Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph
 

http://ecomodder.com/imgs/flea-boat-tail-collage.jpg
Above image clockwise from top left: aluminum frame, cardboard construction; finished
prototype; removed during A-B-A fuel economy testing; still frame from video of tuft-testing

Overview ...

This discussion thread documents the construction & testing of a temporary, proof-of-concept aerodynamic "boat tail" on a 1998 Pontiac Firefly (Geo Metro).


Some facts about the prototype ...

• Weight: 5.5 kg (~12 lbs)

• Length: it adds 1.4 m (~4.5 feet) to the car

• Hard to park? No. The car + tail is only ~30 cm (12 inches) longer than a 2008 Toyota Camry (the most popular car in the U.S.), and shorter than a Ford F-150 (most popular truck). So it would easily fit in a normal parking spot.

• Sturdy? Yes. I could shake the entire car up & down and side to side from the end of the tail.

• Handling: I drove with the boat tail on the car for about 300 km (186 miles) at highway speeds up to 100 km/h (62 mph) and saw no change in handling. (Had to keep an eye on rubberneckers in other cars, though.)

• Rearward visibility: obviously there's no rear window; the next version will have one

• Tail lights: the car's original rear lights (brake, reverse, parking & side lights) were relocated to the end of the tail, as well as the licence plate

• Ugly? Duh! It's a prototype made of cardboard and duct tape! But the idea is beautiful in my eyes. Most production vehicles are aerodynamic disasters on wheels, and I think that's ugly.

Testing the prototype ...

1) First road test: read my impressions of the first drive with the boat tail on the car. Skip down to Nov. 21, post #100

2) Tuft testing: the tail spent a lot of time in the poor man's wind tunnel (ie. being tuft tested). This means observing the motion of yarn tufts taped to the car. The tufts reveal the direction & nature of air flow, and indicate whether the design works or not. Tuft testing results can be seen in the YouTube video, below.

3) Fuel economy testing: I used a ScanGauge to do an A-B-A test (before/after/before) that revealed a 15.1% fuel economy improvement at 90 km/h (56 mph). For testing details: A-B-A testing results posted, Dec. 5th

http://www.youtube.com/watch?v=1sQFtd-bTfw

http://ecomodder.com/forum/attachmen...1&d=1264546553

The original discussion started here ...


These are the goals for the project:

• design a removable boat tail under 5 feet in length
• the disassembled pieces of which will fit inside the back of the car
• lightweight, stiff construction
• minimally "invasive" to the car (discreet mounting points, etc)

I'm imagining 6 separate panels that will somehow snap or wingnut on to a light frame that can be easily dis/assembled.

I've done lots of waffling on materials, but I think I'm going to go with aluminum for the frame and coroplast for the skin. Both for their lightness, and ease of workability. Assuming I can find black coroplast (Home Depot here only has white - $20 per 4x8 - and the sign making shop wanted an obscene amount for a 4x8 black sheet of it).

Runner up idea was a fabric skin - UV resistant boat top canvas (which I can get in black). But thinking about hurricane force wind that will rarely be coming from directly ahead of the vehicle made me think coroplast is probably better.

One task I've actually completed so far: I added 6 feet to the wiring harness of each of my tail lights so I can move them between the back of the boat tail and their stock position as needed.

Twice now I've pulled the car into the garage and fussed around with various building materails, from cardboard to styrofoam to wood to alumimum. Nothing built yet. I've spent a silly amount of time standing around looking at the back of the car.

I think I need to just start building!

http://ecomodder.com/imgs/boat-tail-poll.gif

Also see my second boat tail project:

http://ecomodder.com/forum/showthrea...uft-25223.html

http://ecomodder.com/imgs/tail-wrapped-collage.jpg

http://ecomodder.com/imgs/collage-summary.jpg

.

I vote for boat canvas. If fabric is good enough for biplanes why not a car? You could use brass grommets to hook onto pins. Also works for lacing, cinch up points, or tie-down points. It keeps it flexible to fold up when not in use.

Interesting project. I'm following closely. I need to do something for my aero some day.

