I don't have a picture but I have lost one canoe over the edge of the car.
I used canoe blocks on the roof of the car and tied straps front and rear. I had a third strap in the center going through the door of the car. In rain, this strap wicks water into the car and in upstate NY, as the rain poured down I removed the center strap.
A strong gust, cross wind, pushed the canoe off the top and dragged long the side until I stopped. Needless to say, the canoe was broken in several places beside the damage to the skin.