I use the crank type straps on my wooden boats without damage. I don't think it's where the strap meets the hull that the damage occurs, more likely where stress is concentrated at a weak point like the cockpit. My boats either have no decks or flat ones, they are easier to support and distribute the load than with a peaked kayak deck. For rotomolded plastic kayaks however, I use a pair of wood crossbeams with slots cut in their top surfaces that fit the cockpit edge accurately: this prevents movement as well as distributing forces.

One note: I carry my boats upside down always. They catch less wind and are stronger in that position - and they don't fill up if it rains.

The crank needs to be out of contact with the boat and the straps should have a twist in each section to prevent annoying vibration. They easily handle highway speeds. There is still a need for a backup line in case of mistakes or when things go wrong, cops like that sort of thing as well, and I like a bow and stern line visible to the driver so if a boat starts to move it shows before the boat takes flight. For short trips I dispense with the stern line and hang a flag from the stern that I can check in the mirror from time to time.

To anchor the bow line, which will have to keep the boat close to the car in the event of the straps letting go of the boat, on my last vehicle I popped the hood and found a couple of decent size screws behind the grill, one on each side. I made up a pair of brackets from 1/8" aluminum and attached them with the screws; they poked through the grill and each had a 3/8" hole in the end for a safety line. My new vehicle is going to require attention this Spring to provide something similar, but it has a respectable roofrack which is a good start.

Careful attention to the interface between the boat(s) and the vehicle is essential, with arrangements to provide distributed support to the boat and the roof and plenty of friction to prevent movement. The unprotected top of a car is not an adequate place for a boat. A roof rack is usually OK but they're designed to support weight not resist lifting forces so make sure the boat is not going to lift up to any degree. Slight tension on the bow line will do that; when in doubt run the strap through a window. Too much tension on the bowline, especially with a long, skinny and fragile wood kayak, will cause damage, although not a problem with my relatively short boats.

For extremely long boats I like the idea of a support run down to a towing point, or maybe a trailer. And sometimes you just have to slow down . . .