After paddling my son's shiny new Ocean Kayak, my old, often-repaired Indian River canoe felt like paddling a waterlogged door. So in the interest of chasing fish along the Florida Gulf Coast's oysters and islands, I decided to add a sail. The rules of the project were that I spend a minimal amount of money and that I use what's already around the house as much as possible (it's summer and I'm a teacher).
Traditional sailing is all about performance, but performance is a relative term. My goals were to maintain my normal paddling speed (about 2.5 mph, according to Garmin) without paddling, to keep things simple, and to not ever bail out a swamped canoe.
I did some sailing when I was young, so I sort of knew what to do, and I had some old sailing odds and ends in the attic, but everything I added could be made with materials from the hardware store.

Sailing is all about the balance between the sail's center of effort and the boat's lateral resistance (imagine holding a sign at a windy protest rally - if the stick is in the center of the sign, it's balanced, if it's off to one side, the sign wants to swing downwind like a weathervane). More pressure in front of the leeboards=swing downwind, more pressure behind=swing upwind, so all the rigging needs to be as adjustable as possible for the first few attempts. I ordered the steps by how difficult they would be to change.
1. mast placement
2. sail rig
3. leeboards
4. a way to steer (technically you can steer with the COE/LR balance but the COE changes as the wind speed changes - grrr)

Many thanks to Tim Anderson and thousands of other online canoe sailors and boat tinkerers!

Step 1: Mast Placement

I was going to use an old windsurfer mast so I cut a matching hole in a scrap piece of 2x4 and fit it just behind the front seat (which is really the back seat - you paddle a canoe backwards when alone). I attached it with long screws through the aluminum strip that covers the gunnel, but these were the only holes I made in the boat (holes in boats = bad). I wasn't sure enough of the placement to glue down the mast foot, so I attached a piece of work-out mat to the bottom of a 1x4 scrap and wedged it into place with some 1x4, secured with screws. I attached the old windsurfer mast foot, but a piece of 2x4 with a hole drilled most of the way through would work fine. The decision about placement was made partly for convenience - near the seat is less in the way - and partly because it "looked about right."

Step 2: The Sail Rig

I decided to make the mast about 10 feet, with the tack of the sail about 2 feet up. 10 feet also " looked about right." I tried to go with an approximate golden ratio as much as possible (16ft. boat, 10 ft. mast). My old windsurfer sail cut down to an eight-foot luff (front edge) goes out (foot) about 6 feet, which is on the small side of normal for a canoe sail. Since I can't sew, I hemmed the cut with white duct tape.
The original sail had a sleeve for the mast, which is simple and efficient, but I felt like I ought to be able to power-down in a hurry, so I replaced the luff edge with brass grommets and rope hoops and attached a halyard pulley to the top of the mast. I also ran the other end of the halyard down from the sail head so I can pull the sail down rather than count on graviity. I sealed the inside of the mast with spray foam, just in case.
The boom is a section of wooden closet rod from the hardware store that used to be my push pole. I attached a cut-away piece of PVC at the mast end so the boom snaps onto the mast but still slides up and down.

Step 3: Leeboards

This is my favorite part. Leeboards provide that lateral resistance, but don't require a big hole in the bottom of the boat like more traditional sailboats. They're also easy to remove and swing up when they encounter oysters, weeds and other boat-stopping hazards.
I made mine out of scrap 1/4 in. plywood, which is thinner than typical, but I'm hoping that because they are pressured against the hull when in use, they'll be stiff enough. Length was also a guess. I felt like there should be a little more than two feet under the water, but if I'm wrong, they're easy to replace. There are many, many shapes of leeboards to choose from. I took this shape from Leeboards.com's pictures of Danish sailboats because I though they looked cool. The top circle/bottom circle ratio, as well as the length/width ratio are also 1:1.6-ish

The thwart that holds them is scrap 1 in. sch 40 PVC. I screwed a 1/2 in bolt through a couple of PVC plugs and cemented them with J.B. Weld, then measured the Thwart for the widest part of the boat and glued-in the plugs. I'm securing this in place with straps until I'm comfortable with a permanent position. From just looking and guessing about the COE, I have their position narrowed down to within a foot, but we'll see.
The leeboards can be raised and lowered with lines that lead past my seat. They are attached to the opposite edges of the boards from the direction they pull, so they help hold the board higher when not in use.

Step 4: Rudder/steering Oar

As I mentioned earlier, a balanced sail rig doesn't actually need a rudder. I felt like I could trail a paddle behind me on the lee side of the boat and pry away enough to keep the boat from rounding up into the wind, but I didn't like the idea of reaching back behind me all the time, so I lengthened a breakdown paddle I already had with a section of aluminum shower rod and a PVC fitting for a T-grip. One of my kayak paddle leashes keeps the paddle attached down near the blade and I move it to the other side when I tack. The "tiller extension" PVC allows me to hold the paddle from the center of the boat, as well as temporarily bungee it down when I need two hands for some other sailing task (like getting a sandwich out of the cooler).

Have fun and be safe!