Scarfing joints is a giant task best left to the professional. Any Dunn Lumber can order you a 16' long piece of ply for the bottom, pre scarfed. A company in Oregon makes that stuff. On my old woodie, I used 1/4 UHMW for the bottom. You can buy this for the same price as ply. UHMW is a super hard, super slippery plastic that is virtually indestructable. There are some very specific installation instructions, but once the "shoe" is on the bottom of your boat it will slide off rocks (and the launch) faster that you can spit. You secure it with Silicon Bronze screws into the bottom of your boat. UHMW is available at Laird Plastics in Seattle.

1) Cut the UHMW to the proper size for your boat bottom.
2) Set UHMW out in the sun to get it warm. Leave your boat inside in the shade. The UHMW will warm at a faster rate than your boat, so if you screw it in cold, it will expand and be warpy on the bottom of your boat on a warm day.
3) Place the UHMW on the boat bottom, drill counter sink screw holes in and place screws. You need to work quickly while the UHMW is warm.
4) Once you've got the entire shoe screwed in, back each screw out one at a time and dip the screws in 5200 Marine Sealent and replace them. This will seal the screws in the bottom of the boat.

You will not be dissappointed in this bullet proof bottom. You'll never have to touch it again, and it is slicker that snot.