but many of us have hit logs hard with Royalex boats and not experienced such extensive fractures. Obviously, the only way to find out is to repair the boat and start paddling it.
For bonding linear cracks it is definitely best to "gutter out" the crack using something like an old "church key" can opener as a scraper. In that event you are trying to maximize the bonding surface of the epoxy and diffuse the stress riser that occurs at the interface between the epoxy and the ABS. In your case, the cracks extend obliquely through the entire foam core and both solid ABS strata. My feeling is that beveling the cracks at the surface probably wouldn't gain you much, but you could certainly do it.
I would probably just clean the foam core well using denatured or isopropyl alcohol and allow it to dry well. Don't use acetone or MEK as it will dissolve the foam core. I would coat the exposed foam core with a coat of epoxy applied with an "acid brush" just before mating the surfaces and then use clear plastic packing tape to keep the surfaces aligned on the side of the hull you are not working on. If you use G Flex you have a working time of 45 minutes or so at normal temperatures. If the shape of the canoe does not look fair, you may need to use a sheet of thin plastic or stiff cardboard on the outside of the hull to maintain shape. I have sometimes been successful holding a stiffener in place against the hull with cam straps, but it depends on the curvature of the hull.
G2d who posts on this forum quite a bit feels it is important to remove the vinyl layer from the Royalex before bonding cloth to it and does so with a sharp chisel. His theory is that the bond of the vinyl to the ABS layer of Royalex is weaker than the bond of epoxy and cloth to the exposed ABS would be. I understand the reasoning, but I don't do that. I have seen quite a few fiberglass patches applied by others to Royalex boats delaminate, but I have never seen them take the vinyl off the Royalex when they do so. So I just rough up the Royalex surface with something like 80 grit paper and clean it well before bonding cloth to it. You can use acetone or MEK over the solid part of the ABS as a cleaning agent if the exposure is brief, because it doesn't soak in as it would in the foam core.
After bonding and fairing the hull inside and out with thickened epoxy, I would probably take the time to reinforce over the cracks with strips of cloth about 2-3 inches wide. If you are using fiberglass, you can feather the edges nicely by sanding. Aramid (Kevlar) does not sand well as it fuzzes up, but if you use peel ply over the cloth when wetting it out and remove it when the epoxy is still green, you will get a nice smooth edge. If you do this, try to cut your strips on the bias at whatever angle is necessary so that the fibers in the strips will cross the fibers of your large patches at around a 45 degree angle as that will maximize strength.
After reinforcing the joined pieces with cloth strips, it should be a pretty straight forward matter to apply your large patches inside and out. That will give you a two layer repair over the joined cracks inside and out.
Obviously, you could use E-glass instead of S-glass to save a few bucks, and you could use fiberglass instead of aramid on the inside to save a few more. In my view, your time is going to be the biggest investment in the repair, so I would pay a few bucks more and use the stronger materials. You would not need to completely fill the weave of the aramid cloth on the inside of you didn't want to. You would want to do so on the outside before painting so as to get a nice smooth surface.
Don't use aramid on the outside of the hull as it fuzzes up when abraded.
1 Canoe/Kayak Trailer
URCHIN Portable Anchor
Touring Kayak Paddles
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