W.A.Mair's Boat Tail (1969)

[aerohead]

Originally Posted by freebeard

What I am taking away from this is the acute angle at the rear would be 2x(22 or 23°) or, on average, 45°. Which is what I use on the 3-foot tail in the Collapsible Boat-tail thread.

When you truncate, doesn't a recessed box section work like a half-tonneau on a pickup, and further reduce drag? Is there a rule-of-thumb for determining the depth of the box?

45-degrees is good as long as the curvature leading up to it is as gentle as Mair's or the 'Template' for as minimum.If you use the 3.9 section,it will be even more gentle.
With respect to the half-tonneau,the low pressure of the captured-vortex is communicated through the opening to the inner face of the tailgate,which provides a favorable delta-P across the gate.
On a box cavity there would be no vortex preceding it in which to communicate the low pressure to the the inner face of the cavity wall.