Background (such as I recall it): I was one of a group of partners who bought a Blanchard Senior in early 1977.
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Classic Craft® repairing and restoring classic and antique wood boats for over 40 years. In the case of the new canoe, it appears that the outwales are either covering the upper edge of the hull canvas or is in fact what's holding it in place. I think the answer will lie in refastening the canvas to the frames (copper tacks) before you start on the new outwale. If the worst comes to the worst (and assuming a sound wooden structure,) the whole hull canvas could be replaced -- the technique here being to remove the brass rubbing strip and keel and bilge keels if any, tacking the canvas along full length along the keelson first, then working from one end stretching the canvas up both sides at each station till you get to the other end.
Stretch the canvas tight enough to remove major wrinkles, pull the canvas tight with pliers and secure at each rib with stainless staples, or brass or copper tacks. If you do not replace the canvas now, you will just be delaying the inevitable, and will be kicking yourself in a year or two removing the new outwales that you just put on. On a canoe, the term rub rails usually applies to a strip of wood running the length of the outside of the canoe, some 4-6 inches below the outwale.

And I second the recommendation to visit the Wooden Canoe Heritage Association site, especially the forums.
Also, if you need to do replacement of any of the ribs or planking due to rot, you will want to remove the canvas first anyway. Good reading is The Wood and Canvas Canoe by Jerry Stelmok and Rollin Thurlow available from our hosts.
The mast is somewhat shorter than it might be otherwise, and the gaff significantly longer. To get WoodenBoat delivered to your door or computer, mobile device of choice, etc, click WB Subscriptions.
And further, I think that you should remove the other outwale as well so as to be able to tighten the canvas uniformly on both sides, working from one end to the other, before replacing both outwales. You will need a series of tucks at the ends, and the best way to work them out is to study the original before removal. By June we all agreed that we had to have her hauled-out so we could do some "basic maintenance". The gaff is pulled up much more closely to the mast than in most gaff rigs, sometimes almost vertically.

If the outwale fastening is similar to my rubrails it should be a relatively easy process to remove the screws, remove the bits of rail, and fit a new rail in place of the original.
I am thinking that bending the wood and drilling holes for screws would be the most dificult part. When the boat was lowered back into the water the hull immediately flooded because the strips had shrank.
You could literally see sunlight and down to the bottom through the hull when it was first lowered into the water. The boat sat hanging in the slings overnight, and by the next day the strips had swollen-shut enough to render the hull (reasonably) water-tight, and it could be pumped out.
Repainted the hull, didn't like the color we first chose, and repainted the hull a second time.

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