Wouldn't it be easier to scarf the joint and then inlay a very thin piece of wood along the scarf seam? That would hide the feather and, if you put it on after the boat was stitched and the inside was glued, it would be a snap to line them up perfectly. You could use model ship plank wood (you can get it so thin, you probobly wouldn't have to remove any of the ply) I think it would look pretty cool. A nice piece of mahogony or walnut to accent the ply and no complicated setup.
I have a pocked full of pennies and I'm givin them away two at a time
-David

: Hey Rehd,

: The joint takes out the middle ply almost perfectly, so it does not eaken the
: outer plys significantly. The main reason for this joint is not its
: superior strength, it is its looks. Scarf joints in plywood, show as do
: butt joints. Scarf joints tend to have either a feathery edge or a
: distinct lip. Butt joints sometimes leave discoloration from the epoxy and
: sometimes the glass tape is visible after finishing. The spline joints
: work great. They leave a nice crisp uniform line. The are also very easy
: if you have the appropriate tools. They are also much more self aligning
: than either scarf or butt joints. Don't get me wrong, scarf and butt
: joints work fine for stitch-&-glue kayaks, but spline joints are far more
: elegant.

: Mihke