A good group showed up at Greg’s shop to find out how to stretch wood. He is almost finished making a 16’plywood boat; the problem is plywood is only 8′ long. The answer a scarf joint that is stronger than the plywood. It took him several times to master this joint so the boat is now 15′ 8″. This turned out to be a blessing since boats 16′ and longer require more safety devices. There are two ways to make the joint. One using a router the other using a circular saw. 
Greg chose to use the circ. saw. The first thing was to make a fixture to cut the plywood at a 15 deg. angle. 
You then hold the saw on its side and make the cut on each the end of two pieces of plywood,
then clamp it and glue it together in clamping cauls.
Its hard to believe but the joint is stronger than the rest of the plywood.
He also used this joint on the trim boards but used a table saw jig for this. 
Greg found an article in an old Fine Woodworking magazine showing a scarf joint cut at a compound 15 deg. angle that makes amazingly strong saw horses.