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

: Hi Jim

: The spline joint goes inside the plywood on both edges and is totally hidden.
: I seriously doubt it will help the joint in any way as the plywood,
: whether 3 or 4 mm, will be really thin after cutting out the kerf for the
: biscuit or spline. I think this would possibly weaken the outer sides of
: the joint and mot give much more strength than a regular butt joint. This
: joint is good for adding extra glueing surface when putting thicker ( 1/2
: - 3/4" ) materials together in an end-grain situation. In the case
: where you are covering the materials with glass and epoxy on both sides,
: the actual glue bond falls a ways short of the strength gained by the
: glass sandwich. It's been proven over and over that the simple butt joint
: and/or the scarf joints are plenty strong for this purpose. In my opinion,
: this joint is way more work than the amount of useful strength difference
: it will add to the joint.

: The spline joint is very useful in cases where you are joining sides of a
: cabinet or corners where only the glue joint is holding two pieces
: together, then they affectively tripple the glue area and increase the
: strength of the joint measureably by adding some side grain to the joint.
: It's a good joint and certainly won't take away from the strength of the
: panels, but takes some extra set up time, extra tools and, in my opinion,
: doesn't add measurably to the joint in the glass sandwich layup. Course,
: that's just my thoughts on this and no proof to back it up. ???? Take it
: with a granule of Sea-Salt!

: Rehd