Scarphing 101

The scarph is not the knitted piece of clothing around your neck (that is a scarf), it is a joint used in woodworking.

I'll bet that the scarph joint scares more prospective boat builders away from stitch-and-glue boat construction than the taste of epoxy. But neither is really that bad. Like wheatgrass smoothies, epoxy is an acquired taste. And like holding your breath until you pass out, after making a few scarph joints you start to enjoy it.

The scarph joint is magical. You think plywood is limited to 4-foot by 8-foot sheets? Think again. With the magic of the scarph joint, the size of plywood is limitless. Once you master the scarph joint, you will be unable to resist the urge to create an infinite sheet of plywood. You will keep scarphing and scarphing until that sheet of plywood occupies all of your backyard and half of your neighbor's backyard. At which time your neighbor calls the authorities and you are carted off by the men in white coats.



Start by tapering the ends of sheets of plywood, preferably at an 8 on 1 taper.

½-inch marine plywood tapered 8 on 1

Sea Scull tapers in 5mm RevolutionPly

The tapers can be created in numerous ways. Some boat builders use the old-fashioned block plane

and simply hand-plane the tapers. But that is too serene for me. I need something way more violent.






Ahhh....the belt sander.

Why spend hours creating easy-to-clean-up wood slivers with a quiet block plane when you can spend hours using a deafening machine to turn plywood into microsopic dust that clogs your nose and lungs?







How about a jig on a circular saw?

You can make your own jig or I think you can buy the jigs pre-made. You get the same audio ambience as the belt sander, but, unfortunately, you don't get the pleasure of the microscopic dust. However, the circular saw with a scarphing jig has an olfactory benefit - the sweet aroma of the saw blade binding, overheating, burning the wood and ruining the saw blade. The biggest challenge of the circular saw is the limited depth. A 7¼ saw cannot scarph 8 on 1 in ½-inch plywood.



Better yet, the power planer.

It is 30 decibels better than a belt sander. The power planer can ruin wood faster than any other tool. You'll be so occupied trying to avoid hearing loss that you will plane halfway through your workbench before realizing that you just ruined $300 worth of okoume.



Makita 4-3/8 power planer
with John Henry scarphing jig

My weapon of choice...the power planer with the John Henry Scarphing Jig. I bought my jig over 20 years ago. Last spring when I realized I lost my instruction manual, I emailed John Henry Inc. I received a replacement manual a week later, free of charge.















Don't worry about imperfect tapers. Any resulting voids in a poor joint will be filled with thickened epoxy. The epoxy is stronger than the wood. Like Mo 'Poxy says, "The more epoxy - the stronger the boat." If you could build the entire boat out of epoxy it would be awesome strong. But, you would be broke since epoxy is expensive and the boat would weigh tons (epoxy is denser than water so it don't float too good).

However, if you are building a boat with a bright finish (bright: boat building term for natural wood finish) then pretty joints are desired.



Overlay the tapers and glue them together



A completed scarph joint in crappy CDX plywood.
The joint is there - look closely.




 

Scarph joint in okoume for CLC Chesapeake.
Ugly joint but the bottom of this boat gets painted.

Completed CLC Chesapeake with painted bottm.
Note the CLC Chesapeake under construction in the lower right corner 

Saturate the tapers with unthickened epoxy. Then use thickened epoxy to join the sheets. I thicken my epoxy with silica powder. Sometimes I add wood flour for color and flavor.

Do all of the epoxy work in one session with no epoxy curing between unthickened and thickened epoxy layers. That way you get a chemical bond in the epoxy, which is much stronger than a mechanical bond that results from gluing to cured epoxy.

Be sure to use enough thickened epoxy so that when you clamp the joint, excess thickened epoxy oozes out, indicating that you have enough thickened epoxy. Beginning boat builders will use too much epoxy, which is better than too little epoxy. With experience, you will be able to judge how much epoxy your scarph joints need. If I had a penny for each ounce of excess epoxy that I discarded, I would have a bunch of pennies.


Clamp the joint.

Lady C ½-inch marine plywood panels
with scarph joints screw-clamped

Wax paper - have lots of it around. Epoxy doesn't stick to wax paper. Put wax paper between the work surface and the scarph joint. Put wax paper between the clamps and the scarph joint. Use enough wax paper to accommodate the excess epoxy that will ooze out of the clamped joint. Used correctly, wax paper will prevent you from gluing your panels to the work surface, which pretty much ruins the panels.



Nemah 3/8-inch marine plywood panels with cinder block
weights for clamping. Notice the direction of the face grain
and the wax paper under the cinder blocks.

Almost any clamping method will suffice. If you can remove the excess thickened epoxy that oozed out before it cures, that saves sanding it off later, but, NEVER UNCLAMP THE JOINT UNTIL THE EPOXY HAS CURED.

I used stiff boards and screws to clamp the Lady C panels. Since the Lady C would be painted, I didn't worry about the holes where the screws penetrated the panels. After the epoxy cured, the clamps were removed and the screw holes plugged with thickened epoxy.

I used weights to clamp the Nemah and Sea Scull panels since I would finish the boats bright and did not want any holes in the panels.

Sea Scull 5mm RevolutionPly panels with cinder block weight
for clamping. Notice the polyethylene sheeting under the
panels. Epoxy doesn't stick to polyethylene

The quality of the joint is more important than the straightness of the final panels. Everybody enjoys a good joint. The final cut panel shapes
will be smaller than the scarphed panels with most of the excess panels discarded. So if the panels are not perfectly straight, it doesn't matter as long as they can accommodate the final shape of the boat.



Is the scarph joint strong?

Stronger than the wood. I've destructively tested dozens. After cutting the panels to their final shape according to the boat plans, there are excess scraps with scarph joints. I take the scarph joint scraps, say "Go ahead, make my day" and bend the scraps to failure. The plywood fails at the end of the scarph joint, every time. It never fails in the scarph joint.

The epoxy is stronger than the wood. The Mo 'Poxy, the stronger the boat.

So, there ya go. Scarphing 101