[orbywan]

About a month ago I decided to build a boat tail for my 95 E-350 Ford 25 foot turbo diesel Class C motor home. I read the aerodynamics study NASA did at Dryden Air Force base back in the late 60’s on the squared off E-150 van about 20 times, as well as several other studies, drew up some plans and three weeks later and some serious sweat the prototype is done and tested.
Photos of the process are posted below. I used ½” square tubing for the ribs. I determined the shape I wanted, hammered out a ‘control’ rib, taped it to the bench and then built 28 more just like it, so the four sides have the same shape coming inward. Next I built the rear frame, then made it bigger, then added ribs one at a time until I had the shape.
The basic frame weighs about 75 pounds, which is about 25 pounds heavier than the rear bumper that was removed, so there is a net gain of about 25 pounds plus the weight of 5 sheets of coroplast, and a bunch of aluminum pop rivets.
I tried my best to replicate the rounded corners of the body on this vehicle because it makes sense to me that going from a rounded corner right to a 90 degree square corner wouldn’t be conductive to smooth air flow.
This creates lots of compound curves, which are a pain to cover, but as far as the prototype skin is concerned it wasn’t too bad. The four corners were the worst so I used monocoat to cover them, which is heat shrink plastic made for covering model airplane wings and fuselages, available at most hobby stores. The larger areas are covered in coroplast. You can see in the photos where the panels had to be cut and trimmed to allow for the compound curves. Not much modification considering the shape.
In an attempt to keep the air flow over the roof area good, I also faired the roof A/C housing. According to the tell tales, that worked out really well, although now that I know the overall fairing design is good, now I have to get the A/C to work with the fairing in place. I think maybe if I louver the hell out of the final cover it will be good.
I’ve only done one road test of the new design so far but it was very encouraging. Getting up at 3AM to beat the heat and wind, we (my wife insisted on going, she thinks the boat tail is great, how cool is that?) drove from Tucson to Phoenix, which is about 1,000 feet lower elevation, to Apache Junction, a total distance of 135+ miles, one way, with lots of subtle hills and valleys in between. We didn’t use the A/C and we ran right at 57-58mph, since that is the ‘sweet spot’ for this drive line (7.3L turbo diesel Ford, A4OD auto trans, 4.10 tail gear).
The drive up yielded 17.26 mpg, compared to an average of 13.1 for the same trip sans the boat tail. I have to say by the time the tail was completed I was having my doubts this would make much of a difference so I was really happy with that. The return trip we used the A/C the whole way, and had an intermittent crosswind of about 10 to 20 mph and got 14.68mpg. That works out to a two way average of 15.97mpg. Not bad for a first try under those conditions.
Anyway, it will take a lot more testing to know the accurate numbers of course but I’m very happy with the initial results. I also have a ton more work to do before this is anywhere near ideal or finished. I still have to skin the rest of the under carriage. For this test, about the last 3 feet of the belly pan was installed and I’m guessing it was catching some air in there. So far the cost of this extravaganza is about $225 for materials, which was way worth it just for entertainment value, for me as well as my friends. If I had 5 bucks for every time someone has said, ‘What the HELL is that?’, the materials would be paid for. : )