Best Taper for Boattail on Shortbus Camper

[Tahoe_Hybrid]

Originally Posted by stcyrwm

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.

I suggest a 6.0 liter Hybrid Vortec swap

that will double your MPG on the spot
you will get 18.5 city/20MPG highway

you will have to get a 6" custom drive shaft though as you will SOOOOOOOOOOOOO much torque at the rear wheels.... you will need a Heavy Duty rear differential with AUX cooler as well

remember length = leverage and your adding 20"inches
the wheel base of the hybrid vortec is 116 and that is 136"


since you have the Space you could wire up two batteries





it's about 5500 pounds it's the same curb weight as the GM hybrid it's do able you can also add rear A/C/heat





also look into the rear differential swap i'm sure the stock diff is 3.73 a taller 3.08 will give you better highway MPG (speedometer will need to be Reprogramed) at the expense of acceleration(but the hybrid does not have this issue as it has a ton of torque the 0-60 is 6.5-7 seconds)


or you can put taller tires which will change the final drive ratio ...(normally cheaper to do this)

[aerohead]

Originally Posted by stcyrwm

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.

If you were only dropping from the top,your best angle would be 9-degrees.
If you'll do a 10-degree side angle boat-tailing,plus a 10-degree upswept diffuser you can exaggerate the top slope to 25-degrees.Chamfering the edges will net you the drag minimum.It will be very much like the Aero RV.On a simple 'box' form,this tail delivers a 0.0934 drag reduction (delta-35.6%).This was all tested in 1984 by S.R.Ahmed et al.You'd find it in Hucho's text.