Here is an update on my mortised thwarts. (Identification of the canoe has been discussed in previous threads but with no firm conclusion, so it just tell people it is a Maine guide canoe, early 1900s, unknown builder.)
I was mindful of Zach's comments about the difficulty of staying square to the rail, so I made a jig for the router which kept it level and limited travel from side to side. And of course I did a test first.
The router bit was 3/16 wide and cut 1 inch deep. The thickness of the inwale, rib ends, and planking was enough that the router bit could not reach the outwale. (My outwales were made and installed, but will have to come back off for canvas in the spring.) My thwart ends were all more than 1/4 inch in thickness, so after the first router cut (at the top) I lowered the bit and cut again until the mortise was right.
Of course, the rotary cutter left considerable wood at the ends. I used a drill with a 1/4 Forstner bit to get the end of the slot right and then to chew away at the rest, and finally finished with a round rasp. The end result is not perfect but I'm pretty happy with it.
The photos are out of order, but here is the canoe as I received it. The inwales were clearly original but very fragile, the outwales were poorly executed earlier replacements, but the thwarts were also original and rock solid. I was able to clean them up and re-use them.
And this photo shows all three thwarts installed. You can see I have a flat spot in the gunwales at the stbd end of the first thwart, and a little too much curvature of the gunwales forward of that. When I hang the canoe up for the winter, I going to strap in that bulge good and tight and see if that will fix it by spring.
The other photos show the router setup, etc.
Once again, I want to acknowledge all the good advice on these forums! This job was a lot easier because I was able to draw upon the knowledge of others who did it first.
