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Old 12-18-2018, 10:02 PM   #1 (permalink)
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Boat Tail on a hatchback?

How exactly does one go about doing this?

I drive a 96 Subaru - I have my reasons, no I am not willing to change cars. If you must know what those reasons are, it's that there's a risk of snow 7-8 months out of the year here, the AWD and winter tire combo is invaluable - it's easy AF to work on, the parts and repairs are cheap and, there's a ton of deer here. I hit a deer doing 75 mph a few weeks ago, the deer exploded, and I ended up with nothing more than a cracked bumper and a dented hood.

There's also Elk here. For those of you whom have never seen an elk in person, just picture a deer the size of a horse.

There's also the occasional moose. For those of you that have never seen a moose in person, just imagine driving down the road and hitting something roughly the shape and size of a Clydesdale horse, but about 500 lbs heavier and that's without the antlers.

(They're about the size of a Suburban)

We also have Bison that will occasionally wander across a lonesome highway in the night. If you've never seen a bison, combine a cow and a horse, and give it steroids and a really bad attitude. It will proceed to stomp you, and your car, after you hit it, while you lie bleeding on the side of the road.

The Wild West is a dangerous place for a Prius. <- I might put that in my Signature, haha.

I've seen again and again that a boat tail is by far the best MPG improvement you can put on a car, but I cannot for the life of me figure out how I'd put it on a car such as mine. Is there a way to do it without rewiring the entire rear electrical array? Should I remove my entire hatch and save the extra weight, just installing the lights at the end of the tail? Or can I go around it and create grooves that my lights would still be visible? What do most modders make it from?

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Old 12-19-2018, 12:18 AM   #2 (permalink)
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How much it helps will depend very much on the speed you're driving. At lower speeds it won't help at all. At highway speeds you can expect a realistic 10-20% with a well made tail. On the flip side, the weight reductions from removing your hatch would be basically zero on the highway; it's largely only in stop and go traffic that weight reductions do anything measurable.

Something to think about - half a boat tail provides way more than half the benefit. What you might find much easier to design and install is a kammback, which is a truncated boat tail which can provide (I'm going to make up a number here) 50-75% of the benefit with a quarter the footprint.

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Old 12-19-2018, 09:19 AM   #3 (permalink)
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Hitch receiver, Removable, and light cargo:
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Old 12-19-2018, 09:39 AM   #4 (permalink)
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Most on here (that I've seen at least) are made of coroplast (corrogated plastic, think cardboard box material design, but plastic) sheets, usually with a coroplast "frame" underneath. Coroplast is easily obtainable from a local sign shop , usually in 4'x8' sheets. Typically they will also use some form of plexiglas in the center to maintain visibility. MetroMPG can tell you all you will ever need to know about how to do it, he's done it so many times I'm pretty sure he could build one while sleeping.
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Old 12-19-2018, 11:42 AM   #5 (permalink)
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Quote:
Originally Posted by DeliveryGuy89 View Post
I've seen again and again that a boat tail is by far the best MPG improvement you can put on a car, but I cannot for the life of me figure out how I'd put it on a car such as mine. Is there a way to do it without rewiring the entire rear electrical array? Should I remove my entire hatch and save the extra weight, just installing the lights at the end of the tail? Or can I go around it and create grooves that my lights would still be visible? What do most modders make it from?
There are a number of ways you can do it. My most recent tail was a foamboard/fiberglass composite, spaced out on a cargo box hitch mount so I could still open the hatch:



It turned out the x-braces were completely unnecessary. The lights were plugged into a trailer light harness wired to the stock lights. According to the people behind me at the GGP, they were obnoxiously bright.

Before that, I had a coroplast tail I threw together on a Friday afternoon before driving to Connecticut:



Both of these designs retained visibility of the stock lights.

I'm building the next version now out of acrylic attached to the hatch itself, so it will open with it. It will be a 3/4 design, open on the bottom.
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Old 12-19-2018, 12:31 PM   #6 (permalink)
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most modders

I second the motion on following MetroMPG's cardboard and duct tape rapid prototyping.
A receiver hitch cargo carrier,or lashed-on bicycle carrier can support a mockup of a tail for testing.
A weekend of fabrication can get you something tough enough to withstand road testing.If you like the results,then you can slow down,and invest the time and resources to fabricate something you'll enjoy,without wondering if it will all be for naught.
Since the performance of a boat tail is predicated upon quality airflow coming at it,you might want to consider an under-body cleanup,and full,rear wheel skirts,to help reduce any obvious turbulence upstream of the tail.
Canoe fairings for the rear wheels would also help for 3-seasons driving.
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Old 12-19-2018, 04:57 PM   #7 (permalink)
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Quote:
Originally Posted by aerohead View Post
Since the performance of a boat tail is predicated upon quality airflow coming at it,you might want to consider an under-body cleanup,and full,rear wheel skirts,to help reduce any obvious turbulence upstream of the tail.
Canoe fairings for the rear wheels would also help for 3-seasons driving.
That's definitely something I'm wanting to do, I'm just not sure how to do that one either. Bolt on some aluminum around the tire seems easy enough - I had rust above my rear wheel wells, so I cleaned it up, sanded it down, coated it with plastidip and bolted on some tread plate above it and painted it black, it actually looks really cool - I've got quite a bit left over, might try that. I feel that the diamond pattern might drag down some aerodynamics, but I also feel it would still be functional, and I wouldn't have to purchase anything new.
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Old 12-19-2018, 05:02 PM   #8 (permalink)
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functional

Quote:
Originally Posted by DeliveryGuy89 View Post
That's definitely something I'm wanting to do, I'm just not sure how to do that one either. Bolt on some aluminum around the tire seems easy enough - I had rust above my rear wheel wells, so I cleaned it up, sanded it down, coated it with plastidip and bolted on some tread plate above it and painted it black, it actually looks really cool - I've got quite a bit left over, might try that. I feel that the diamond pattern might drag down some aerodynamics, but I also feel it would still be functional, and I wouldn't have to purchase anything new.
Even a box-cavity would do some good.And they don't require any curves,especially compound curves.
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Old 12-19-2018, 05:05 PM   #9 (permalink)
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Quote:
Originally Posted by Ecky View Post
How much it helps will depend very much on the speed you're driving. At lower speeds it won't help at all. At highway speeds you can expect a realistic 10-20% with a well made tail. On the flip side, the weight reductions from removing your hatch would be basically zero on the highway; it's largely only in stop and go traffic that weight reductions do anything measurable.

Something to think about - half a boat tail provides way more than half the benefit. What you might find much easier to design and install is a kammback, which is a truncated boat tail which can provide (I'm going to make up a number here) 50-75% of the benefit with a quarter the footprint.

I like this idea, although it seems that it would reduce my rear visibility quite a bit. I'd have to play with it to get it to a point that I wasn't sacrificing safety.

Some quick research says that "To gain most of the effects described by Kamm, a carís tail has to be tapered to about 40-50% of its largest cross-sectional area."

I have no idea what this means, could someone break down exactly what I should be measuring here?
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Old 12-19-2018, 05:06 PM   #10 (permalink)
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Quote:
Originally Posted by aerohead View Post
Even a box-cavity would do some good.And they don't require any curves,especially compound curves.
Box Cavity?

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