Hitched Detachable Boat Tail finished. With tests!

[wagonman76]

Aerohead said the same thing to me which was true. The cold temps really do kill most aero gains. But I did get a noticeable difference when we actually had warm days here. About a 5-6 percent gain.

[brucey]

Took another picture for this. You can also see the little sides I've done. Not a lot of progress lately honestly.

I'm thinking getting a smooth transition onto this might be too difficult with a single hitch attachment. Short of making an aluminum frame, this is going to be very difficult to remove and install.

IN fact, I just cracked some of the glass trying to put the thing on. I was balancing it and it fell on me. Its still solid, but damn.

I'm thinking a clear kamm permanently attached to the car and a boat tail from there would be a MUCH better way of doing this. At least it wouldnt be that difficult to scale it back. Just more work.

[orange4boy]

Don't give up on it. You may have coincidentally filled up with winter gas. The angle looks fine to my eyes. I had my kamm at a steeper angle and still got attached flow according to the tuft testing.

I also had problems with intermittent sticking brakes which drove me crazy before I fixed them, then all the benefits of my work became apparent.

Mainly, cold weather hit just as you finished so you would probably be getting worse mileage than baseline without it anyhow.

[Dan9]

awesome project, as stated above, do your testing and maybe...establish a cold weather baseline for your car if you will keep testing now.

[MetroMPG]

FYI the "proper" use of that half teardrop template as I understand it from aerohead:

Size & overlay the teardrop so its base lies on the car's ground plane (where the tires touch). Size/position the highest point of the teardrop with the highest point of the vehicle.

Examples: see this image.

[Piwoslaw]

Brucey, I measured the top angle of your boattail and it looks like more than 15 degrees. If the airflow doesn't detach at the hatch/boattail transition line, then you'll probably see attached flow down the back, but that doesn't have to mean that everything is OK. From what I've read, the optimal angle is 10-12 deg. As it gets steeper the airflow stays attached but lift increases, until about 26-28 deg. when it detaches. In this case, detached may be better for fe than generating lift. Would it be possible to tilt the boattail upwards slightly?

This of course is a worst-case, I'm with the rest that you dip in mpg's is probably weather/fuel/random.

[BamZipPow]

When you build these boat tails...what tools are you using to calibrate the angle?

I've seen things like the Denali Digital Angle Protractor, the Johnson Level and Tool Angle Finder, and the Wixey Digital Angle Gauge. I think I would lean towards the Wixey Digital Angle Gauge because you can zero it out.

[Piwoslaw]

Originally Posted by BamZipPow

When you build these boat tails...what tools are you using to calibrate the angle?

I use a very complicated tool



It's nothing more than a level ruler with a 12-78-90 deg. cardboard triangle taped to it. Cheap and reusable/recyclable. I only used it only to check the angle, the actual planning was done with heavy use of highschool trigonometry (even cheaper, and even more reusable).

[bennelson]

Just an idea for a similar project on other vehicles:

Some SUVs have a "swing-away" rack for mounting the spare tire on the back of the truck.

Wouldn't that be almost the perfect mount for a custom boat-tail?

It's big enough that you should be able to easily mount to it with three points of contact, and the whole thing could swing away, so you would still have access to the back of the vehicle.

I'm still waiting for someone to make the inflatable boat-tail!

[BamZipPow]

Originally Posted by bennelson

I'm still waiting for someone to make the inflatable boat-tail!

How about using one of those seasonal/holiday lawn inflatables?