Best Taper for Boattail on Shortbus Camper - Fuel Economy, Hypermiling, EcoModding News and Forum

stcyrwm · 10-11-2019, 12:57 PM

In the next week I am planning on building a boattail on my 2004 GMC Savana 3500 Shortbus Camper. I am looking at doing something in the 2 to 4 foot range for length. I would like to keep it on the shorter side but could be swayed based on fuel savings. At the moment I am leaning toward a compromise of 3 feet or so.

I am currently getting 11.25 mpg. The engine is the Vortec 6L Short Block V8. Overall length of rig is 20'.

I guess my main question has to do with whether there is an ideal taper for roof, sides and base? My memory from reading through a lot here in the past is that maybe 10 or 12% would be ideal but I wanted to double check that number.

All other factors being equal I would lean toward a smaller taper for storage space reasons but that is definitely secondary to potential fuel savings.

Any help here would be greatly appreciated!

Thanks,
Bill

PS
I did read the "Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)" thread and noted that several folks thought his taper was too aggressive but I couldn't find what would have been better % taper in that thread.

freebeard · 10-11-2019, 04:48 PM

How long is the short bus? Asking because reattachment. If it's long enough consider Mair:

There's a better diagram I should find. Mair worked on torpedoes so it translates to box vans and such. I fail to understand how people define a curve with an angle (central angle vs departure angle), but the terminal angle shouldn't exceed 22°, close enough to 45° for a full boat tail.

In your case a truncation, a box cavity or a base plate could be considered. Details in Search.

Hersbird · 10-11-2019, 07:19 PM

I think the degree needs to start at zero and then taper.
Take a far away side shot and then use this tool. Just cut off the template at whatever point you want the overall length to be.
https://ecomodder.com/forum/tool-aero-template.php

stcyrwm · 10-11-2019, 07:32 PM

Hi Freebeard,

Thanks for responding. The overall length of the bus is 20 feet.

Sorry, I guess I had been thinking in terms of straight lines not in terms of curves - for simplicity of construction. Going back to the RV Aero thread I can see he did curves also. Hmmm, I wonder how much of a problem that is if I stay with straight lines?

I could probably figure out how to do curves but I would still need to know how to design it all ie how to calculate the size of the rear end of the boattail. Again any sugguestions on how to calculate that? And then how to calculate the taper needed for the curve. I get that there might be an answer in the graph but I have no idea how to translate that into my situation.

Thanks again for the help,
Bill

freebeard · 10-11-2019, 07:32 PM

The Template and Mair are antithetical. One relies on a blunted fore-body and the other does not.

edit:
Simulpost! at 4:32

"In your case a truncation, a box cavity or a base plate could be considered. Details in Search."

A TrailerTail™ is a flatted low-res truncation. Box cavities can be curved or flat. A base plate is flat.
https://ecomodder.com/forum/showthre...ies-21204.html

stcyrwm · 10-11-2019, 07:47 PM

Hersbird, Do I have this right here? I was a little unclear about lining the 0 mark up with the apex of the bus but this seemed to make the most sense:

Hersbird · 10-11-2019, 08:16 PM

make the template a little smaller so it never is taller than the bus. The bottom line is right. Then move the template back and put the zero right at the back as that is the highest point and furthest back.

stcyrwm · 10-11-2019, 08:16 PM

Freebeard most of the links referred to in that thread are no longer working but I will search around a little more.

Hersbird · 10-11-2019, 08:20 PM

I tried what you were thinking initially, just straight lines and made a storage box out of it all. It did help, but I couldn't hardly put anything back there as it took too much weight off the tongue and caused trailer sway.

freebeard · 10-11-2019, 09:09 PM

I think the thread was about the broken links. I'd refound some of them, but that was then.

Look at the Template from fore-and-aft. It's hemicircular.

One good resource is AutoSpeed - Technology, Efficiency, Performance. And here's a slide from a UofCoventry presentation on boxed cavities. The Cobra coupe is another example.