Scarphing Techniques for Wood/Epoxy Boat Construction

Developed by C. Tracy O’Brien Copyright 1995

Of all the processes associated with the building of wooden boats, scarphing has to be the most feared – and the least exploited. Prior to the advent of structural epoxies, scarph joining of both plywood and solid lumber required elaborate clamping schemes and careful temperature control. With the proper use of epoxy bonding systems, excellent scarph joints can now be produced under a variety of conditions with minimal clamping. Just how good are these joints? Under most circumstances, good enough to forget they are there!

Besides structural integrity, the other aspect of scarphing that concerns many builders is the actual preparation of the joint: how to cut long, knife-edged tapers that match each other. Before we get into the "how to" of cutting the bevels, though, let’s look at the question of bevel angels: for most application, a taper of 8" for each 1" of thickness has proven more than adequate. For spars and the like, a taper of 12"for each 1" of thickness can be used.

Now back to the question of joint preparation. While I found that most of the usual cutting methods (hand planing, routing, etc.) seemed to work, they all proved to be too slow and painful for any kind of production situation, even in a small custom shop such as mine. Experimentation eventually led me to develop techniques that use power saws to perform the majority of the cutting. These techniques involve the use of two simple fixtures, one for plywood, and the other for applications where long lengths of solid stock are required, such as rub rails, strip planking, and spars. I'll describe the construction and use of each of these fixtures separately.

For cutting plywood I use a circular saw that has what amounts to an auxiliary running surface attached to the saw’s shoes: this second running surface projects out from the shoe at an angle of 83 degrees, and just barely clears the blade at its base. When the saw’s angle adjustment is set a 0 degrees, the blade is at a 7degree angle to the face of the fixture. The fixture is made of plywood and can be either clamped or bolted to the saw.

(See figure 1. For construction details.) When the fixture is properly positioned on the saw, the saw’s movable guard will be inoperative: I found that by completely removing this guard the saw could be fitted with an 8" diameter blade, rather than the stock 7 ¼’ blade. The larger blade extends the depth of cut of the saw, and so leaves less of a ridge to plane down on thicker stock.

In use, the saw is run against a straight, stiff board that is clamped 3" in and parallel to the edge of the plywood. You will need to maintain pressure on the upper surface of the fixture (see photo) to keep the saw running evenly. A couple of practice cuts will give you a "feel" for the technique. This type of edge cutting is quite hard on blades: a freshly sharpened steel crosscut blade will leave a smooth, splinter-free surface, but will only hold its sharpness for 50 to 75 feet of cut.

Because of the limited depth of cut of the blade, the saw will leave a bit of a ridge on 3/8" and thicker plywood. This ridge is easily removed with a block plane run with one edge always in contact with the cut surface of the panel.

The key to a full-strength joint is the use of a two-step gluing process: Step one is the liberal application of an unthickened epoxy/hardener mixture to the cut faces of the two panels to be joined. This can be down by brush or ruler. The purpose for this primary application of resin is to pre-saturate the exposed end grain so it won’t wick the resin out of the glue mixture when the joint is assembled, leaving a glue-starved joint.

After the initial application of resin has had at least 5 minutes to penetrate, you may apply a second "coat" consisting of your resin/hardener mixture blended with a gap filling substance, such as milled cotton, etc.

To join the panels lay the first with its cut face up on a wood floor (or other surface that can be nailed into) and place a strip of waxed paper under the edge of the panel. Place the second panel slowly down over the first, taking care that the waxed paper is not blown over into the glued face of the first panel. Align the panels by both sight and feel: inspect the mating of the grain at the edges of the panel, and feel for flushness of the top surfaces. Plywood is not always consistent in its edge thickness: you may have to split the differences. When you are satisfied with the alignment, tack each end of the joint with a light staple gun. Place a second strip of waxed paper over the joint followed by a strip of scrap plywood, which is nailed down, using 4d-5d bright box nails in a double row on 3" centers. Let cure 24 hours, remove nailing strips and carefully belt sand the joint until it is level. Any surface depressions left along the glue line can be filled and sanded, as can the nail holes. In certain situation, such as plywood on frame construction, where the panels are well supported by battens, you can join the panels as they are bonded to the hull. The procedure is the same as bench assembly.

The preparation of long lengths of solid stock is most easily done on a table saw. The ideal fixture for this purpose should require as little of the operator as possible, as fat as having to make adjustments, etc. The fixture I currently use in my shop requires only blade height adjustment. The rip fence need not be removed from saw: merely slide it enough away from the blade to provide safe clearance.

The construction of this fixture is straightforward: if you fabricate the cleat first and then attach it to a slightly over-width piece of plywood, you can run the assembly through your table saw with the cleat riding in the mitre-guage slot, which will give you an exact fit. You need only then attach the ¾" X 1 ½" clamping block as shown, and the fixture will be complete.

In use, the stock to be scarphed is positioned on the fixture with its end flush with the forward end of the fixture and clamped to the clamping block, taking care to locate the clamps out of the path of the blade. Position the fixture on the saw with the cleat in the mitre-guage slot and feed slowly into the blade.

To assemble the joint follow the two-step gluing procedure outline above, and clamp as follows: Place both pieces to be joined on the edge of a bench or long 2X4 laid flat on horses, etc. with the glued-up faces vertical. Place a 12" square of waxed paper under the joint and align the glued faces. Clamp both pieces to the bench with two C-clamps 15" or so apart. Fold the waxed paper up to cover the sides and place a ½" plywood clamp block on each side of the joint. (These blocks should be a s high as the stock to be clamped and slightly longer than the joint itself). Clamp these blocks onto the joint using only enough pressure to keep the joint from moving. You can then remove the clamps holding the assembly to the bench. Allow to cure for 24 hours before removing final clamps. If the finished stock will be subjected to extreme loads while being fitted to the boat, you may want to give the joints 3-5 days to cure prior to installation. Edge nailed strip planking can be joined as it is installed, which results in almost no wastage.

Once scarphing becomes a standard practice in your shop, you will note two things: First, you will see significant savings in material costs due to the ability to use standard sizes of plywood and solid stock, and by assembling off-cuts into usable pieces. Secondly, you will be able to use materials more creatively: for example, several of my designs utilize the bottom panels that are assembled with their face veneers oriented across the width of the hull, thereby increasing panel stiffness to a remarkable extent.