Boat Tail Construction-Civic Sedan - Fuel Economy, Hypermiling, EcoModding News and Forum

bs25942 · 06-10-2014, 07:07 PM

After viewing the 'redneck boat tail' I have decided to make a boat tail. I have overlayed the template on images and they seem alright. Can anyone tell me if they are wrong in an way? I was not sure if the side taper was done correctly.

For materials,

I will probably build the frame as a tubular structure. I was thinking Metal conduit or PVC piping? For anyone with PVC experience, do you think it would be strong enough to support the weight? This seems easier since I can use a heat gun and bend it to the right shape.

I want to build the frame and then cover it in foam. After sanding/filing the foam, I want to fiberglass it with probably two layers of 4 oz cloth. Does this sound reasonable?

For the rear section, I will probably use 10mm coroplast that I have lying around.

Please give suggestions and point out problems if you can

aerohead · 06-10-2014, 07:27 PM

Here is the Cd 0.12,1959 MG EX 181 streamliner and its contour.

Here's 'Template' plan-view I'm recommending

redneck · 06-10-2014, 08:05 PM

Quote:

For materials,

I will probably build the frame as a tubular structure. I was thinking Metal conduit or PVC piping? For anyone with PVC experience, do you think it would be strong enough to support the weight? This seems easier since I can use a heat gun and bend it to the right shape.

I want to build the frame and then cover it in foam. After sanding/filing the foam, I want to fiberglass it with probably two layers of 4 oz cloth. Does this sound reasonable?

For the rear section, I will probably use 10mm coroplast that I have lying around.

Please give suggestions and point out problems if you can

I think PVC is to flimsy. It is also heavy and would be hard to attach. A 10' stick of 1/2" EMT weighs around 2.2 lbs. is rigid and can be bent easily, but will have to be welded.

A big plus for using 1/2" builders foam is that one side has aluminum foil and the other side a plastic film. The aluminum foil side will keep the fiberglass resin from making contact with the foam and melting it.

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ecomodded · 06-10-2014, 08:17 PM

Man, a 10' length of EMT sure is light , I guess being a 1/2" it is very light.
Sweet

bs25942 · 06-10-2014, 09:58 PM

I unfortunately don't have immediate access to a welder. Do you think I could braze/solder it or would there not be enough adhesion?

redneck · 06-10-2014, 11:20 PM

.

It should be mig welded. Mig is easy to learn and can be used for many different welding projects. There are 115v flux core mig welders for $200 or less that will do the job.

Or, find a friend who has one and get him to help for beer and burgers.

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Xist · 06-11-2014, 04:37 AM

Could you not just build it out of foam and then fiberglass it?

http://ecomodder.com/forum/showthrea...ect-13533.html

bs25942 · 06-11-2014, 02:07 PM

For the side taper:

I have overlaid both my original template and the suggested template on the car. Will the original not work because it is too aggressive of an angle for the air to stay attached? The suggested template seems pretty flat or maybe I did not line it up correctly. Obviously I would rather have a flatter angle with no air separation versus a steeper one but I also want the cross section to be as small as possible at the end.

aerohead · 06-11-2014, 04:01 PM

Quote:

Originally Posted by bs25942

For the side taper:

I have overlaid both my original template and the suggested template on the car. Will the original not work because it is too aggressive of an angle for the air to stay attached? The suggested template seems pretty flat or maybe I did not line it up correctly. Obviously I would rather have a flatter angle with no air separation versus a steeper one but I also want the cross section to be as small as possible at the end.

If your car was actually a half-body,the sides would be identical to the top.With this,the flow velocity and pressure along the sides would be closer to the top flow.
With the CIVIC,the airflow over the greenhouse is faster,and at a lower pressure than at the sides do to the way the streamlines are divided at the nose and windshield.
With less body camber,and higher pressure,the side flow cannot support attached flow over as radical a curvature as on top without triggering separation.
The more gentle curvature of the 2-D strut of minimum drag (second image,center table) has the sectional density necessary for a more gradual pressure rise which will not jeopardize the boundary layer.
A number of the lowest drag cars ever constructed happen to use this more gentle 2-D section profile for their plan view boat-tailing.
Until more data comes along I believe it to be the safest bet.
Here is Kamm's research model for Jaray-type rear ends.The longest, most gentle plan-view contour netted him the lowest drag.

Superfuelgero · 06-11-2014, 06:43 PM

Quote:

Originally Posted by bs25942

I unfortunately don't have immediate access to a welder. Do you think I could braze/solder it or would there not be enough adhesion?

Most schedule 10 pvc will bend with time and heat cycling. Schedule 40+ may work, but is very sensitive to uv.

For the emt:
Straight sections: Sleeve, epoxy and rivet
Joints, flatten, then epoxy and rivet. Works best if pipe continues to on (think a continuous square, flattened at corners).
Removable sections rivnut instead of epoxy and rivet.