I will be driving from Michigan to Montana with my 2 kayaks. They’re basic sit on hardshell kayaks. I have a Thule rack.
I’m looking for recommendations on how to tie these down for a 1600 mile trip at the end of February.
I figure strap them down to the rack and bow and stern lines to front and rear bumpers.
Anything else I should be looking at? I’ve never hauled the kayaks that far. Figure I’ll be driving 300 miles +/- a day.
Remember to check your lines daily and retension as needed.
I recommend underhood tie-downs. You want short, straight bow lines with one line pulling the boat left and the other pulling the boat right so in your case four bow lines (2 per boat). In the rear the bumper is fine…you want the lines anchored to the corners of the bumper.
I often double strap the boat to the rack too.
The kayaks would have to be pretty long to need this, but if they stick out any on the sides of the vehicle or more than 4 ft off the back, you need safety flags, either 12 x 12" up to 18 x 18" depending on state laws you travel through, and possibly lights if driving after the sun goes down (some states it must be lighted some minutes after sunup or before sundown).
It would be interesting to know what position you will have them on vehicle (bottom down, bottom up or in J cradles)? It is always a good idea to see what your boat manufacturer recommends.
If you use cockpit covers when bottom down or sideways (j cradles) make sure you use the strap (seals cockpit covers use them) that goes around the entire kakak (in addition to front retainer clip) to make sure it stays on at highway speeds. I have had one come off in rainy weather…scary noise it made.
Check the lines frequently during the day. Some cordage may stretch if used as bow or stern lines and need to be retightened.
If your rack’s crossbars are attached to factory roof bars, strap them to the bars as well as the rack. Watch where you park at night, brightly lit by the motel entrance discourages thieves.
I’ve driven many 500 to 1000 mile trips on interstates and mountain roads with sea kayaks on flat Thule bars on Volvo wagons, Subaru Outback, Hyundai Santa Fe and Mazda CX5. You have the right idea with the 1" cam straps and bow and stern lines. I second Kevburg’s recommendation that you ALWAYS wrap the straps around the vehicle factory bars if your rack is attached to them. I prefer to haul my kayaks upside down – seems to reduce wind buffeting and definitely is better if you end up driving through heavy rain. You will reduce drag by putting cockpit covers on them but be sure to fasten the covers to the boat somehow so they don’t get blown off with the wind. My boats are all flat decked Greenland style – you may need to invest in cradles to carry them inverted if you have peaked decks.
If I am going to be parked at a motel during a trip or otherwise be leaving the boats on the vehicle unattended, I l secure them to the factory rack with a cable lock. My boats have fittings that the cable can run through. They can be added if you don’t have anything like that on your boats by drilling a pair of holes to attach a stainless steel U-bolt through the hull (somewhere that it won’t interfere with self rescues) and put rubber washers or silicone on both sides of the fasten-through.
When I had a shiny new and costly boat that might attract a thief I would carry a lightweight 6’ by 20’ canvas painters’ tarp and “mummify” the boat overnight by wrapping it up using rope or long bungee cords
I’ve driven across Montana (ND state line to Butte) in the winter several times, and have noticed that they use some of the coarsest road “sand” I’ve ever encountered. Most of us would call it gravel. I expect RM hulls would hold up fairly well but would want to protect anything else from getting sandblasted.
Two straps at each usual point rather than one. For that distance you do not want to be worrying about a strap wearing.
Winter driving ,…be prepared to wash both vehicle and boats. Winter road salt solutions will coat both and need to be removed at the other end.
On the plus(??) side, winter driving can mean that the whole kit and caboodle ices to the car and you don’t have to worry about theft. 
(Obviously not a suggestion to rely on, but it is a potential complication upon arrival.)
Same as 100 miles, but have to check your ropes and straps when you stop.
We do 5000 miles on two round trips a year. We store hull side up and take an extra set of straps just in case there is a wear issue or one of the cams gets jammed.
We travel from Maine to Florida… Next trip next week. So far we have never had anything other than straps ice up.
Washing should happen with road salt but its not been above 20 for a while and I hate frozen locks. That is an issue boats or no. Your boats will of course deserve a bath.
We do find that if it rains or gets warmer than here the straps can stretch so the rule is check tension after the first five miles and then at every fuel stop thereafter. And check that your rack fittings are tight. Some need periodic tightening with allen wrenches.
I just did a 900 mile trip in stormy winter weather. When you are going to be driving at highway speed, I stop after about 20 -30 minutes and check that everything is holding together the way you think it should and slightly tighten the cam straps if needed. I stop often about every two hours and check the straps and tie downs and stretch my legs too. Sometimes with some boats/straps/wind conditions you have an unruly tie down that you need to check often. On my 900 mile trip nothing loosened much after the first check up.
If you do have a sun roof, it’s very helpful.
Also we learned from living in a Class A for a few years that you need to pay attention to fellow travelers and any “funny looks.”
Also important to walk around the entire rig and observe fairly frequently.
We have also had a couple times when I decided it was too windy and vetoed the captain. Be conservative.
As @Celia has said, we just double up on both the main tiedown straps and the bow and stern lines. If possible run the bow and stern lines to opposite corners for each kayak. If hauling upright use a cockpit cover and secure it. Our front tiedown straps go over the grab loops on the cockpit covers which are in turn clipped to the foredeck bungees.
If you have factory rails make sure that the main tiedown straps are fastened to the boat, rack, and roof rails, which will hold down the rack in the event of a rack failure.
One inch cam buckle straps in good condition are preferred for the main straps. Easy to use, tremendous tensile strength, and they don’t stretch. They shouldn’t loosen, but a brief check every time you stop is a good idea.
A cable lock or something similar is a good idea to secure the boats and racks to the car.
We took our kayaks from Illinois to Wyoming last year and not only used the straps on our cradles and under-hood tie-downs, but also lasso-like cables that send a “lasso” around each end of the boat, then sit inside each other with the ends tucked into the car. They not only provide another layer of security for driving, but they help make theft more difficult. kayak locking cables
re bow and stern tiedowns. We run our straps as vertically as possible. This makes for far less wiggle room if there is a rack failure. Had we had tiedowns going over the hood of the vehicle we would have had a catastrophic event in Colorado when a rack tower failed… With the bow line running vertically to a hood loop nearer to the windshield we actually could limp along on ranch roads( we got off the Interstate)
We car-topped 2 sit-inside poly sea kayaks roughly 1300 miles each way - from SW Ohio to PEI, Canada this past Summer. Two straps over each kayak (at the two rack/kayak support points). We also had bow and stern lines from both ends of each kayak - to the front and rear bumpers. We drove a mid-sized pickup - so the racks were spaced (one crossbar at front of cab and one at the rear end of the bed) about every third of each kayak’s length.
No problems at all. Most times we stopped for gas, food or to have our dog potty we would check all of the rigging. You probably already know - but stopping soon after you start out, to check the rigging, is a good idea because although it was secure at home - the first acceleration, braking and highway speed air forces can move things enough that some things might loosen.
Can’t comment on whether there are any considerations to be made due to the highway speed windchill in the Winter.
As others have suggested, I would also double-up on the camlock web straps.
In over forty years of hauling motorcycles on trailers, bicycles, and most recently kayak on roof racks, my rule of thumb has always been to check all rigging after one mile, ten miles, and every 100 miles (of course every time you stop as well).
Take all tools, allen wrenches, etc needed for crossbars and rack (jbars, saddles…) incase they loosen and extra cam straps and bow/stern tie downs. From experience I know this!
