(Update of past cold crack experiences)
I have fixed a couple of wood gunwaled RX boats with cold cracks, and seen a few more. The only firsthand exception to the RX with wood gunwale rule was a very old RX Explorer with vinyl gunwales that cold cracked, not at the pop rivets, but for several inches at each and every floatation lacing hole drilled through the hull.
I was happy I went to look at that inexpensive used Explorer 20 minutes from home, and happier to have walked away.
Secondhand I have seen photos of a Royalex hull with vinyl gunwales that developed cold cracks below the pop rivets. But, those aside, it is almost always wood gunwales and RX.
The actual why is more of a mystery. All of the cold cracked wood gunwale RX canoes I worked on have been long term residents of the mid Atlantic region. It gets cold here, but nothing like further north, so the actual degree of cold may not be that much of a factor. And plenty of folks in the frigid northcountry seem to store their wood gunwale RX boats outside or in unheated areas, without backing out the screws and without problems.
So it is not just the cold, and it is not every canoe.
Why would some few RX canoes cold crack and many others, same make and model, stored in the same conditions, endure cold winters without any problem? I believe that beyond the usual RX and wood gunwale combination the root cause is an occasional manufacturing defect.
Quoted in the past tense, Royalex sheets were custom made in a process that combined sheets of vinyl, ABS, and foam and then vulcanized them together. The Royalex started out as a flat sheet, which was then thermo-formed. During heating, the core expanded, forming closed-cell flotation within the hull. At the proper temperature, the sheet was removed and placed on a platform, the mold is lowered on top of it, and the sheet was vacuum-drawn into the hull shape.
I think the important phrases are during heating, the core expands and at proper temperature. I had a Royalex canoe in the shop years ago in which I suspect at proper temperature was not achieved, the foam core never expanded sufficiently, and the vee hull bottom had all the rigidity of an inflatable.
My suspicion is that cold cracks are the result of some miscue during the manufacturing process. Not hot enough for long enough, too hot for too long, even some problem with the canoe being vacuum drawn into hull shape.
It seems like there was a lot that could go wrong in molding Royalex sheets, and a whopping load of RX boats were made. No surprise if the occasional hull escaped with hidden manufacturing defects.
