Like M, I have had my kayaks (17' and 18') on the roof rack set upon several Hondas and even a little Toyota Corolla. I don't see any reason why a 21'ter would work as well. Use bow and stern lines to secure the ends. For windy conditions, I'd add lines that went to the sides from the bow tip to the wheel area using clips that anchor on the body of the car. The clips would be J'shaped and foam lined to protect the car from scratches. Hope this helps.

Oh yeah, I have sped along at 100 mph with the kayak on the roof rack (bow line only) without troubles.

Robert N Pruden