Have you considered the Fiber Reinforced Plastic sheets at Home Depot, (the bubble texture shower wall stuff) It can take high heat, can be formed and is quite strong. Easy to cut and very thin. $27 for a 4x8 sheet.

Oh! This is gonna be interesting to watch! You might wanna check Rona too. Here 4x8 coroplast sheets are 16 and change at Rona.

I was thinking of a collapsible aluminum frame (all one piece, but can be folded in on itself/collapsed into itself, like when you lower crutches) with the boat canvas stretched over some chicken wire. The chicken wire will add enough stiffness that the canvas shouldn't deflect that much, and the whole thing can be removed/collapsed on itself, taking minimal space in the interior.

If built intelligently, a piece of the removed canvas could also double as a windshield sun-shield, as well as a couple pieces for the front windows. That would take some real engineering time, though.

You may not need aluminum framing except as stiffeners. A bent coroplast corner is quite stiff. Modern plywood boats are often frameless, and better for it.

Cool, I'd like to see what you come up with.

I used metal skin on mine because it was free, and because welding it to the frame made it rigid with not as much work.

Didn't get a pic of mine with the extended tailpiece before I took it off, but I've gotten my downstate mileage back after removing it.

Metro,

I would go for the Styrofoam!!

The stuff is easy to cut/glue/form/sand/paint and so on.

Go for panels that are 1.5 inch thick. I think cost is around $15 per sheet.

After the shape is complete, add one layer of 6 ounce fiberglass cloth and epoxy to the foam, using the wet layup method. Make sure to add about 3 layers on all your hard-mount points, and embed wood blocks in the foam in these areas.

For a boat tail, you will probably go through about one gallon of resin, and if you use West Systems, it will set you back about $100/gallon. Cloth, about $40 or so.

The cool thing about foam is, after your done, it will look just like an automotive finish. Of course don't ask about how much time the sanding/glazing/priming and painting will take. But this is something you can do anytime you wish. Just take them off the car and start sanding. And you can still drive the car in between.

Just my $.02, Jim.

I've been pondering this as well. Not a priority at the moment with two other projects on the burners, but maybe something worth it for either of us to cobble something together as a mock-up for the AMEC run. Don't forget the use of strong magnets and/or suction cups! I really feel like neodymium magnets or strong suction cups could be at least two of the supports for a removable boat tail.

Except there should be no corners on this boat tail. All the "sides" of the boat tail will be rounded (think: more of a cone than a pryamid, so will need some kind of framework to support the shape. Correction: there will be hard edges where the boat tail terminates at the "transom".

Christ: the chicken wire idea to support fabric is a good one. But I decided against fabric because chicken wire or the supporting "tent pole" framework to get the polygonal "resolution" I want will be as bulky for storage or as much work as doing coroplast surfaces, I think.

Anyway, here's all I have to show so far. Should have enough length here to go almost to a point if I want to take it that far. (But I don't plan a point - it'll have a flat rear surface to mount the lights/licence plate.

http://forkenswift.com/album/7-tail-...s-ext-rear.jpg

http://forkenswift.com/album/7-tail-...s-ext-side.jpg

Jim: this is why I ruled out a foam/fiberglass approach. The kammback is a resin-soaked cardboard/fiberglass sandwich, and I spent 75% of my time trying to make it look nice after it was functionally done. The body work tested my patience :).

Also, I don't think the finish turned out all that great relative to the time I spent. To add insult to injury, it's peeling/bubbling in spots!

...I'd get a replacement backlight/hatch assembly from a junkyard and attach the boattail permanently to IT, then simply bolt / unbolt the whole she-bang to do A-B testing.

But then it wouldn't easily fit for storage in the car...

I think the point is that in this case, it may be much easier to slap something on there permanently to test it and see if the results make it worth the time and effort to make a fancy one that will detach and fit into the hatch. A whole hatch may be pretty cheap too depending on the junkyard. I know there are plenty of that generation hatches in the yards around here.

Heh, I think we've all been there.

Can't wait to see what you come up with. I think you will find that the internal bracing can be minimal if you let the coroplast skin carry the load; i.e. monocoque construction.

