With all of my boats, I tend to lean toward scarf joints when joining two sheets of plywood together. Butt joints are quick and easy, but they also require adding a butt strap or at least some fiberglass to each side. A scarf joint done reasonably well will hold all of the strength it needs right in the joint. In addition, it leaves just a single seam on each side of the wood. It also preserves most of the natural flexibility of the plywood where a butt joint can create a flat spot in the bend.
To start, I typically go with a 1:10 ratio (the acceptable range is 1:8 to 1:12). This means that the length of the joint should be 10x the thickness of the plywood. For 1/4″ plywood, your scarf should be 2 1/2″ long. I will start by drawing a line 2 1/2″ in from the edge of each sheet of plywood. I will then clamp them one on top of the other with the top sheet stopping at the 2 1/2″ line on the bottom sheet. The goal here is to shave down both boards at the same time (pictured right).
Once they are lined up, I will either clamp or tack them down and begin taking down the edges. This can be done with a planer or a **sander. I usually opt for a belt sander, but be careful. A belt sander can rip down layers pretty quick and you don’t want to go to far. If this is going to be painted surfaces after, then just be concerned with getting the correct angle to provide the maximum gluing surface. It doesn’t have to be perfect if it will be painted. If it will be finished bright, you will need to take your time and make sure you are spot on.The thin edge of the plywood must be thin and uniform across the whole board. If you try to sand that down after the scarf is done, you will reveal the layers of plywood and finishing it bright will look pretty funky. As the edges come down, I use a small straight edge along the angle to view my progress.
Once everything looks uniform, it is time to glue. If you have an option, choose the strongest glue you have. This is a joint that may bend and flex alot and if it ever gave way would likely mean a complete failure of the boat. For scarfs I use West Systems Epoxy, but any of the epoxies I’ve tried are fine. Start by putting down a strip of 2″ packing tape right at the top of the joint you just made on both sheets. If the line you drew at 2 1/2″ is still there, the tape goes right along that line. This will keep cleanup time reduced. Dry fit the joint together and see how your joint looks. Clean up any spots that look off with a sanding block. To glue, I will coat boat edges with a thin layer of un-thickened epoxy. I will then thicken the epoxy to ketchup consistency and coat one of the surfaces liberally. No holidays here.
Once the glue is on, I will bring the two boards together at the joint and get them all lined up. ***IMPORTANT: Make sure you have a sheet of wax paper under the joint. Otherwise you will end up gluing your sheets of plywood to whatever is underneath it. You should use a long straight edge (I use a Johnson 6′ level) to make sure the board line up perfectly. Remember, you will only be able to see one side of the boards at this point because you don’t want to move things around too much. Once lined up, you will want to clamp or weight the joint. You also need to make sure that the boards won’t slide off each other. As you can see in the picture above, I do all of this work on a couple sheets of sheathing. Once the joint is together with glue, I will use a couple of small nails right through the joint to keep it from sliding off. Once that is done, pile on the heavy stuff and wait until morning to see your clean, flat plywood scarf.
Just because I tend to overbuild my boats a bit, I like to take the strength of the scarf a step further and I will usually throw a strip of fiberglass over the joint on each side. Nothing special about that process. Just make sure the joint is flat before glassing and that you sand down the edges of the fiberglass after it sets up so it won’t cause a bump in your final layer of fiberglass.
**Note: The belt sander that I linked to is not the sander I have used and tested. I hate my belt sander (Craftsman “Dragster” made by Black and Decker) and am likely going to replace it with the one I linked to.
Thank you for reading!