: There's another explanation. Which way up was your boat? Boats are
: designed to carry a load in the middle. If you mount it on a car
: rightside-up as so many folk do then and tie it down at the ends
: then all the loads on the boat are reversed. Even if the boat
: survives, this treatment can cause hogging.

: I always mount my boats upside down; they don't need covering that
: way, a frame shaped to fit the top of the boat is simple enough
: to make and the straps go over the boat midships where it is
: strongest.

: I do put a safety line down from each stem to the car but that's so
: I can see if the boat moves; it's an early warning system but it
: will also keep the boat with the car if it does come loose,
: preventing it from causing an accident to a following vehicle,
: and there is not much tension in those lines.
I mount my boats on the car the same way you do. I keep my stem lines a little slack just to make sure I'm not yanking down on the ends of the boat. I didn't cinch down the center tie down straps as tight as I could have (the wet wood flexes more so it's harder to get a really tight tie down) and the boat shifted slightly right before it broke. I think this may have been a contributing factor as well (a little twist can do a lot of damage at the wrong point) but not nearly as much as the size of my stringers and crosspieces.

FrankP