useful for stitch-and-glue kayaks, such as the Chesapeake Light Craft boats I’ve been building.
One problem the kayak builder faces is that Okoume plywood is typically available in sheets 8 feet long, but the boats we build are often twice that in length, or more. Which means that the panels used to form the bottom and sides of the hull as well as the deck need to be joined end to end. The best way to do that is to scarf together blanks cut from the plywood sheets. The easiest and most practical scarf joint for the amateur builder is made by tapering the broad surface of the blanks and glue them together with epoxy.
It is vital that the scarf’s taper be long enough to ensure that adequate surface areas are glued together. The scarf should be about 8 or 12 times longer than it is thick, and it is imperative that the scarf of the mating panel be an exact mirror image. That may sound difficult and intimidating to the first-time builder, but it’s actually fairly easy to acheive.
Take the panels to be mated end to end, and measure out the length of the scarf. Assuming an 10:1 scarf using 3 mm. plywood, the scarf should be 30 mm. long; with 4 mm. plywood, the scarf should be 40 mm. long. Mark each panel accordingly with a line paralleling the end of the blank. Remember that the slope of the scarfs of the two pieces must be complimentary, e.g., if slope of the scarf cut into the piece on the left slopes downward from left to right, the scarf of corresponding piece on the right must also slope downward from left to right.
After doing that, stack the panels being scarfed, with each one set back from the one below by the length of the scarf. (That’s where that line marking the scarf is useful) and clamp them together with all the ends to be scarfed together. This means that every other panel should be upside down. For instance, if the inboard side of the forward blank is up, the inboard side of the mating after blank should be down. When they’re all clamped together, the end of each blank should be offset from the blanks beneath it by the length of the scarf. The end of the clamped pile of blanks will have the appearance of a long, shallow staircase.
To cut the scarfs for the Mill Creek’s hull, I had a stack of ten blanks – 2 forward and 2 after lower panels, 2 forward and 2 after upper panels, and 1 forward and 1 after bottom panel. As I’m using 4 mm. plywood, the stack was 40 mm. high, and the combined length of the scarfs was 400 mm. long.
Side view of scarfs. Note the stack of blanks.
Then take a small power sander with a pretty coarse grit (I used a palm-sized orbital sander, but a random orbital, sheet or even belt sander could be used; one can also use planes or rasps, but the work would be slower and more difficult) and begin sanding down the “steps.” I found it to be most effective if the sander bore on three of the steps simultaneously, as that kept the slope of the scarf constant. Be sure the wood is supported, and cannot flex away from the sander. As you go, you’ll start cutting through the plies of the plywood; if the work is progressing evenly, the joint between the plies will form nice, straight lines running perpendicular across the blanks, parallel to the ends being scarfed. If those lines are not straight, the wood is not being removed evenly. Working slowly and carefully, it’s pretty easy to achieve good, even results. Stop shortly before the scarf edge is razor thin, as it will be pretty fragile. If you do that, you’ll find that your 10:1 scarf slope will produce a mating surface that’s about 8:1, which is good.
Finish up with a sanding block, also with a pretty coarse grit. If the “stripes” of the scarfs cut into your pile looks like the those in the photo, you’ve done well and can move to the next stage, gluing the blanks together.
