Sorry I didn't see this earlier! I am one of those who made a kayak paddle out of two cheap "Walmart specials." Have to say that I was in a hurry, being impatient by nature, and didn't worry very much about precision, just getting it done. After all, it was a couple of $12 paddles.
A perfect scarf joint? I don't know. Nice thing about epoxy is that it can make up for a multitude of short-comings.
First, I cut the flared ends off of both paddles. I simple held (by hand,) both paddles stacked one on top off the other, the ends even, and used the table saw to cut both at once. This was to keep them the same length.
I knew I wanted the length of the joint to be about 4 times the thickness of the wood, so I made a mark where the cut would start and another further down the shaft, about 4 times the diameter of it. I thought that looked a little "short," so I moved the second mark a bit further down. When that one looked about right, I used the first paddle as a template to mark the second one. Then I camped the two paddles together with a pair of C-clamps, so that the top mark on one paddle was even with the bottom mark on the other. This way, I cold cut both paddles at the same time, and even if nothing was exactly right
, both cuts would be exactly the same
, which seemed to be the most important thing to me.
The only hard part was was cutting a piece of scrap wood to help me hold the paddles against the cutting guide at the correct angle so the cut would begin at the top mark on the first paddle and exit at the bottom mark on the second one. I didn't want to cut them one at a time, as I did want the two cuts to match a closely as possible.
I'm not sure if or how this would work for feathered paddles.
Once the cut was made, the rest was easy. I gave both cut faces an application of epoxy with no filler. When that was about cured, but still a bit tacky, I laid out some short scraps of 2X4's like railroad ties so I could lay the paddle down and be able to put some clamps on them to hold them together. This time I mixed in some wood flour with epoxy and buttered up one of the paddles with it (just a thin layer,)and laid both halves on the "rail bed." and brought the two halves together mostly by eyeball. The finger tips did the final alignment, feeling the far ends of the joint. Nothing more than that.
When the joint felt right, a pair of C-clamps was tightened just enough
to hold the two parts together without squeezing the thickened epoxy out of the joint,
and then the hard part, simply waiting for the joint to dry.
It has bee quite a while since I made that paddle, I can't quite recall, but I think I did use a single wrap of fg cloth around the joint itself. I has bee that long. I did make a second double paddle a little while after that. Both paddles are still holding up just fine. As for strength, that joint is the last place those paddles could break. The both have gotten heavy use and abuse, being used not only as paddles, but as push-poles, levers to move things, pry an over-weight 15 foot (+) pirogue out of the mud, that sort of thing.
Actually the part that needs re-enforcement is the blade end of those cheap paddles. The tend to splinter and crack. A wrap of fg cloth and epoxy around the blade would help a lot, and a bit of epoxy & wood flour protecting the ends of the blades wouldn't be a bad idea, either.
Might not be the info you were looking for, but maybe it will send you in the right general direction. . .
Spring Hill, FL