Inflatable boat tail
 

http://i429.photobucket.com/albums/q...e-boattail.jpg

Your requirement of having the boat tail stowed in the car was a challenge. We had earlier concepts of rigid or semi-rigid boat tail appendages mainly supported by extended hitch connections. So a folding inflatable could be attached ala bike rack straps & a rigid spar to hold the rear lights/license assemblies. The inflatable form could theoretically have the optimum airflow shape. Inflatable aerospace tech has been updated from earlier Goodyear airplane studies. The exciting 'Mars airplane' studies depend heavily on reliable inflatable airfoil engineering solutions. The tech has already been used in the Pathfinder projects.
http://www.youtube.com/watch?v=WZbR3...eature=related


Goodyear have taken early tests & concepts of inflatable airplane platforms. A compact rescue package designed for downed pilots/operators that could be air-dropped. They could then be deployed & flown to their freedom and back to friendly lines. However watch out for catastrophic deformation at extreme speeds.

http://www.youtube.com/watch?v=ywPkL...eature=related

I've thought that a junkyard hatch would be a good thing. Speaking just of the Kammback: I could integrate it better into one that I didn't mind welding/cutting up/bondo-ing/etc. It could be lighter, plus I could lose the rear glass and enclose with plexi.

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I'll probably skin the boat tail with cardboard for testing before doing anything more complicated. (removable panels, coroplast)

EDIT: but I won't be able to work on this any more until the weekend at the earliest.

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Cd asked me via PM if I knew the car's current Cd. I don't. I went out one day this summer to do the Instructables "measure your Cd" coastdown test, but the numbers I got didn't fit very well in the curve used to figure out Crr and Cd, so I didn't bother finishing it.

Botsapper: inflatable would be fantastic.

These ideas just keeps going around. Let's see some eco-prototyping!!!

FuelSaver by ATS™

Could you make it so that you open the hatch to be the "top" of the boattail, and the rest of it extends/unfurls from inside the car?

Unfortunately not. The top of the hatch is actually about 6 inches of the end of the roof. Opening it even part way increases frontal area and hammers Cd.

http://www.metrompg.com/posts/photos...ch-compare.jpg

Semi-related: Video: Illustrating the Vast Importance of Aerodynamics (open hatchback experiment) - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

Bubble butt
 

http://i429.photobucket.com/albums/q...metro-boat.jpg

Here's another view of the removeable tail. The clear/body color PVC panels welded into a rear-body conforming airfoil. The inflatable would have multiple high pressure tube spars either longitudinally or transverse (interestingly the transverse 'bumps' offer some aero benefits, Mythbuster testing?)
The temporary structure could be held down with straps & there is a rigid spar coming from the trailer hitch, holding the signal harness, plate & rear light assemblies. It also tucks in & tidy up the inflatable tube ends.
At the end of the day, it could also serve as the children's slide on the front lawn.

bowling for fuel
 

I'd set up bowling pins on the curb and then see how many can be knocked over with a left turn!
Wow,the imagery is just too good!
I do think maybe Kamm has a point with "practical" length issues.Something this long might want to be half car/half trailer.I'm to squeemish to attempt something that radical in pure cantilever.Not sure what a strong momentary sidewind gust might do.
We do need an EcoModder wind tunnel!

The area swept by a turning car is symmetrical fore and aft of the rear axle, so overhang, especially if tapered, will not hit stuff.
Most cars can use more tail area for crosswind stability. The center of "lift" is usually about 25% of the way back, since the front is redirecting the air, while the center of mass falls naturally around 50% back. The tail does generate more side force, but it is on the back axle, which has traction that otherwise makes the front end wander. Even with long tails, several streamliners have added a tail fin for stability. More side force, within the ability of the chassis to support it, may even give a boost by working as a wing-sail.

...pardon my simplistic analogy, but it sure "looks" like a really bad case of the "...tail waggging the dog..." situation to me.

Not if you ran some strings to opposing sides of the car, that hooked on the inside of the balloon.

i.e. top left hooks to bottom right

With something as useful and beautiful as this, why would you want it removable?

Okay, I suppose it would interfere with your ability to load akward cargo and parallel park. But if you drop the requirement that it be able to be stored inside the Metro, it becomes easier to design and build, since you can use a single rigid frame. Storable in the garage is almost as good. You'd use the boattail on road trips, and leave it at home for commuting.

