Having such a stack required me to switch from being rope guy to a strap guy. Ropes didn’t allow for a totally secure load without shifting. I used two straps per each stack. I tie off my buckles with a quick knot with the strap in case it slips. Then tie extra strap to the rack, just in case.
Since I paddle both kayaks and SUPs, often I have to carry both depending on where we’re going. Long before SUP, I had been carrying 17′ sea kayaks on my racks often for long distances. The enclosed pic shows how i mix both craft for a secure load. If i had more kayaks, I’d add my Yakima Kayak Stackers, which are vertical bars. In this case, the kayaks would go on their sides, giving me more space to add more boats. In windy conditions we tied the nose/stern down to the bumper to prevent shifting. I’d recommend doing so if you’re tying down long race SUPs as well.
My local paddle/surf shop Urban Surf graciously donated the rack pads which have worked well in replacing my former insulation pads which didn’t last long.
More Useful Rack Links: