Best Taper for Boattail on Shortbus Camper
 

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.

How long is the short bus? Asking because reattachment. If it's long enough consider Mair:
https://ecomodder.com/forum/member-f...1215134663.jpg
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.

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

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

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

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:

https://ecomodder.com/forum/member-s...o-template.png

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.

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

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.
https://ecomodder.com/forum/member-h...6-imag0252.jpg

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.
https://ecomodder.com/forum/member-f...31-1-26-08.png

Hersbird, Here is take 2. Is this right? Pretty minimal taper for the first few feet:

https://ecomodder.com/forum/member-s...o-template.png

Let go of the Template. It has a very narrow range of application, and is not fit for your purpose.

In 2-4ft, pace Mair, l/d (~3/6) give a 50% reduction in Cd. Of this one component.

The top and sides are straightforward, with a curve, slant or box step; the bottom needs special consideration due to the wake from the rear wheelwells.

If that's your vehicle here's a drawing I did for someone else. Hersbird proved out the pillars on the front corners.
https://ecomodder.com/forum/member-f...-1224-copy.jpg
...the theory (from others on the forum):
https://ecomodder.com/forum/member-f...01-2-04-41.png

So if I let go of the template then I need some help from someone on how to size the boattail, which I feel more confused about at this point than when I started (not necessarily a bad thing given that I might have been on the wrong track in the first place???).

The original idea was to do something similar to the RV in the thread titled "Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)". Several people commented that the angle of the curves he used were too much which brings me back to how does one calculate how to build something where the angles are not "too aggressive"?

I don't honestly know what this means other than maybe I am way over my head trying to do this, because of my own confusion reading this? I do get that a 50% reduction in Cd is good but everything before that is basically like another language, that unfortunately I probably don't have the time to learn right now.

Hmmm, "straightforward" but again if I let go of the template I have no idea where to begin other than eyeballing it based in the other thread I mentioned or going back to the original percentages I mentioned???

:confused:

I am going to try asking this a different way.

From what I have learned about boattail tapers it seems they could be as minimal as 5 degrees (based on bus diagrams that are from Hucho I think) and according to Hucho/Buchheim & Mair shouldn't ever go steeper than 22-23 degrees (that is via Aerohead).

Orbywan in his "Aero RV" thread that I referenced earlier in this thread seemed to get pretty good results from a taper on his boattail even though it was over the recommended 23 degrees max.

Assuming I am on the right track here I am trying to figure out which would be the best taper if I did something similar to orbywan's boattail.

Here are the 2 extremes that I am looking at (this is view from the rear of bus/camper):

https://ecomodder.com/forum/member-s...aper-calcs.png

Option A is with a 5 degree taper and is more inline with the aerodynamic template.

Option B is with the max taper recommended by Hucho/Buchheim and Mair.

Obviously I could also do anything in between.

- So do I stay with the 5 degree taper that is closer to the aerodynamic template and go with option A?
- Do I go with the max recommended degree taper and go with option B, because at least in that case I have orbywan's experince to fall back on?
- Or do something else???

Thanks for any help,
Bill

Given a 2 to 4 foot extension (Averaged as 3ft), following (pace being italicized because Latin) the work of Mair (shown above at Permalink #2) a three foot extension on an approximately six foot wide vehicle would approximate the first data point shown. Interpolating the vertical axis gives a drop from ~0.17 to ~0.06Cd. This result must be combined (somehow) with the Cd of the other aspect of the aeroform.

Paging aerohead!

I still haven't found a good example of a stepped box. See the wakeboards here:
https://ecomodder.com/forum/member-f...dynamics-c.jpg
and this:
https://encrypted-tbn0.gstatic.com/i...0hSi1vlUi_GOsQ
https://ecomodder.com/forum/showthre...tml#post603215

The general idea is that at the truncation you choose, there is a line on the Template that can be achieved with a flat/slanted piece, a curve that starts smoothly from the forebody, or a straight piece inset appropriately. The latter can be useful to exposing taillights. In each case the trailing edge is in the same place.

You bus ain't no teardrop. Instead of the Thee Holy Template, consider the NASA van.
http://ecomodder.com/imgs/knox/nasa-van-z.jpg
https://ecomodder.com/forum/showthre...tml#post323990
There'll be more about it if you dig. But this profile gives the gist, and follows Mair.

Hi Freebeard,

Thank you for your help and your patience!

The NASA van photo is definitely helpful. I could easily use that as a template for what I am doing. Interestingly enough the taper on the NASA van seems pretty darn close to what orbywan used for his RV Aero mod and he had really remarkable results according to his reports.

For what it is worth I did some rough calcs on the taper of the NASA van and came up with 14 degrees. Then I went back to the oerbywan thread and while there isn't as good a photo to measure from, his taper came out by my calcs at 16 degrees - roughly equivalent to the NASA van.

Okay, feeling much more confident to give this a stab...

Thanks again, Bill
:turtle:

Okey-doke.

That covered, look at Taking aim at air drag again. Ignore the 20% lost to the trailer gap. You still have the proportions of front vs rear vs undercarriage. Things like the high momentum mudflap and vortex strakes apply to you situation more than something borrowed from race cars.

