Wallyworld

Sailing, Watertribe and boat building

Scarf Joints

One thing common to boats is a long, skinny hull. Available plywood is limited to about 8 feet long, but I need a hull closer to 20 feet… So one quarter inch sheet of plywood sheet is cut in half and joined between two other sheets, making a rectangle 20’ x 4’ x 1/4”

The joints are special.
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In order to assure strength, a few techniques can be used. Firstly is a simple butt joint, where the ends of the plywood butt up against one another, and a backer board carries stress from one sheet to the next across it’s spine. This joint creates a lump and a significant hard point in the hull, where bending will not happen. It also means that bulkheads must jump over the butt board. Finally, it isn’t pretty.

Another type of butt joint is to layer both sides with a layer or two of fiberglass tape, which adequately bonds the two boards, but again, adds thickness to the joint area. Subtle, but real. Sand it away, and there is no strength because the tape’s threads are cut.

Anatomy of a scarf joint

Finally, you can create a scarf joint, a diagonal cut at the end of the boards to be joined that match up. This reduces the length of the joined boards by 1/2 the length of the diagonal cut, but ensures that only the same thickness of wood is present across the joint as the thickness of the wood being joined. The epoxy between the boards is actually stronger than the boards to a degree, so a “hard point” is created, but that can be located at a bulkhead where the wood wouldn’t be bending much, anyways.
Scarf joint
The only problem making such a joint is… how do you cut that diagonal, anyways?
Scarf jig
The solution I tried was to create a box to support a router, and move that box incrementally up a pair of angled boards which straddle the plywood sheets. After much trial and error, this was found to be reasonably accurate, and was then touched up with good old sandpaper. Turns out I should have skipped the preliminaries and gone with good old fashioned sandpaper to begin with, but that’s another story.
Once the diagonal end cuts are made, then it’s a matter of stretching plastic sheeting across the tables (2 eight foot tables and one 16 foot table provided support for this LOOOOONG glue-up) and prewetting the joints with simple epoxy. This coat soaks into the end grain of the boards, preventing a dry joint when gluing up. It turns out that capillary action can suck the epoxy right out of a joint and starve it of support. Next, a mixture of epoxy and wood flour (basically dust collected from sanding wood) is mixed and spread onto the joint much like mayonnaise across bread. The two boards are fitted together, and compressed from above by weights placed on boards which are on plastic sheeting stretched over the joint. The plastic sheeting thereby prevents bonding the eventual hull to the table or to the weights above.

Then, you sand, find and excavate any irregular areas, fill them with more epoxy, and sand again until you can’t tell by feel where the joints were. The result is impressive. A 20 foot by 4 foot sheet of 1/4” plywood!