Montana Riverboats Test Domain Roadkill

Scarfing Plywood

Scarfing Plywood     

Full length plywood has become impossible to find. Even if you can find it the price of the plywood is high, and so is the shipping. It's better to buy plywood in 4x8 foot sheets and do the scarfing yourself. To make a single length out of two shorter pieces, you make two slanting beveled edges at one end of both sheets. Then you spread glue over the joint and clamp it together. This reduces the overall length of the two sheets by approx 1-1/2" This is OK. Later on in the plans, when I refer to 4x16 plywood, I really mean 4x15'x10-3/4"'s not much of a difference, and 4x16 is easier to say (and write). Note too that AA marine plywood sometimes comes a little longer than 8'. In that case, you might well end up with a full 16' It doesn't really matter. In a later step, when you layout the side plywood for temporary rib stations, start the layout for both side pieces from the same end: front or back end, take your pick. Just make sure you don't start measuring from the front on one side and then from the back end on the other side.

West System sells the above tool at many of its retail outlets. Instructions come with the tool. I bought one of these scarfers nearly 20 years ago, and still use it today. The literature that comes with this tool claims it will successfully scarf plywood to 3/8" thick (approx 9mm in Okoume). But it works fine for 1/2" too. To scarf 1/2" plywood, use a new, full-diameter 7-1/4" blade (not sharpened down to a smaller diamter) and then be prepared to sand or hand plane a small ridge of wood approximately 1/2 a surface ply thick, where the saw blade doesn't quite make it through the panel. (These plans don't call for using 1/2" plywood anyway, I just added that for perspective).

After making any such scarf cut, build up a long flat working surface with 3 or  4 extra-straight 16' 2x4's layed over saw horses. Then put scrap plywood or chip board over the 2x4's and then cover the chipboard. Then make the scarf cuts. Then put the panels aside temporarily. Then cover the chipboard with visqueen. Then lay the gluing panels on top of the visqueen. Wet the scarf joint with unthickened resin. Then mix up some standard gluing putty with resin and micro-fibers and cover the face-up side of the joint. Press the panels together and align one 16' edge with a string line. Then drive drywall screws right through the still wet scarf joint, down into the scrap chip board below the visqueen. Back the screws out 4-8 hours later. That's it! You now have full length plywood.

Ok, now we need to talk about plywood and what to buy. The materials list (in part-one) lists 2 4x8x1/4" panels as needed to make the sides (or two 4x8x6mm okoume, or two 4x8x5mm sapele...sapele is stronger, so the thinner panel is ok). To make the side panel, you follow the above scarfing instructions to make one 4x16 panel. To do this, you make a pair of matching scarf joints at the ends of two 4x8 panels and glue the two together, end-to-end.

To make the bottom panel takes a little more explaining. The Honky Dory is wide boat. It's 56" wide in the middle, which is (obviously) wider than a standard 4x8 panel. Why? The extra width is a godsend when rowing. The Honky Dory is extra-stable. Fishermen can walk around in the boat, change ends and even sit a little off-center in the boat without problem. The Honky Dory breaks big waves like an arctic ice breaker, and yet it holds very well and it turns on a dime. The wide bottom makes it one of the best all-purpous fishing boats ever built. Building it does require a few extra hours of elbow grease.

For a long time, for each boat, I would buy two 5x10 panels I got from Harbor Sales in Baltimore or from Edensaw in Port Townsend Washington. (links are on the boatlinks.html page). The extra plywood left over after gluing the two 5x10s together went into seats and dry storage lockers, so nothing was wasted. This is still a good way to build, but 5x10 is hard to find now, and it is getting very expensive.

Another approach is to limit the high cost of 5' wide plywood to one sheet of 5x8: IE you can build the bottom panel from one 5x8 and one 4x8. Edensaw usually has 5x8 panels in stock. To build this way, you scarf both 5' ends of a 5x8 sheet. Then you split the 4x8 into two 4x4 pieces, and scarf one 4' edge onto the center of each end of the 5x8.

Finally Given the current cost of 5x10 plywood, the smartest way to build the Honky Dory bottom is from three sheets of 4x8. Two full size sheets are scarfed together to make a 4x16. Lay this over top of a recently completed strongback and trace a pencil line around the edges, at the ends, where the panel is wider than the boat. Cut the corners off, with a skill saw set to 25 degrees. Now you have a bottom panel that fits nicely everywhere except the middle of the boat, where there is a long skinny, double pie shaped piece missing from each side, approximately 4" wide at its widest, tapering to a point at both ends, approximately 60" long. Clamp a straight edge on the bottom panel, on each side, so you can make a standard, sharply slanted scarf cut on each side, where the new pie shaped piece will go. Rip two full length pieces off the remaining 4x8 x 3/8 bottom piece you still have, that are 6" wide. Measure and cut those to the right length (approx 60"). Scarf one edge of each 6" x 60" piece and glue those onto the sides of the main bottom panel. Now you can lay that back over top of the form, scribe one more time, and you now have an ultra-strong bottom panel, ready to install.


West System Scarfer

This scarfing jig can be purchased from West System. After drilling appropriate holes in the bottom of the saw plate it is bolted onto the saw.

Home made scarfer

This home made jig is a rectangle of 3/4” chip board with a 2x4 screwed and glued the the long edges of the rectangle, where one such 2x4 is slightly angled on the table saw before assembly.

An 8:1 scarf in 1/4” inch plywood makes an angled surface approximately 2” inches long in 1/4” inch plywood. A 7-1/4” inch blade will cut through 1/4” inch plywood but not all the way through 1/2” inch plywood. But that’s OK. You can cut almost all the way through, break the plywood (it will only miss by about 1/8” inch) and clean the resulting nubbin up with a block plane. I use and 8” inch blade (and saw) for my scarfing jig.


Here (below) a home made scarfing jig (bolted to an 8” worm drive saw) glides against a straight edge temporarily clamped to the end of the panel that needs to be scarfed. After cutting slowly and steadily with a freshly sharpened blade the joint can be used as is. But it can also be smoothed up a bit with a razor sharp hand plane, running the blade of the plane at a slight angle to grain of the scarf cut.

This is a 7-1/4" worm drive bolted down through the foot plate onto a home made scarfing jig.  I have since replaced that 7-1/4" inch worm drive with an 8" inch saw.  Adjust the angle on the top 2x4 so you get the longest angle that still cuts through 1/4" inch plywood.  A 7-1/4" inch saw at that angle will not cut all the way through 1/2" inch plywood but that's OK. You can clean up the nubbin left after the cut with a sharp block plane.


Here is the same jig now with an 8" inch worm drive bolted on, held up tight to straight edge clamped on the end of the panel, at the appropriate minimum distance.