Plan for my cargo trailer

[aerohead]

Originally Posted by PaulB2

I had seen that illustration in other threads, but was not aware of the 10 degree sides and didn't notice that the last item was the lowest Cd either (I was concentrating on the 3rd and 4th from the top). I also do not see what is different between the last two items. What does the cross-hatching represent?

I think the taper on a boat tail would be pretty easy (no need to compromise with that square box), but getting the transition smooth like the NASA van would not be. Hope that doesn't hurt too much. Also I have a feeling that adding the bottom part of the cone is not worth too much without also having a belly pan on the trailer. Or maybe a dam at the front of the trailer and skirts on the side, to exclude air? A belly pan would be pretty easy on this trailer, just hang some plastic roofing under there. Skirts and dam would not be that tough either, provided I find some material that can take a beating (I was thinking of plastic baseboard molding).

Right now in the Ram I have a home-made half-tonneau. I realize the gap is a big problem and probably is a lot of work to fix, which is why I'm thinking of just winging it with a lip on top to separate the flow for now. A vortex trap or two might not be too difficult, for what it's worth; but I'm guessing they don't help unless the Ram has a high cap.

freebeard nailed it,as the cross-hatching does represent a 10-degree plan taper,absent with all the other tails.
With the upper images you can see that the up-swept 10-degree diffuser was good for a 0.025 Cd reduction,or,8.7% (4.3% mpg).And yes,that's with a smooth belly up ahead.
*Another tidbit of info: the 1980s GM 'OPTIMUM' boat tail was very much like the image we're looking at,with the exception that GM's has graduated edge radii for all horizontal edges and constant radius rear vertical edges.A lot harder to construct,but they eliminate vortex formation,something Scott Funderburk (SP?) of Texas Tech reported.GM's tail is about 10'-6",but we can cut it off wherever practical.
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I don't know about an air dam on the trailer,but sides,which mimic'd a diffuser slope off the back of the RAM would be good.
*Tuft-testing around the fenders would show if you have any attached flow in that region.If so,you might be able to create fairings to smooth the flow there.