Unprotected ash is simply not very rot resistant when exposed to the elements. It has a relatively open grain that soaks up moisture. Repeated exposure to damp opens the grain more and eventually the wood splits. The ends of outwales, where the end grain is exposed, is particularly prone to this process, and obviously, any hold drilled into the wood is a point for water entry into the heart of the wood.
Scuppered deck plates and inwales do help promote drainage some. It goes against the grain a bit, but it is a good idea to drill a 1/4" hold right where the tips of the inwales join together at boat stems, as close to the molded hole as possible to allow drainage of trapped water since this is the lowest point when the canoe is inverted.
I am going to have to rerail an old boat I just bought and will probably use ash. Since this is basically a flat water boat, I might go ahead and use a more durable treatment than penetrating oil to avoid some maintenance work in the future. I am thinking about applying several coats of System Three clear coat epoxy (very low viscosity with allegedly good wood penetrating power) and then covering it with varnish or polyurethane. I have generally preferred oil finishes on the gunwales of my boats, because the way I use them they tend to get scratched, so this would be something different.
Full Size Sail Rig
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