“Fixing the problem"
Here's what we found after our problems (broken ribs, buckling, etc.).
Above is a picture of the last two ribs after the trip. On the right I've removed the stern-most rib for replacement. You can see the damage in the next rib also.
These are "coupons" I made using the new double-bias Kevlar, the old Kevlar, and either US Composites or West Systems epoxy. The top coupon has the US Composites 635 epoxy with 2:1 hardener, the bottom has the West Systems 105 epoxy with 206 hardener. They were laid up the same, Kevlar and 6 oz s-glass, sandwiched between wax paper and allowed to cure. The thickness of the US Composites coupon is consistent with what the new canoes are (~40 mils). The West System coupons are 55 and 73 mils! Naturally, the results are are consistent with the canoe performance:
Double bias Kevlar (from 2006) Twill Kevlar (2003)
The other thing I noticed is that the double bias Kevlar with the US Composites epoxy actually continues to bend. We saw this on the trip - the later in the day it was, the more buckling we experienced.
Here I'm removing one of the ribs. I've added reinforcement for the seat area in the second picture. I removed the seats for reinforcement, and for the rib work.
All set for repairing the ribs (left). I used 6" wide 10 oz s-glass tape from RAKA. A total of three ribs were completely replaced, all others were just covered with the tape. I used RAKA 900HP with their 631 hardener. I mixed up enough to do three ribs at a time (about a pint) - this is a fast hardener (12 min pot time). It's 80 F for the first day of October, so it did indeed set up pretty fast. I made sure to get it out on the canoe as fast as possible. The greater surface area avoids the "smoking pot" syndrome.
You can see here (left) that the 'glass is not yet totally wetted out (a little milky looking). One hour later the ribs are rock hard. The slow USC epoxy just does not dry hard enough for the strength we need here. Not their fault, but a faulty application on my part.
So, what did we learn altogether?
Shrink wrapping the form worked great. You get a wonderful finish on the inside. The rib material bond very well with the shell with just a little roughing up.
You need to use a thicker epoxy, or maybe a faster hardener with USC. Their medium hardener (3:1) may have been OK, but I was afraid of the shorter working time. I also liked the idea of a non-blushing resin. One possible solution is to use the slow hardener with the Kevlar, then the medium for the s-glass and fill coat. If I make another one, I'll probably use RAKA or West.
I'd like to try peel-ply to get a better finish on the ends. It should work.
Cutting the ribs with a hotwire setup worked fine, but may not be worth the trouble. They do look nice, however!
Use s-glass tape for the ribs, and use a fast hardener, at it's normal working temperature (75-85 F). It will cure up harder, and you really need the strength for the ribs. You could dent our ribs with a finger using the slow hardener with the thin epoxy.