Boattail
 

It's easy to see how a boattail works, it closes down the size of the hole you just made in the air, and does it with minimal detachment.

Say the rear of a car from bottom of bumper to top of trunk measures 2.5' and the width from side to side is 4.5'.

Which is more important to close down, the vertical or horizontal?

I am getting ready to start on my first version that consists of a trailer hitch exiting the middle of the rear bumper instead of hanging in the breeze below the car. My idea is to come straight off the trunk deck level with the trunk to about 4' off the car, then have the bottom portion keep the same angle as the rear bumper and then have the sides taper in as well.

All will be in an effort to reduce the departing air to half the size of the rear of the car, or less.

Am going to test before and after for flow over the rear of the car.

top/sides
 

If you do a boat tail,working only with the roof curvature,you'll lose 60% of the streamlining benefit.
You need to do the plan taper as well to see significant gains.
Today's Bugatti Veyron will serve as a proper model for your roofline,or use the Template.
You'll want to pull your sides in like GM's EV-1 as best you can.

BHarvey- Glad to see another tail project in the works. Make sure you like talking to strangers, because you WILL get a lot of questions... and pictures... and double takes:D.

I've tapered both vertical and horizontal on mine, though, with the plummeting temps, I can't quantify the effect.

I've also read somewhere(Aerohead) that smooth air is needed to properly feed the tail( I made sure the belly pan was in place before the tail). I am trying to get someone on standby for the next snow storm to do some snow wake videoing.

Looking forward to your build thread:thumbup:.

I tested my 600mm long boattail with aba test @130 km/h speed
Here are my todays boattail test results:
on N 4.7l S 4.0l
on N 4.6l no data
off N 4.9l S4.3l
off no data S4.3l
off N 4.8l S4.2l
on N 4.7l S4.1l
on N 4.7 l S4.1l

avg on N = 4,675l avg S 4,066l
avg off N= 4,85l avg S 4,266l
diff N = 3,6% diff S= 4,7%
Total difference 4,15% less fuel in this test. My test route was not perfect due to hilly motorway, but the difference is clear. Also my cars fuel gauge shows only 0.1 liter accuracy which was also little problem. That is the reason for so high speed in the test. Also during driving the differences came in the uphill sections. Without the spoiler fuel consumtion rise much more...