"Installing Thwarts, Building the Seat, Finishing the Gunwales"
The seats and thwarts were cut from alder left over from my Wee Lassie paddle. Dimensions are about 1”x¾ “. The boards for the seat will be oriented with the long axis vertical for additional strength (due to the width of the seat), and constructed with mortise-tenon joints. The thwarts will be similarly oriented.
The yoke is made of ¾” stock cherry. The only real question is where to mount it. I located the center balance of the canoe and lightly marked the gunwale. I then moved the yoke about 6” aft of center, and located the equivalent weight of the seat boards 18” aft of that point. I then re-checked the balance with my guess – it was right on. I’ll assume the bow and stern thwarts (and bow/stern breastplates) will balance out, and the wider, mid-bow thwart will not make much difference either.
I made a mockup of the placement of the seat (9” off the keel) and portage yoke (12” off the keel) with the 18” gap between. I then sat on the mock seat to assure I could be comfortable with the ergonomics. Looks like it should work (but doesn't paddle well at all!).
Now, what about the balance of the canoe? If I assume I weigh 200 lbs with all my gear on, at a distance of 18” from the balance point, that’s a moment of 300 ft-lbs. I can balance that off with a 50 lb pack 6 feet forward of the balance point. That should work too. A heavier pack can move aft more, and there’s room for a lighter pack to move forward. With no pack, I’ll have to kneel, which means I’ll need a pad for the front of the seat (which now serves as my butt-rest).
For the bow and stern “handle” thwarts, I placed them in about 20” from the stem on the gunwales and drew a pencil line to get the correct angle. Make sure the width at the gunwale line matches your form. I then adjusted the table saw blade angle to match and cut to the right size. Since this ends up being a compound angle (tilts in towards the vertical center line and back from the keel line), I’ll use the belt sander to match it up the rest of the way. This same technique will be used for the seat.I cut the forward (#2 thwart) 24” long and finished the ends as before. Again, make sure you match the width with the width of the form at that point. It ended up being about 6’ from the bow stem. Using a router and 1/8” radius rounding bit, round off all the thwarts, except where they mate with the gunwales. Using the correct bit, drill a hole through both gunwales into the thwart. Countersink and install the screw (I used 2” gold-colored deck screws). Repeat on the other side.
I cut the short boards 12” long assuming I’d use a 3/8” tenon. This makes for 11” between the long boards. Cut the mortises and tenons such that there is an 11” “hole” in your seat (or more, depending on your size). I cut the tenons 3/16” all around and ½” deep.
The mortise location was drilled with a 11/32” bit in two spots, and chiseled it out until I had a tight fit. I also rounded off the edges of the tenons slightly by hand with sandpaper. Using silica-thickened epoxy (or wood dust, in this case), clamp the seat together and let it cure. Once it’s cured, round off all edges with the router. I'll use lawn chair repair webbing for this seat, and will probably convert it to cane sometime later.
Clamped up Cured Load Test
I split some scrap cherry with the table saw, ending up with two ¼” thick plates. I then laid them on the bow/stern and traced the shape on the underside. You may have to sand the underside to get a tight fit. The plates are then screwed into the outwales (I'll use ¾” brass wood screws again). Then round off the edges with a 1/8” rounding bit and sand flush.
I oiled (Formby’s® Tung oil) all inaccessible wood parts before mounting (Kevlar side of gunwales, underside of breast plates, ends of thwarts, seat, etc.). I “painted” on the Tung oil for the seats to assure I got enough on before the webbing was installed. Everything else was rubbed on with a lint-free cloth.