Flying boats! I have seen it happen and it has happened to my kayak on one occasion. The sinking feeling you get as you see your boat fly off the top of the car isn’t very nice.
In the case of my kayak there was a flapping sound, followed by the clunk of the buckle hitting the car roof followed by a whoosh as two kayaks flew off down the road! Fortunately on both occasions no one was hurt and the damage to the boats minor. If it had been later in the day it could have been a different story.
Tying your boat onto the roof rack correctly is critical. It could save both your kayak and car from serious damage and keep other unsuspecting motorists that little bit safer. Using good quality straps or rope is a must and if the strap/rope is worn then they must be retired.
I use Palm kayak straps (which incidentally now come in easy to spot colours). I always use two straps per kayak as it was a single strap that had failed when my boat went solo down the road. When ever possible I use the shape of the boat to my advantage, i.e. place the straps in such a way that the boat cannot slide forward or back. The loose end of the straps are always given a couple half hitches before tucking the end of the strap out of the way so it doesn’t flap.
Whistling straps can be an issue and can prove to be very annoying on long journeys. Putting a couple of twists in the strap before tying it off can often solve this.