roscoe1974 said..I've always had the opinion that a SUP or surfboard etc. placed bottom-down would create more 'lift' than top-down does (ie. the rocker line creates a larger presentable surface to the airflow from forward movement). Always have mine upside down, with the fins to the front, and in 22yrs+ of various types of board on the roof, have never had an issue with boards coming off nor damage to racks, car or boards themselves

Yeah, I've always done the same with my shortboard surfboards too (bottom facing up) but with the fins at the back. I understand there is a school of thought that fins at the front would prevent the board from sliding out under the straps but have just stayed with the fins at the back. I guess it looks better too, LOL.
My style above has worked so far, touch wood. That said, back in about '93 or so my mates were doing about 80km/h (the speed limit) on our way to the beach when one of the roller roof rack brackets snapped. I'll never forget that sound nor the image when we turned around in our seats to see of all our boards in mid-flight.
Luckily there weren't any cars behind us but the carnage of the boards was brutal. Deadset, my 3 day old Byrne copped it in a bad way by landing on a paddock fence post. I hadn't even surfed it yet.
As a result of the smashed rail, I had to sit out that surf session. And to make the day even worse, the surf was bloody cranking too.