Joe, that sway is what I mean about the Tail Wagging the Dog.
On the other hand, a boat on the roof is something of a highway hazard too, particularly with lighter cars.
Leeneia, many lighter boats, and particularly in Oz and NZ, are made from aluminium. Or aluminum, if you prefer. See Nomimal's post for one.
They are so light that rowing them against the wind is hard work.I remember once..... the next land was Asia!