This page has some semi truck aero research data.
Aerodynamic Resources : Aerodynamic Resources for Tractor/Trailers
On boat tails. There's plenty of research from the 60's when they were all the rage on race cars. Early wind tunnel tests found that a sharply cut off "Kamm back" tail worked almost identical to a full tail, as long as the angles were correct to keep the airflow attached.
Look up some pictures of those old racers, especially the Shelby Cobra Daytona 427 Super Coupe, and you'll see that a concave back panel was often used. The Daytona Cobra Coupe also used a sharply cut off tail with a concave panel, but the effect was spoiled by the addition of a spoiler on top to counteract lift.