@Botsapper: That's a very nice render. But if you taper the sides, bottom, and top at the same angle, you preserve attached flow with shorter overall length. Actually, with the Metro taillamps being almost half as wide as the rear of the car (based on counting driveway tiles), you probably want a square rear truncation, with the license plate mounted above or below the taillamps.

MetroMPG: How long do you think you're going to make the boattail?

The bottom is not so well supplied with air, even with a great belly pan and rounded. 4 degrees on the belly pan is usually as conservative as 15 deg on the top and sides. If the tapers were continued to a point at the back, it should be only slightly higher than the stagnation point at the front. A bit of rake will help cancel the lift and induced drag if the air goes over the top faster than underneath.

Nice to hear from you again!

If I am reading what you have said about the relative lack of air on underside of the vehicle compared to the sides and top, it would seem that you could conclude that the taper on the underside of the aerocivic is too steep.

I remember reading in that thread somewhere, that you asked Mike if he had performed tuft testing in that location. How much higher can the 4° go before air detachment would be a concern in this location?

Jim.

That would depend upon ground clearance and other issues. If departure angle is a problem, turbulators can be added just as the angle is increasing.

You're right about a rigid frame being easier to construct and more stable, but being able to fold a boattail and stow it in the trunk is very useful. Not all road trips are of the 'drive to grandma's ranch' type. This summer we drove to Romania and a boattail would have been awsome on the 1500+km trip each way, but once there we were driving around, sightseeing, city driving, parking, etc. The boattail would only be in the way, and we couldn't just take it off and leave it at the border station for two weeks. But if it deflated and packed into a size not much bigger than the rear light assembly in Botsapper's pic, then it could just get stuffed anywhere until it was time to return.

The underside of my boattail is steeper than I would have liked it, but I have a sharp approach to my driveway and didn't want to risk dragging the bottom of the boattail. I haven't tuff tested the underside of my boattail. I can't see it when I'm driving and don't have any aeronut neighbors to follow me, observe, and make sense of what they were seeing. But I have noticed that sections on the rear of vehicles that have turbulent flow tend to collect dust and dried deposits of dirt from water/salt spray. The underside of my boattail doesn't collect get dirty, so it can't be too turbulent under there. The side taper helps in allowing air to fill in the space between the road the underside of the boattail.

Hi,

I'll post the image I just made for my thread here as well, because I think it illustrates the issue.

1st possibility is relatively short and curved to a point
2nd possibility extends out to nearly the point where a 15 degree top slope intersects with the 4 degree bottom slope
3rd is a truncated version of the 2nd

http://img.photobucket.com/albums/v7...orBoattail.png

And a gardening hothouse!

Another amazingly good image, botsapper - thanks.

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I have nothing to report other than when not occupied with a few unrelated tasks last weekend, I sat on my duff. Haven't done any construction yet (aside from the virtual kind in my head - done lots of that!).

And you can call it the BubbleButt:p

hahaha brilliant!

dirt could be good
 

If the underside of the tail were a little dirty it might have an advantage with high speed stability.
Some race car designs have purposely slanted tails which communicate the low base pressure under the shelf of the tail to the cars underside.
The distance out from the rear wheels acts as a moment arm creating a torque at speed,creating useful downforce albeit with some drag penalty.

I remember back in 1989 when a Yugo got blown off our Mackinac Bridge. Hard to say what really happened, everybody's stories were contradicting, some say she was speeding, some say she stopped, some say she started swerving... but one thing was for sure, it was a tiny car driving in high winds. Since then they installed anemometers and close the bridge in high winds.

I can understand one might want to remove the tail. Taking a trip to the city, it can be a lot harder to maneuver in tight spots with a tail.

So what about an umbrella frame deal?

Couple pieces of tubing that slip inside each other to make the main "handle" section, a ring that slides up the tubing to open the "chute" section, could be made of thin rods and fabric, would fold up in seconds and could be stuffed in the trunk with no problems. A quick PCV tube frame could be attached to the hatch to connect the loose ends of the fabric.