Scarfing Rub Rails

"There is nothing -- absolute NOTHING -- half so much
worth doing as simply messing-about in boats."
From Kenneth Grahame's "The Wind in the Willows"

Tapering Jig for table saws or radial arm saws.About $20 at Amazon.com

Scarfing Jig from The Sailor's Sketchbook by Bruce Bingham

Using the Scarfing Jig, from The Sailor's Sketchbook by Bruce Bingham

The Sailor's Sketchbook
by Bruce Bingham, NA
76 great do-it-yourself projects to improve your sailboat ... like building wooden blocks, designs for trailboards, rigging secrets, etc.

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How Scary is Scarfing?

The Weekender is simple to build. In the basic plans, the only essential tools you need are a circular saw, jig saw, power screwdriver and a few miscellaneous hand tools. No difficult, exotic or fancy woodworking skills required. That's a good thing!

That being said, several builders have found it difficult to find stock long enough to make the rub rails in one piece. They can find 12', or even 14' stock, but not the 16' long stock needed. Making each rub rail up of two pieces presents its own problems ... a simple butt joint would be hard to match up on the curved side of the hull. And besides, part of the strength of the hull is the stiffening effect of the un-broken rub rails.

Scarf joints are often used in boat building, and they provide an easy solution to the rub rail problem. A scarf joint is a long angled joint. The long angle provides a lot of "glue surface" for the joint, making it very strong. For a 1 x 2 intended to be bent around a hull, a 1 to 12 scarf joint is ideal ... meaning that for every inch of thickness, you need 12" of length. In the illustration below, we would be looking at the 1" edge of the rub rail stock in a top view, so "X" would equal 1" (well, probably a little less, since most 1" stock is really about 3/4" thick). The "7 to 12 X" gives the recommended range of scarf joint lengths ... from 7 to 12 times the thickness of the wood. For joints where there is stress ... like bending around a hull ... its better to use 12x's the thickness of the wood.

From The Sailor's Sketchbook, by Bruce Bingham

Looking at that joint, you realize something right away. You lose 12" of length off each piece because they overlap. So for our rub rails, which have to end up at 16', you can't use two 8' 1 x 2. You could use one 8' and one 10' 1 x 2 ... butted end to end they would be 18', of course, but after the angle cut and overlapping them to make the scarf joint, you end up with 17' long rub rails. A little long is better than too short!

There are a number of ways to cut this joint. First, you could use a tapering jig on your table or band saw. Adjust the jig for a 1:12 cut and taper the end of each rub rail piece. For a scarf joint to be successful, the mating surfaces must be smooth. Some light sanding is in order to remove saw marks from the mating surfaces. This is best done by using a sanding block to evenly sand them; hand sanding may produce low spots and weaken the joint.

Think Safety!
Boat building involves inherently dangerous activities, and each builder must be responsible for his own safety. Information presented in our articles are "builder to builder" communications, rather than from experts in the use of tools or techniques. We encourage all builders to read manufacturer’s instructions, follow all safety precautions, and be pro-active in seeking out more information on reducing the risk of injury or illness.

A simple jig can be built to use a hand plane or router to cut the angle. The left column includes a jig illustrated by Bruce Bingham, a naval architect, in his book The Sailor's Sketchbook. The jig is built with angled sides that produce the 1:12 ratio. The jig and rub rail stock is clamped to the bench, and a hand plane used to trim down the stock. Note that the plane is held at an angle to the jig, so that the sole in front of the cutter rides along one side, and the sole behind the cutter on the other side. A similar jig could be built for use with a router.

Gluing up the scarf joint is not hard, although some builders have trouble with the joint slipping. To prevent that, mate the pieces together without glue, clamp them temporarily, and drill a 1/16" hole from the back in the middle of the joint. You only have to go deep enough to start the hole in the front rub rail piece. Slather both pieces with Weldwood plastic resin glue or epoxy, put the mating surfaces together, and drive a finishing nail into the hole just drilled. Now apply clamping pressure. When using c-clamps, use small blocks of wood to help spread out the clamping pressure, and use at least two clamps, one on either side of the "center nail". For epoxy, don't apply too much clamping pressure; you don't want to force all the epoxy out of the joint. Let the joint cure at least 24 hours ... 48 hours is better.