On November 5, 2009, Darren Quick put up this post in Gizmag, about how a Boat Tail on a truck saves fuel. Even though it was not smooth and graceful, but rather slab sided and boxy - it still turned in "fuel savings of 7.5 percent".
He goes on to say - "The fuel savings, which also means a cut in emissions, were realized by the boat tail dramatically reducing the drag caused by the lower-pressure effect that occurs in the wake of a vehicle." The key thing is the trailing edge or tail end of the vehicle, is the moree critical aspace for airflow control, and wake managment.
He Shows that these were not just one-off, random tests, but long running hard comparisons, averaged out over time - with various changes to the Boat Tail length: "The lorry was driven for a period of one year with a boat tail of varying lengths and one year without a boat tail. The improved aerodynamics were dependent on the length of the boat tail, with the optimum length found to be two meters (6.5-feet)."
In his story, he also points out the many other aero-structures that Trucks have added to them today to save on fuel: "Trucks already sport a range of modifications designed to improve the aerodynamics of the vehicle, such as side panels, aerodynamic mudguards and mirrors, and roof deflectors. Of these only side panels are more effective at cutting fuel consumption (9 percent) so it’s hard to imagine we won’t be seeing trucks on the roads with a little extra junk on their trunks in the near future."
However - there are a few things I have discovered by attending a presentation at the University of Toronto, on Vehicle Aerodynamics - putting small vortex generators on those cab side panels causes the air to spin up and tighten up the air flow over the gap between cab and trailer that still exists, and cuts the interference drag at this area caused by air trying to escape from between the trailer and the cab, making a new wall of air, so the air stays attached - from cab to trailer.