In certain parts of the world boat theft is a serious problem and given that most whitewater kayaks are close to £1000 it’s a pretty serious and probably very lucrative business.
My boat storage at home is pretty secure but I do feel that my kayak is at its most vulnerable when it is sat on the car. My current car has a roof rack that securely bolts to the car which means that it is fairly easy to lock my kayak to the rack and know that it is relatively secure.
I have used one of these really simple Playboater Rackguards for a few years. It’s great for quickly securing the kayak to either the cars roof rack or even the cars tow loop/wheel if the boat is on the ground.
The key is to wrap the cable around/through parts of the kayak that are hard to loosen off or remove. I like a loop around the seat pillar and through one of the rescue loops/handles as they tend to be fastened on using security bolts anyway.
The ball can also be trapped inside of the car, although I do know of a couple of instances where doors have been wedged open to release the ball.
Given the thickness of the cable a determined thief could get past it, however if it slows them down and increases the chances of them being caught then it’s worth using.
There are also a number of other products on the market that are worth looking at and centre around the roof rack straps having some sort of inbuilt locking system.