Can't see your rack picture on your post. I carry a SW17 right side up, on a Thule rack, on my BMW 328i at 80 MPH between FL and Me. with no problem. I've carried my Ches. double at freeway speeds also. I tie the bow and stern down for freeway driving.... With good racks I'd suggest right side up in fine weather. SEEYA Jack   

   Here is another try at posting the image of my rack:

   JACKNLIN: Do you have some sort of cover for the cockpit opening?Thanks!

   I use Thule saddles very similar to those. Like Jack I've driven all over the east coast and even to Texas (I'm in Florida) at interstate speeds with one or two kayaks on the roof and no problems. Always right side up and I do have cockpit covers, mainly to keep the rain out. I also always use a bow and stern tiedown. Most cars have tow rings, if not you might have to get creative. 

George K

I'd also recommend right-side up, especially if you've put hatches in the deck. Between that thin coaming and the hatches, the deck is not a good loadbearing surface.

You definitely want a cockpit cover, especially if there's any chance of rain. The last thing you need is a 16-foot bathtub with a 1/4 ton of water in it over your head.  It will also keep out road debris, like leaves, trash, insects, birds, etc.

Boats in general and CLC's especially, are meant to smoothly slice through a fluid. From a physics point of view, air is a fluid, just like water, only less dense, so a low-speed boat will go through high-speed air very nicely. Just be sure to keep the boat as closely oriented as possible with the direction of motion.

Cross winds are another story. If you get hit by a wind gust from the side, such as crossing a high bridge or getting passed by a truck, the boat will definitely feel that. If it's not properly secured, it could be blown loose. Fortunately, that's an easy problem to solve and to check for. Just use enough tie-downs and, when you're done attaching the boat to the car, make sure that you can push anywhere on the boat without it coming loose. You should be able to move the car on its springs by grabbing the boat's stern with both hands and moving it around. The boat shouldn't move at all.

All that new stuff up on top of your car will increase air resistance. Because of the shapes, the racks will actually cause as much or more resistance than the boat. The main effect will be that you will need a bit more power to maintain your speed than without the racks and boat. You can make up for that by driving about 5 mph slower since drag is proportional to the square of the velocity. Full math available on request :-)

Good luck,

Laszlo

   One thing I've learned by sad experience is to stop after driving a few miles and check all my straps. No matter what I do at the beginning, I almost always find one or more straps to tighten. You want things nice and secure before you get to highway speeds.

 I do have a cockpit cover and for a large cockpit such as yours I'd suggest a fore and aft batton to keep it from flapping. SEEYA Jack 

   Where can I get a cockpit cover for my ML 16.5 ? Also can you point me in the right direction about the battens? Thanks for all the good info!

I generally use Yakima systems or a canoe trailer. Carring my kayaks right side up eliminates problems with hatch covers and other deck mounted things.  Upside down on the car and the hatch cover straps flap against the roof of the car.........anoying and paint damaging.

I get most of my soft goods from NRS or a lead from Paddle.net. 

You might check out these guys.

http://www.oakorchardcanoe.com/search.php?search=cockpit cover&y=21   You'd have to call to get the size right.  You could always have one made by a canvas shop.

 As Grumpy says any sail maker or canvis shop can make a cover and will know what I suggest about a batton. If none in your area you can make a templet and order by mail. SEEYA Jack  

Be carefull with these cradles as they are only put together with friction pins that can vibrate out over time. I bought a set and ended up throwing them in the trash.