Working With Short Strips

Making Scarf Joints

As noted in Chapter 8, planks long enough to be ripped for the 18-foot lengths required for the Virginia are not readily available, nor would they be easy to transport, handle, and cut if they were. It also seems that whenever a long plank is found, it will have a defect somewhere that either renders it useless for boat building or would require cutting around the defect, defeating the premium length. There are many more acceptable shorter planks available than there are perfect long ones. The task, then, is to build a boat from wood that is shorter than the finished boat. Joining short pieces with scarf joints to make the requisite lengths is the generally accepted method of doing so.

Scarf Joints

In this joint, the ends of the stock to be joined are tapered to a chisel point and the mating surfaces are glued together. In a contemporary strip built boat, the joint is cut and glued together on the bench before the now full-length strip is mounted on the hull. Gluing the joint as the two strips are mounted on the hull is possible, but misalignment or a poor joint is a distinct possibility. The bending and twisting stresses that strips undergo requires a joint that is at least as strong as the wood itself. A properly made scarf joint satisfies this requirement, and can be placed almost anywhere on the hull a natural full length strip would be used regardless of bend or twist.

A good scarf joint is almost invisible, and certainly does not draw the eye to it. When the joint is made using book matched strips, grain and color matching can be nearly perfect. If the joint is made with raw cut strips before any planing or milling of the bead and cove edges, the scarfed strip is virtually indistinguishable from a natural full-length strip.

To make a scarf joint, the ends of the strips are cut at an angle much shallower than a simple 45° miter. The actual numerical value of the angle is not important; rather a ratio of the thickness of the stock to the length of the cut is usually stated. This ratio can range anywhere from about 1:6 to 1:12 for joints where the cut is made across the wide edge of the stock. In practical terms, a 1:8 scarf means that for ¹/₄" thick stock, the length of the taper is 8 times ¹/₄", or 2 inches^[1]. A 1:6 scarf would have a taper length of 1¹/₂", and a 1:12 would be 3 inches long. A 1:8 scarf is recommended, which provides a sufficiently large glue surface while still being able to be easily cut with precision. The actual cut need not represent a given ratio exactly, but both strips to be scarfed must be cut exactly the same.

Which way do you cut the taper for the scarf? Do you cut through the narrow edge of the strip, or the wide edge? Almost without exception, scarf joints for strips, planking on traditionally built guideboats, and gunwale stock are cut across the wide edge. The resulting joint, when seen on a strip or plank, is vertical or at a right angle to the length of the strip. Scarfing across the narrow edge of the stock is sometimes used for joining different colored woods for decorative purposes on canoes and kayaks. The wide scarf has a gluing surface significantly greater than the narrow scarf, which is an important consideration when the joint will be stressed, as on a curve.

Making the Scarfs

Cutting the tapers on strips, either before or after milling the edges, is not at all difficult. The main points of concern are 1.) The strip must be firmly presented square to the cutting tool, and 2.) Both pieces must be cut exactly the same. The chisel edge should finish square across the strip. A disk sander with 80-grit paper and shop vacuum dust collection or a table saw with a simple sled jig can be used satisfactorily. In both cases, the disk and saw blade must be square with the worktable. In a pinch, a belt sander can be mounted with some sort of jig to firmly hold the strips at the desired angle. Safety is more of a concern with the table saw, since the strips hanging off the table require them to be tightly held in the jig. While surgeons have performed miracles reattaching severed limbs, it is rather doubtful that scarfing fingers back together was covered in medical school. A clamping arrangement on the sled is a must.

Figure A4-1. Cutting a scarf in a cedar strip using an angled guide block clamped to a bench-mounted disk sander.

In Figure A4-1, a "homebrew" disk sander is used to cut the scarfs. A block of wood is clamped to the table to guide the strips into the sanding disk at the appropriate angle. The strip is first held against the guide block and tight to the table, and then fed slowly into the 80-grit disk. When the chisel end is developed, stop feeding but hold the strip firm for a few seconds. When cutting action has all but stopped, keep the strip tight against the block and firm on the table and pull it to the left. This release action slides the strip along the angled guide block and lifts the cut surface away from the disk without damage.

Figure A4-2. Equipment required for scarfing strips.

