Question: How are you going to get the boat to the water? Seems more logical to use a proper car roof rack that way you already have boat and can go launch almost anywhere you want.
I have my sit-on-top kayaks on top of my trucks canopy on a Yakima kayak rack. Two straps for each...I have put many, many miles on that set-up without bow/stern straps and no problems to note. A useful hint for loading on top of your SUV, bring along a 3' high a-frame ladder or similar. Makes it a lot easier loading/unloading.
