When traveling any distance with my boats, I have always used  Thule or Yakima racks with the canoe  brackets.  Once the brackets are fitted to the gunnels and strappend down, those boats don't budge. The canoes brackets are worth there weight in gold! I then tie them down at the bow and stern.  Solid as a rock.  They have gone over the Mackinaw Bridge and the Duluth Bridge in high winds and don't even budge.   

On the other hand, I don't paddle aluminum canoes.  If I did, I probably wouldn't care as much if it came off the car (as long as it didn't hurt anyone).  From the way I see people drag those boats over the rocks in the back country, I'm surprised people don't just drag them home behind the car.   :-?