I haven't noticed any problems with my trailer when I have the canoes on top and they're higher than the top of the vehicle. I would think an upside down canoe would be about as good as you can get at cutting through the air. One thing with a utility trailer to keep in mind is that the tongue of the trailer will likely be too short to get a good balance.  With the 17 ft. canoes on the trailer the center of gravity is too far back for my trailer and there is not enough tongue weight to keep the trailer from "pounding" up and down on the hitch (being aluminum, the trailer itself is very light).  I can't move the canoes forward without running into problems with turning. I could try extending the tongue but I solve the problem with 5-10 gallons of water on the front of the trailer when going on long trips.  So, my suggestion would be to plan for some easy way to get some weight on the front of the trailer when needed.