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A boattail tapered only horizontally OR vertically would greatly reduce the drag, but has an increased wetted area compared with a horizontally AND vertically tapered boattail, so it would have slightly higher drag. It is obviously a successful solution, witness the multitude of horizontal boattails (rays, flatfish) and vertical boattails (sunfish, angelfish) in nature. The Aptera has a combination of horizontal and vertical tapers. The Avion has a only a vertical taper to its boattail.
There is an advantage to having some horizontal taper to the boattail. Since it sticks out some distance behind the rear axle and moves in an opposite direction compared with the front of the vehicle, having some horizontal taper would make it less likely for the tail to clip nearby objects when making a sharp turn.
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