For the fairing for over the windshield, look to Luigi Colani's trucks.
http://www.uni-leipzig.de/~mielke/ma...sich/truck.jpg
https://ecomodder.com/forum/showthre...html#post70669

Model it after what the tractor trailers are doing with their boat tail.

Model it after a vegetable steamer.
https://images.crateandbarrel.com/is...WSilicnFtROF16
Go from four-sided to eight-sided in the first two feet, then have a collapsible box cavity that extends from 2 to 3 feet when it's opens like flower petals.

Yeah, it looks like the template wouldn't work well for you.
Here it is on my VW showing how VW followed it, and how to continue the lines If I were to make a boat tail or aero trailer for it.
https://ecomodder.com/forum/member-h...1-17-40-19.png
On of the guys on here is the Airflow Truck company builder. This is their latest truck and the trailer tail looks like it doesn't have much of a taper either coming off of a box.
http://www.airflowtruck.com/wp-conte.../2018/07/1.jpg

Don't forget that the taper in the top view will be different, as it's applied at the centerline of the vehicle. Applied like that, it's only ~4' "tall", so you'll get a much more aggressive taper on the sides.

Still on schedule?

Hi Ennoread,

Are you saying that I might be underestimating the taper based in my calcs from the side photos? That I might be thinking it is 15 degrees or so when it is actually more?

If so, could you elaborate because I don't quite get it. I did wonder about that because I am calculating from the photo as it is a 2 dimensional object and obviously it is not. Is that what you are referring to by the view from the top? I don't get the ~4 feet reference?

Thanks, Bill

PS
I cut a piece of plywood for the base last night at the 15 degree mark and it feels like the taper is less than the NASA truck or the RV Aero vehicle I mentioned, so I am inclined to think you are on to something here.

I started building base yesterday. Moving slowly but that is okay. I am hitting road soon so at some point I have to just commit and build it but I am still questioning my numbers a little.

As I said in my other post the taper I cut at 15 degrees last night "looks" less than the NASA photos or the RV Aero thread I referenced. I do realize at some point I just have to commit and build and hope for the best.

My daughter is home until Tuesday so having time with her is more of a priority right now but I plan on hitting the road soon - so if it is going to happen, it is going to happen soon!

Bill

It's possible to overthink things.
Undershoot and you have a bigger wake; overshoot and you have separation creeping up your posterior.

edit: You might do well to focus on construction details. Are your panel's materials flexible or rigid. Curved is better than flat, but has to be fabricated, for instance.

This help?

http://ecomodder.com/forum/member-or...5-top-view.jpg

You'd need a top view to actually fit the template to, but it's pretty much a box.

Point is, when applied to the side view, the template barely curves in at all in 36". In the top view, it would curve in much more. Still not as much as the Dryden van, or aero RV, but more.

It's the same curve.

Side taper is double the top taper, but it's a half-body. The bottom (being a different case) upsets the symmetry with the top.

If you truncate The Template you find there're diminishing returns at the pointy end. Which leads back to Mair.

The pertinent question is how tight can the initial curve be (given the truncated length). It's not simple — case in point the greenhouse of the GM EV-1. It's 'too fast'.

Once again, 'premature optimization...'.

I went back and re-ran my calcs based on looking at the NASA van and the RV Aero Mod boat tails from the rear view and reconfirmed that the taper is in the 14 to 18 degree range.

Having said that, I am going to forge ahead at this point with construction. My daughter heads back to college tomorrow so that will free up some time. Here's hoping the weather also cooperates!

1). How many miles per annum will this RV be used?

2). At a 12% improvement, what is the annual reduction in gallons used? (45?)

3). Even at $3/gal, and the RV’er average of 5,000-miles per annum, it’s maybe a little over $100 in savings.

A bus like that really isn’t a highway vehicle. Metro loop, point to point, is about it.

Look at the differences in school buses and highway passenger service buses. What sticks out is ground clearance. A school bus doesn’t benefit from lower cladding . Adds weight and makes inspections more difficult.

A Trailer-Tail has a LOT of force being applied to it while underway. Find a truckstop and walk up to have a look at the structure. It’s the right idea, but to have it stay attached may preclude use of rear emergency door.

Straight trucks (26’ Penske rental as example) have some of the very worst aero imaginable. And it’s that exposed underside that does it. The Fed-Ex Custom Critical guys blow everyone off the road (find a pic).

The key to lowering motorhome fuel burn is planned use. I can show that a 5-mpg Moho can use less fuel on a trip than something that gets four times that much: it’s about capacities for propane, water & food.

The important question to answer is: How many nights aboard before a re-supply run is necessary.

That’s the fruitful avenue.

The genuinely low transportation fuel cost route is where the family car is used to tow an aero travel trailer. Again, capacities matter. But the real savings is reducing the headache of a second drivetrain to maintain & repair. Not enough annual miles accrue to stave off dry rot with Mohos.

The cost of an RV is the total expenditure over (X) years divided by nights aboard (Y) in that period.

Step back for the big picture.

(If you keep it, see Roger Marbles site on RV tires. Mandatory reading).

.

more like how a audi loses it rings?

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 :eek:
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)

tail geometry
 

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.