How to Scarf Plywood for Boat Building
Benefits of Scarfing Plywood
Today most of us can only find 4' x 8' sheets of marine plywood. In order to build a boat longer than 7' - 8' we need to lengthen the sheets of plywood. By scarfing the plywood together we can make a piece of plywood as long as we need with seamless joints after finish work. By properly scarfing our plywood we can join it without butt blocks or any other form of reinforcement at the joint, unless the plans call for it. A proper scarf joint will be stronger than the plywood itself. If tested the plywood will fail on a proper scarf before the scarf joint will.
Step #1 Start with Good Quality Marine Plywood
Try and find the best marine plywood you can buy. Building a nice wooden boat is a big commitment of time and money. When you factor in plywood, epoxy, fiberglass, sanding paper, disposables, primer, paint, hardware, and rigging; spending a little more on good quality marine plywood is worth making sure your entire investment will last many years to come. I use the best marine plywood I can find in my area. I like Okoume Marine BS 1088 for my boat projects. Note that Okoume marine plywood is not the same as Luan found at our local home center.
Step #2 What Tools are Required?
There is more than one way to scarf plywood. You can use a simple hand operated block plane. A block plane is slow, but going slow does make sure that you don't remove to much material to fast. If you are only going to build one boat a block plane is probably the way to go. If you are going to build more than one boat or have a large boat project in mind, then you might want to use a power tool. You can use a grinder with a sanding disk and sand the required bevel. A hand held power planer is a good choice which works the same as a block plane, just faster. You can also use a power saw with a jig attachment. Using a power saw gives you a nice accurate bevel and fast.
Step #3 Cutting the Scarf
In order to make a strong scarf you need to make the bevel 7 to 1 or 8 to 1. I like using 8 to 1, for 3/8" plywood you multiply 8" x 3/8" = 3". So you need to measure 3" back and bevel the plywood to the mark. If you decide to use a power saw with jig, it will not cut the bevel completely as shown in the first picture below. You will still have to use a hand block plane to complete the bevel.
Step #4 Gluing the Scarf
Once the bevel cuts are ready then you can setup to epoxy the joint. I like to epoxy the scarf while the the plywood is on my shop floor. I chalk a straight line on the floor as a reference to make sure my sheets of plywood will be as straight as possible. Then I lay down some wax paper under the joint. I mix up some epoxy and wet both pieces of plywood. After both sides are wet out I mix up some thickened epoxy with silica and apply the thickened epoxy to both sides. Once the thickened epoxy is applied, I flip over one sheet and line up the joint. I apply another piece of wax paper to the top of the scarf and press the joint down with a straight 2x6 and some bricks. You want to make sure you apply enough pressure to hold the joint and allow for some squeeze out. You do not want to apply to much pressure which can cause most of the epoxy to be forced out of the joint. I then like to let the joint sit 24-48 hours before handling.
Step #5 Finishing the Scarf
Once the epoxy has cured you can remove the weights and inspect your joint. I like to go ahead and fair the scarf which will save me some work later. I mix up some thickened epoxy using microballoons to a thick syrup consistency, then spread evenly on the joint being sure to fill all low spots. When the epoxy has setup I sand it lightly with a belt sander until the joint looks and feels seamless. Do both sides and then you are ready to cut out your boat hull panels.