I'd suggest the flat mount type. If you get one that is just plastic, I attach some felt so it is easier on the hull.  I had a j-craddle. It's nice for saving space but not as stable. I had one snap off of my rack during a heavy cross wind on the highway. I was real lucky that I also had my yak attached to the rack by Laso locks. If not for the laso lock my yak would have gone flying down the highway and caused unknown havoc.

Bow and stern lines are also a must regardless of other factors. My 17Lt has been introduced to I95. Go thing I noticed it and was almost stopped.

I advocate a flat type rack, with the kayak hull side up. That way, most of the air pressure is pushing the boat down, rather than trying to rip it off the car. I have Thule racks with "Set-To-Go" cradles, which are niceley adjustable to fit the boat.

I have always carried my MC 16.5 right side up with just foam pads on the load bars.  That works fine for a flat bottom boat.  I initially carried my Shearwater sideways using Malone Auto Loader racks.  I recently changed to Thule cradles with rollers aft for the Shearwater, carrying it right side up also.  The main reason for the switch is to simplify solo loading.  With the autoloaders, I needed to stand beside the car, on a step stool, and lift the boat up into the rack.  After it slipped once and nailed me in the jaw, I went looking for a better way.  The rollers let me load up over the back of the car.  With the original setup for both boats I made two road trips from S. Calif up thru Utah, Wyoming, Montana, Idaho, and back, with never a problem. - Always used bow & stern lines.

PAG

I guess I am bucking the crowd but I use Yakima BowDowns that have the yak sitting on its side. I like the padding, and the space saving aspect. With long bars I can load the yak, a canoe (using gunwale brackets), and the rocket box all on top of my rigs. I have never had a problem at freeway plus speeds (yes I have been known to hit 80 plus mph when passing) or in cross winds. As long as your rack is secure and the boats are secure to the rack and you use bow and stern lines you, your boat, and your car are fine. My rigs are either a Honda CR-V or a Tacoma with a canopy or pop-up camper on it.

I've tied many, many dozens of boats onto vehicles over the last forty years+. Never, ever skip the bow and stern lines.

I sold both Thule and Yakima products for five years. The J-hook racks from either are good for maximizing space on your roof, especially if you have two wide boats to haul. OTOH, the rollers and saddles are good for loading a boat alone, especially once the weight exceeds 50 pounds or thereabouts or the combination of your height and the vehicle roof height makes a J-hook arrangement less than optimal.

The solution if you haul one boat sometimes and two boats sometimes: Buy a J-hook rig for one side and saddle and rollers for the other. When you have two boats, you will undoubtedly have another pair of hands to help load onto the J-hook side. Yakima makes the best rollers and Thule makes the best saddle, IMO. You can easily mix them in this fashion. If your budget allows, the Thule Slipstream with a very good roller on the back and a sliding frame for the whole setup is very hard to resist. I love mine.

I think the solution of one of each style mentioned above is a grreat idea.

For me though I have moved onto a Malone trailer. I carie two Wood Ducks a sassafras 16, and still need the rooffor the Kahalo!

No heavy lifting to the roof at all!

David

I have a chesapeake 18 and carry it on an F150 with Adarak Truck Bed racks. Yakima saddles in front and Yakima rollers on the rear rack. Both the saddles and rollers work well. But I do like the look of the Thule saddles with flat pads and they might work better for the chesapeake's hull shape. I didn't get them because my truck racks have a fairly large square cross section and the Yakima's were easily adaptable.