Interesting Bill - perhaps we can all learn by sharing our experiences. What kind of hull construction was involved and how the boats were carried? Is it the deck, cockpit or main hull that fails?

The wood boat I mostly use is a 12' solo canoe 22lb finished with a (fairly hefty) seat. It’s built from 3 mm marine ply with cedar chine logs and small decks for and aft. It is not glassed, has no bulkheads and a single thwart. I carry it on a roof rack inverted with molded foam blocks between the gunnels and the roof rack cross beams. They do more to prevent dents than to spread the point load. For long trips I use 2 x 4 crossbeams which are stiffer. The boat doesn't even creak when I rachet the straps. The ply is under pressure along its width with very little cantilever force.

So where’s the difference? Is it the way the boats are mounted, the material, or is a kayak more vulnerable than a canoe? Maybe it’s just a question of how much tension is applied. I suspect ply may be more resistant to compressive loads than strips - glassing mostly provides resistance to tension. When (if) I get my stripper finished I guess I will find out

Maybe strippers intended for car-topping need strategically located bulkheads. I used to use ropes and trucker hitches, but I found I had to stop and re-tighten fairly frequently. The straps are quicker and easier to use, and I only have to stop once after about 10 minutes on the road to check them then I can drive to my destination without worrying.

The strap material is intended for the purpose: could be there are good and bad rope materials for this job. However, I don't try to get more tension than I got with the ropes. But the wisest words are yours: if conditions get bad, slow down or stop!