This project requires four 1/2" thick by 4' wide panels. Two of them are well under 16' and two are just over 16'. Scarfing two 4'x8' sheets of plywood together will leave me with a 15'7" long panel because I am using a 5" scarf joint. I start by setting up three saw horses and laying down a sheet of plywood to make a large table. The first sheet of plywood to be scarfed is set on the scarfing table. The reason for the "table" is to support the first layer of plywood to be scarfed, as is gets thinner on the end there is tendency for it to bend down as pressure is applied with the scarfing tools, the table keeps it flat through out the scarfing process.
The scarf joint is 5" so a line is marked at 5" from the end. At about 6" from the end, three decking screws (about 1 1/2" long) are used to at each edge and the center to screw the panel flat. I check each panel as it is laid down and if there is a curve along the end, I always place the bottom of the curve in the center so there is extra pressure holding down the center of the stack. The next sheet of plywood is set on the first at the 5" line. Is it also marked and screwed down like the first sheet.
I need a total of eight sheets of plywood scarfed, so I will do them all at once. The last sheet also gets marked liked the others. The line is used as a reference while scarfing.
A 6" power plane works best, but the smaller 3 1/4" power plane will also work. If a power plane was not available it could all be done with a grinder and a little more time.
The planer is run down the "steps" of the plywood working back and forth until steps are about 75% gone then I start backing down on the amount of wood removed with each pass. It is important to start getting all the cuts even, using the layers in the plywood as a guide. When about 95% of the material is removed I switch to a grinder with a hard pad and 36 grit sandpaper.
When complete, all the sheets of plywood should be evenly tapered with the plies all running in straight, even lines. The ends should all be straight and ground down to nearly nothing. It does not always work out this way and often there are damaged ends and corners where it has been ground to much, the planer tear it out or sandpaper digs under one of the sheets of plywood tearing the edge up. These small blemishes will fill with epoxy and are not a problem.
A row of saw horses need to be set up so each sheet of plywood has at least three for support to keep the panels from sagging. If the floor is uneven it is a good idea to shim the saw horses as needed so they are all on an even plane. On each sawhorse I add a strip of 3/4" plywood and on the center saw horse that supports the scarf joint I use a 12" wide strip and screw it to the saw horse. Strips of 6 mil plastic about 20" wide need to be cut as a barrier to keep the panels from sticking together.
I like to stack all the panels, flat or on edge, so I can apply epoxy on them all at once. The epoxy should be thickened a bit to a honey like consistency and rolled on or brushed on with a thick coat. The first panel is set down so the scarf cut is up on the center saw horse (with plastic over the saw horse). The next sheet is laid scarf cut down over the first. If the panel is laid down to fast, the plastic can be blown into the scarf, which is why I like to use the thicker plastic.
I use a piece of wood (about 1"x2"x12") on edge pressed over the joint to see if the second panel is to high or to low and adjust as necessary. The most critical part of gluing up the panels is to make sure the edge is straight. I will run a string from one corner to the other if more than two sheets of plywood are being glued together, but for only two sheets I will use an 8' straight edge at the scarf joint to make sure the panel is straight. When the panels are aligned properly, I tack them in place with nails on each edge of the panel at the scarf to keep the two sheets of plywood from sliding apart. Plastic is laid over the scarf joint and the process is repeated until all the panels are glued up.
Plastic is laid over the top scarf joint and a 6" wide piece of 1/2" plywood is laid over that, covering the scarf joint. Long duplex nails are driven through the stack to the saw horse below.
Two of the scarfed panels that will be used for the bottom hull panels will be a little short so I am using a simple spline joint to add to the length of the panels. When the boat is finished, the area that has the spline joint will be in the motor well and cut out. The spline joint is not a good structural joint for hull panels and should not be used as such.
A spline joint is made by using a slot router bit on each edge of the surfaces to be joined. Brush un-thickened epoxy on all the surfaces and fit the pieces together and "clamp" with duplex nails just like the scarf joints.
This is a Devlin design, for more information on this design and others offered by Devlin Boat, go to devlinboat.com
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