A couple of sailor's hats crossing Lake Champlain:
It is my usual practice to tie both the bow and stern down when car-topping -- see the 16' OT Ideal above. And for a long haul, I often also tie a thwart directly to a crossbar, something that can't be seen in the pictures above. But sometimes tying the bow and stern down is not practical or possible -- the cocoon protecting the one canoe left no practical way to run lines to the canoe itself. If I can't tie down both ends, I like to at least tie down the bow, even if it means improvising an attachment point.
However, you will notice that the amount of stern of both canoes overhanging the rear of the rack is longer than the amount of bow overhanging at the front, in the pictures on the ferry. This is quite purposeful -- a canoe wants to act as a weather vane and with the longer stern overhang, the airflow tends to keep the canoes aligned properly. If the bow overhang is the same as (or worse, longer than) the overhang at the rear, the overhanging bow becomes a lever arm that the weather-vaning airflow work with to wrench the canoe off. On this trip, since the canoe in the cocoon was not mine, and was particularly valuable,
I kept my speed well below my usual brisk pace this past summer while taking it from Assembly back to builder Steve Cayard, who has been doing some touch up maintenance for Ken Kelly.
The canoes in the pictures posted above by Benson all have a longer rear overhang. I forget what the issue with the blue Morris was, but the best I could do there was to have equal overhangs -- making the improvised duct-tape bow tie-down very important.
On a truck rig like Craig Johnson's, the carrying bars are far enough apart (8 feet) to greatly lessen the overhang and therefore greatly diminish the leverage that wind can bring to bear, making bow and stern lines unnecessary; the lever arm of the bow and stern is not long enough to create a problem -- most canoe livery trailers also have the carrying bars far enough apart that such lines are not necessary. But on a car-top rack, the bars are not that far enough apart (on my Subaru, 38 inches), so bow/stern lines become useful, and critical on a car like a Mini.