During the testing phase as well as during transport of the Space Shuttle on top of a 747, a boat tail was used:
Here I found that this was mainly to reduce the turbulence that would strike the tail of the 747, thus increasing yaw authority.
However, I can also imagine that the boat tail would reduce the base drag of the Space Shuttle, thereby reducing the fuel penalty of carrying the Space shuttle.
Is there any research that shows that the boat tail also reduced the drag, rather than just providing a cleaner flow to the vertical tail?