Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)

[orbywan]

Originally Posted by skyking

What are the measurements?
My truck runs from 10.5" to about 13", for example.

It measures 15 inches in the front, to 18 inches at the rear of the camper body, 25 feet back, and then up to 28 inches at the rear of the boat tail, 4 1/2 feet or so further back at the end of the boat tail.

The steep departure angle is necessary because of the off roading I do with it. Before I modified and extended the hitch, the bottom of the boat tail was about 34 inches off the ground, so that part of it is 'improved' dramatically.

I'm surprised the difference is only 3 inches over the length of the actual body, I thought it was much more than that.

[skyking]

I don't see a downside there.
If you want more from your mod, you know what to do. Fold out a 3 panel tail off yours, take it even farther. Cut it out to allow for the boat, etc. folds flat when you want it to.

[orbywan]

Originally Posted by skyking

I don't see a downside there.
If you want more from your mod, you know what to do. Fold out a 3 panel tail off yours, take it even farther. Cut it out to allow for the boat, etc. folds flat when you want it to.

You're giving me too much credit here, I don't know what to do. 'Fold out a 3 panel tail off yours...'. Do you mean extend the three sides of the boat tail, left, right and upper? Kind of like the sort of boat tails you see on semi's occasionally? Why not do the bottom also?

[skyking]

Leave a hole for that trailer. I see your posts, and observe that when you really use this RV you are towing.
Yes, make a frame for each panel, hinge it, and have the ability to fold it flat.
Put an x-brace across the corners at the back to stiffen it up. If you do it as nice as you did the first work, it could be deployed in couple of minutes.
It will not block the tail lights of license plate either.

[orbywan]

Originally Posted by skyking

Leave a hole for that trailer. I see your posts, and observe that when you really use this RV you are towing.
Yes, make a frame for each panel, hinge it, and have the ability to fold it flat.
Put an x-brace across the corners at the back to stiffen it up. If you do it as nice as you did the first work, it could be deployed in couple of minutes.
It will not block the tail lights of license plate either.

How far would you extend it back?

[skyking]

As far as was practical. pull some dimensions off the two trailers and see what and how you can live with this. You did not have any perfect side-on photos of the boat and RV. One of those would be great for visualizations.
If it is for bare RV, then you'd have to add tail lights on it if it went back more than 3', at least in my state.
I can just imagine how it would smooth flow onto the boat.
Add some longitudinal framework from windscreen to transom, and a tight tarp on that. Essentially a boat cover
You could do no more than that.

[orbywan]

Originally Posted by skyking

As far as was practical. pull some dimensions off the two trailers and see what and how you can live with this. You did not have any perfect side-on photos of the boat and RV. One of those would be great for visualizations.
If it is for bare RV, then you'd have to add tail lights on it if it went back more than 3', at least in my state.
I can just imagine how it would smooth flow onto the boat.
Add some longitudinal framework from windscreen to transom, and a tight tarp on that. Essentially a boat cover
You could do no more than that.

To be honest, for now, I'm not interested in making the boat more aerodynamic, there just isn't time for that, and when I tow something that heavy, I'm OK with not getting great mileage, for now.

I'm going to concentrate on the RV for now, but thanks for the suggestions, I'll give the extensions some thought.

[orbywan]

That should have been KamperBob, sorry.

[KamperBob]

Hey orbywan! No apology needed. I enjoyed meeting too. You were busy then (a good problem) so thanks for making time.

I really don't know how much strakes would help. Maybe none but others use them and you seem the build/test type so it was a logical suggestion.

IIRC most of your MPG gains followed the boat tail extension; not so much the belly pan. I could easily be wrong but given overall dimensions maybe there isn't much room for improvement underneath. The rig is so many times taller than that gap that I suspect the tall sides are the best point of focus. Have you already tried tuft testing with a chase vehicle and camcorder?

Looking forward to future updates on whatever AeroRV work your schedule permits.

Rock on!
Bob

[orbywan]

Originally Posted by KamperBob

Hey orbywan! No apology needed. I enjoyed meeting too. You were busy then (a good problem) so thanks for making time.

I really don't know how much strakes would help. Maybe none but others use them and you seem the build/test type so it was a logical suggestion.

IIRC most of your MPG gains followed the boat tail extension; not so much the belly pan. I could easily be wrong but given overall dimensions maybe there isn't much room for improvement underneath. The rig is so many times taller than that gap that I suspect the tall sides are the best point of focus. Have you already tried tuft testing with a chase vehicle and camcorder?

Looking forward to future updates on whatever AeroRV work your schedule permits.

Rock on!
Bob

I didn't video tape it but I did take several photos of it going down the road the first time it was driven. My driver almost crashed it taking an off ramp. Didn't see another car right next to him. Anyway, the photos are on the original post, I'll dig them up this evening when I've got time to breathe. The flow was very good on the sides, excellent on the top and really crappy on the bottom.

That was before I dropped the floor of the tail down about a foot to accommodate the extended hitch. In the process I'm sure the air flow is better down there. I haven't retested the bottom yet. It's as good as it's going to get. I can't drop it any lower without getting into my departure angle which has to be high enough to accommodate frequent off road forays.

Next up is improving the air flow up front, adding an air dam, and I've taken it this far so I'm going to finish the wheel fairings and see where that takes it. Depending on the results, I may go with a lower ratio tail gear. These 4:10's can't be helping, but I'll lose some towing grunt in the process. 1800 rpm is supposedly ideal for mpg on this 7.3 diesel, that shows up at about 52 mph, that's too low for optimum mpg, but it's doing so well even with that handicap I'm not sure I want to jeopardize the towing ability.

The problem is, with the gearing that high, when I get into the higher rpms, the boost on the turbo kicks in about 65 and the mileage really goes to hell at that point. It achieved 15.4 on the way to Phoenix a while back doing 60 to 65, but I think the side winds I was experiencing had to have some tailwind action going on also because I was amazed to see it that high.

That brings me to another point. It's hard to know for sure when you've achieved a 'full' tank of fuel, especially on a diesel vehicle. The fuel foams so bad it kicks the pressure valve on the nozzle and turns it off way before it would with unleaded fuel. I try to fill at the same station on the same pump and fill it with the same exact procedure, but I'm sure that's lacking in accuracy sometimes.

Has anyone ever heard of a reasonably foolproof way, or mod for a fill neck, or device, that will eliminate some of that guess work? There's got to be a better way, I just haven't found it yet.