Figure A4-2 shows the equipment required for the glue up and includes the glue bottle, two small C clamps, two short lengths of scrap strips used as gluing cauls, and a pencil. The glue is the same as that which will be used for gluing strips together on the boat. Each of the two cauls should be coated with wax (candle, paraffin, bees, etc.) to prevent the glue from sticking to them. A pencil is also required, and the gluing bench surface should be clear and flat for at least a few feet either side of the area where you will be making the joint.

Begin by aligning the joint - do not use any glue yet, just hold the joint together. Slide the pieces back and forth while at the same time feeling for thickness and overlap. This is difficult to describe, but once you have done a few, it becomes quite simple. You want to get the thickness through the joint to be exactly the same as the rest of the strip. Once you have the correct alignment, make a pencil mark for alignment reference across the top (Figure A4-3).

Figure A4-3. Once the joint is aligned dry, mark the top for alignment during gluing.

Now spread some glue on the cut surface of the joint - not a lot, just enough to completely wet the joint and have a little squeeze out. Using the waxed cauls, loosely clamp the joint. Before tightening the clamps, make sure the edges of each strip are flat solid on the bench, assuring a straight joint, and the pencil mark is in alignment. Now tighten the clamps to get a little squeeze out, but do not over tighten and starve the joint (Figure A4-4).

Figure A4-4. The scarf joint glued and clamped between waxed cauls.

Let the glue dry for about an hour more or less, and remove the clamps. Clean up the joint with a quick scraping to remove any squeeze out. If you are using strips with bead and cove edges already milled, immediately turn the joint over and clean the excess glue out of the cove before it has a chance to harden. Hardened glue in the cove will prevent the strip from fitting correctly when it is mounted on the hull. A simple clean-up tool can be made from a scrap piece of beaded strip with the end cut at less than 90°.

If scarfing is done on raw cut strips without the bead and cove edges yet applied, the strips can be sized with a thickness planer prior to milling the edges

Scarfing Gunwale Stock

The principles of scarfing thicker stock such as that used for gunwales are the same as in scarfing strips. The cuts must be square to the stock, and both cuts must be identical. The preferred glue is epoxy, mainly because of its strength and water resistant properties.

To glue a gunwale scarf, mix some resin and hardener only, without a thickening agent. A slow curing type is preferred to allow sufficient time for soak in before excessive curing inhibits it. Spread the mixture on both pieces to be joined and let it soak in. If it all soaks in, add more. Thicken the remaining mixture with the additive of choice. Wipe off any excess soaking epoxy and glue the joint with the thickened mixture. Apply only enough clamp pressure to hold the alignment and still get some squeeze out. Applying too much clamp pressure will force the mixture out and starve the joint, possibly leading to later failure. Rather than using flat gluing cauls as was done with strips, the wedge-shaped off cuts from cutting the scarf joints can be used to direct clamping force at right angles to the joint. Be sure to wax them first.

A simple sled used for cutting scarf joints for gunwale stock on a table saw is shown in Figure A4-5.

Figure A4-5. Scarfing sled for gunwale stock.

The sled base is ³/₄" plywood about a foot wide, so that it will span from the blade to beyond the miter gauge slot. A piece of hardwood is dadoed into the bottom to act as a runner in the miter gauge slot. The position of the slot is such that the edge of the plywood at the blade is actually a little beyond the blade. It will be trimmed later. A hardwood fence is positioned at the desired angle as shown, and temporarily left hanging over the edge of the base. Screws and glue hold the fence down. When assembly is complete, place the sled in the miter gauge slot and turn on the saw. Run the sled past the blade for a final precise trim, including the overhanging portion of the fence. A strip of coarse sandpaper glued to the inside surface of the fence will help hold the work piece from sliding. As mentioned previously, there should be a clamp on the base to hold the work piece against the fence and to keep your fingers from getting scarfed. Although not apparent in the photo in Figure A4-5, a stop on the fence prevents sliding the clamp into the saw blade.

[1] This is not strictly true. The length of the wide cut surface in a 1:8 scarf is actually ¹/₆₄" longer than 2 inches, since it slopes from one edge of the strip through to the other side. From basic geometry, it is the hypotenuse of the right triangle formed by the thickness of the strip, the length from the end of the strip to the start of the cut, and the length of the cut surface.

Appendix 4

Making Scarf Joints