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Boattails
The name came from attempts by early body-builders of expensive cars to produce and aerodynamic product. Examples are the speedsters by Dusenberg, Cord, Auburn and others in the 1920's and 1930's. These cars had rear ends shaped like the rear hulls of fast boats. The expression eventually came to mean a gentle closing of the rear body to reduce drag by achieving a pressure recovery that increased base pressure and reduced the base area; both of which lowered drag. A good example 9is found in modern high-velocity bullets and artillery shells that have a tapered rear profile to a small blunt base. Interestingly, the optimum 1/2-closure angle is 15 degrees, which is the best angle for the boattail designs beginning to be seen on van trailers.
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