Why a simple butt joint instead of a fancier puzzle joint?
Plywood comes in 8′ lengths so the the centre spar (aka: keel, stringer) requires joining. The paddle board CNC fishbones are joined with a simple butt-joint because a butt joint is much stronger than a puzzle joint when sandwiched with plywood. Since most of the sandwiched material is removed when routing the hole for the handle the sandwiched material becomes part of the handle blocking.
Tools required for this step
Titebond 3 or similar glue
Block Plane or something else that is sharp
Steps for joining the Fishbone
- Align spar along the edge of a bench or table with the notches all pointing in the same direction. The parchment paper ensures glue only sticks where you want it. Prepare sandwich material. A rectangle is fine since it is ok if it doesn’t exactly follow the rocker. The overlap is easily made fair with a block plane.
- Cover piece “A” with a layer of glue.
3. Clamp the fishbone to piece “A”. If you do one side at a time you can visually see that the alignment is perfect. Make sure piece “A” is clear of the notches or it will be a pain to insert the ribs later.
- Wait 1 hr or until the glue is dry before adding piece “B” to prevent the first glue-up from moving. Use a wooden wedge or something similar (screwdriver pictured) to clean glue squeeze out from the notches, as it is much easier to remove before it has cured.
5. Clamp piece “B” over night before handling the spar.
6. Use a block plane to clean up glue squeeze out and “fair” the sandwiched plywood to CNC frame.
I have tried fancier methods of joining the fishbone frames but this is the strongest and the easiest way of achieving perfect results.