Most of my trips are local, so short distance drives and low speeds. I always ensure that my straps are tight and that the boat is secure, but I don’t do bow or stern lines. On a long or highway trip, I would use them.
I imagine there is a mix of never, sometimes and always.
If you use them, what’s your setup?
Always bow lines. No matter what. It is the best way to see that something has shifted up there. I criss-cross them across the car, bow line for the boat on the left side goes to the right hood loop and vice versa. That way the bow lines are pulling the boats against any interior braces like the highest part of the cradles.
For long trips I find one strap at each location to be on the foolhardy side. Two straps at each point hence four per boat, so no major issue if one fails. Goes for the Hullivator side too, only diff is that the second strap is run after I have the cradled on the roof.
You can put me in the never column, but with one clear caveat; the racks on my vehicles have always been very solid and properly attached. I’ve always used crossbars with cradles adjusted to fit the specific boat they’re carrying. The boats are solidly tied down with straps and they don’t move, even on windy days at highway speeds. Speaking of which, I keep my speed down on the highway, both for safety with the boats and for better gas mileage.
This has worked fine for me, but if you have any concerns about the security of your rack system, by all means add bow and stern lines.
I use cross bars with gunwale brackets, so my canoes are incredibly secure. Once strapped down, I can shake the entire car by the canoe, without the canoe moving in the slightest
I always use them. Why? It is the old “belt and suspenders” approach.
The you drive your car? Do you wear a seat belt? Why you have air bags, but better yet, you hard ever get in an accident.
I use bow and stern lines for the exact same reason, it handles the case of unexpected failures.
As with most things, there is a point of diminishing returns and/or a point where the effort outweighs the results.
I assume you don’t have a roll cage, racing harnesses and a fire suppression system in your daily driver. You probably don’t put on a helmet every time you climb a flight of stairs or a ladder. Why not? They all protect you in the even of unexpected failures.
I get that they add a margin of safety, no question there.
Every single time since I had a J-cradle failure a couple of years ago. A plastic piece failed on the folding Yakima cradle I was using which caused the boat to twist out sideways which ultimately sent the boat down the highway. I’ve hauled boats thousands and thousands of miles without issue…except that one time.
Having bow lines could have either prevented the piece from breaking or given me enough time to see the issue and get shut down.
There is a lake about a mile from my house with a 30 mph speed limit. That’s the only place I don’t use bow and stern lines. I’ve had a rack system come loose on the highway, and the bow lines not only warned me, but kept the rack on the car long enough to pull over, and fix it.
A friend of mine who completely refused to believe that they worked had his rack and two NDK Explorers come off his car on the highway in traffic. Could have killed someone.
So yes, I always use them except to go to the nearby lake.
Bow lines pretty much always; unless it’s a short trip and I’m not driving over 30-40 MPH. If I’m traveling a ways at interstate speeds then I also use stern lines. I’ve had trucks pass me and the whole car shook from the air blast hitting the canoe. I was glad to have the additional security of the bow and stern tie downs.
I use Yakima round bars and gunwale stops. The towers that clip to the car door frame are OK but I don’t trust them as much as I do the towers that clamp to the factory roof rails. I’ve driven thousands of miles at highway speeds and never had a problem with either. Still, things fail; i.e., sh!t happens.
Tom
Absolutely always.
Our kayaks sit on two Hullavators, with two cam straps each, but we also tie bow and stern lines to loops under the hood and under the rear hatch. With loop to loop connections, Thule ropes with ratchet snaps, and a couple of half hitches for the excess it takes literally just a minute or two to put them in place.
Without the lines the kayaks seem solid but I’ve been rear-ended a couple of times - once really hard - and I don’t want my boats to become projectiles.
Always bow lines, seldom on the stern lines. The bow line is in case of wind shear. That can happen with either weather or a truck passing at speed.
I would rather do a little to prepare for the unknown than to be caught by it, unprepared.
Only time I don’t use bow/stern lines is when I’m moving the kayak on my C-Tug. 
Almost always on the bow line. Usually but not quite as often on stern lines. The very rare exceptions involve trips of under 10 miles on nearly deserted roads.
Based on bad experiences not specifically involving boats.
I have a Hullavator and Thule QuickDraw lines. Fast and easy.
Only time I haven’t is moving a boat down the public access to the lake where I live. But that’s been closed because of high water.
BTW, it’s snowing here. 
Most of my kayaks and canoes I always have bow and stern lines. Mostly because I can see if there is any movement up there without having to look up.
I don’t use them on my Epic because in it’s ultra layup I simply don’t want to put that direction of stress on the hull while it’s up there.
I only used them when rooftopping, which rarely happened. I don’t use them with my usual transport method of trailering. It is easy to check for shifts of position by looking in the rear view mirror. And now that I paddle a surf ski, strapping it down and having NO movement is easy without overtightening any straps. The shape of the fore-footwell lip and the tapering shape, combined with correct placement of the straps, means that if I set tightness for the maximum circumference at each strap location, any shifting at one end toward a smaller-circumference location is stopped by the greater-circumference end’s strap tightness. Similar action to having a rooftopped kayak placed such that the shape of the bow and stern lines always goes inward—toward the vehicle—instead of parallel.
I use both bow and stern lines for car topping (hullavators) in all cases except one. That is when I drive to our city beach to launch … about 1/2 mile at less than 25 mph. Through my front windshield I can see if the kayak(s) are wandering around. I avoid windy days.
I use a bow line when going on the highway, not if I’m staying on local roads. As someone else mentioned, I do also put on a second set of straps if going on a very long trip - I usually run those through the side rails on the outboard side as an extra backup.
One thing I used to always mention when I did a roof rack install (worked at a kayak shop for 7+ years) - most of the bow and stern line setups that are store bought have an open hook at the car end. If the rack starts to fail and the boat moves enough so that the line goes slack, there is a good chance that open hook will come off. I swapped mine out for a small carabiner. If your car doesn’t have a closed loop that you can snap the carabiner into, might want to use hood loops instead. Most cars made overseas will have stout metal loops on the underbody used to chain the cars down on the delivery ships. Domestic built cars often don’t have them, which can make finding a secure attachment point for a bow/stern line challenging.
Almost always. Only exception is when using my 8 foot support spread on the Goodboy Rack, I forgo the bow & stern lines for four launch sites within 10 minutes of the house on local, 30 mph roads. Always, always at highway speeds, or if using the 2 ft 8 inch to 3 ft 6 inch support spread of many car top/SUV racks I’ve used.
Always bow and stern. For one thing most manufacturers require them for their warranty to cover damage to your boar, rack, and car if the rack fails for any reason. Doesn’t happen often, but it does happen.
For another, if you use the car’s OEM roof components with your rack and ever watch crash tests you will often see the crossbars happily tumble off of the roof as the body flexes. In an accident the roof rack or other components will almost always fail before the straps holding the boat. Bow and stern lines can often reduce the risk of the boat becoming a lethal missile and injuring someone else or damaging another vehicle in addition to destroying your boat.
The following is an example of two errors. The owner was interrupted when strapping down his boat and failed to correctly fasten the front strap. Never let yourself be interrupted when strapping down a boat. In addition he did not us bow and stern tiedowns. The result was art highway speeds the front of the boat rose up and tore the entire rack off of the SUV. In the picture the boat and rack have been retrieved from the middle of the road and are now leaning against the side of his van. Note that the boat is still securely strapped to the rack at the stern. Fortunately no one was following close behind.
