This how I canvas a square stern. Have done about a dozen with very good success. See pictures attached.
1. refer to the attached sketch , in the end the transom will have 2 layers of canvas with bedding compound in between.
2. Important to varnish the transom first, especially the perimeter, but I do the perimeter and the outboard (back side) as well, makes cleaning off any errant bedding compound easier. Bedding compound (I use traditional Dolfinite) should not be applied to raw wood because the wood soaks up the liquid portion causing the bedding compound to dry out with an eventual leak, The varnish prevents this keeping the bedding compound soft and "flexible" lasting for years.
3. With the boat upside down, lay the canvas out over the hull (see first picture)
-Mark the center line the entire length of the canvas. This is important because depending on the hull, the canvas (72" in most Old Towns) will just barely cover so need to keep it straight on the boat
-mark the center line on the rear of the transom on a piece of tape
-make sure there is enough canvas hanging over the transom that when the canvas is held against the side of the boat it goes beyond the transom at the sheer (a couple inches is plenty)
4. Picture 2. Mark the outside edge of the transom with a pencil. This is critical to get right. Be sure to hold the canvas tight to the sides when marking
5 Picture 3. Trim the canvas to the pencil line. Transfer the center line to the underside of the canvas (an inch or so is enough) at the transom, this is extremely important for step 7. Starting at the bow, fold the canvas back beyond the transom. The canvas is now upside down, off the boat and extending back from the transom. The trimmed edge facing the transom.
6. Picture 4. Apply bedding compound (liberally) to only the perimeter of the transom outside the rabett (none on the planking)
7. Picture 5. Tack on the canvas to the transom. Align the center lines. Starting at the center, place the edge of the trimmed canvas at the edge of the rabett, covering the bedding compound. Nail at the center line with a single nail or tack. I have used 5/8" ring nails, escutcheon pins with good success. Working outward from the center making sure the trimmed edge stays in line with the edge of the rabett. nail the canvas every inch to 1 1/4 inch all around the transom and at 1/4" from back edge. Make sure the canvas stays flat on the transom edge. May need nails closer together at the turn of the bilge. It will look like this will never work because the canvas takes on this weird shape, but trust me!
8. Picture 6. Apply second layer of bedding compound on the canvas just nailed on about the same width/depth as before
9. Picture 7. Flip the canvas forward to its original position. Don't pull too hard yet. This creates a fold at the transom. Pull gently or enough until the fold is flush with the back edge of the transom (start at the center and work outward). Proceed with caution, if pull past flush it almost impossible to reset the fold. Press down on the canvas to "flatten out" the bedding compound inside the canvas, pull a little more if necessary to keep the fold flush. Once you are happy.....starting at the center nail as before about 1 to 1 1/4 inches apart 1/4 inch from the back all around the transom. Now ready to finish the canvasing.
10. Temporarily clamp the canvas to the sheer with spring clamps in several places. Flip the boat to the upright position. Secure the transom end to something solid. See Picture 9. I use a couple C-clamps (with pads to protect the transom) and chain it to a post. Note the block of wood between the clamps to keep the clamps from twisting. Attach canvas clamp to the canvas at the bow like doing a canoe. Stretch the canvas using a come along likewise attached to something solid. It takes quite a bit of force to get the canvas to conform to the shape of a square stern, don't be scared! Keep stretching until the canvas curls over the entire length of the sheer, otherwise it is not tight enough. Tack off the canvas as with a canoe Picture 10, finish the bow like a canoe. I have found the weight of the boat is usually sufficient to keep the canvas tight, although some extra weight or a post to the ceiling wouldn't hurt.
Hope this helps. Questions? Give me a shout.
Bruce Larson