Buy a new 5th wheel, ripe project for a boat tail

[aerostealth]

This is our new 5th wheel (a 2012 Keystone Cougar). The frontal area is basically 97 sq ft, but the rear reduces down to 65 sq ft from the manufacturer. I have looked at a 4 ft three sided panel boat tail using 4 ft x 8 ft x 1/2 inch foam board and it looks possible to reduce the wake area to 37 sq ft. If my towing arrangement gets 8 mpg stock such a boat tail should improve the mpg to 9.76 mpg. that would be a 1.22 x improvement.

[aerostealth]

A closer look at the numbers on frontal area, wake area, and projected boat tail numbers of the Keystone Cougar 5th wheel trailer adds 8 sq ft for tires. This would make those sq ft numbers FA 105 sq ft, WA 73 sq ft, and 50 sq ft respectively. This leads to a reduction in wake area of only 32% for a 16% increase in mpg. That would be a increase from 8.2 mpg to 9.5 mpg. Still significant, but not the holy grail of reaching 10 mpg. There may be a few tricks to reduce the wake area even a few more ft. Actual mpg testing full tank to refilled tank has yet to take place, so we do not know the the final numbers yet.

[ennored]

Percent of wake area reduction is NOT proportional to drag reduction. There's some plots of the difference here at ecomodder - somewhere.

You need to draw up some ideal and proposed boattails then compare that to a plot of drag vs. boattail truncation. Throwing random numbers out: 20% of an ideal boattail is worth a 50% reduction in drag (but only like a 10% reduction in area). Again the real numbers are here at ecomodder - somewhere.

(I'll search a little. See what pops up for me.)

[aerostealth]

Go for it. We are just trying to get a feel for what a flat panel tail will do. The actual numbers will come after we build it and test it. We sre just using a rough rule of thumb for now.

[vskid3]

How does the F-150 do towing the trailer? Is 9100lbs the empty weight of the trailer or is that all filled up? Is that the El Paso Camping World? (that's where we got our fifth wheel)

[aerostealth]

9100 lbs is the empty weight. The hitch weight is 1500 lbs. It tows this fine. The truck is rated up to 11300 lbs.

[aerohead]

Originally Posted by ennored

Percent of wake area reduction is NOT proportional to drag reduction. There's some plots of the difference here at ecomodder - somewhere.

You need to draw up some ideal and proposed boattails then compare that to a plot of drag vs. boattail truncation. Throwing random numbers out: 20% of an ideal boattail is worth a 50% reduction in drag (but only like a 10% reduction in area). Again the real numbers are here at ecomodder - somewhere.

(I'll search a little. See what pops up for me.)

At the bottom of this table is the boat tail which AeroStealth will be constructing.

Here's a relationship between wake and Cd derived from Mair's boat tail research.If you look at the 'skinny tires' line,you get a sense of what the wake's relationship to drag can mean.

One unknown that we're aware of is,that the Prowler's roofline is cambered downwards,unlike the Ahmed body from which the tail is derived.So it IS an unknown quantity.But without a $50,000 research grant from DARPA,it's the only thing that will fit Aero's budget.It will be the 1st 5th-wheel in the world to be streamlined this way.We'll get some valuable real data from it.And it won't cost taxpayers a cent.
We know of better boat tail designs,but they're 2-magnitudes higher in fabrication complexity and expense.

[freebeard]

At the bottom of this table is the boat tail which AeroStealth will be constructing.... It will be the 1st 5th-wheel in the world to be streamlined this way.We'll get some valuable real data from it... We know of better boat tail designs,but they're 2-magnitudes higher in fabrication complexity and expense.

This implies construction details not evident in the thread.

Apparently a boat tail with plan taper? It appears to have a rear bumper extension. Depending on the camera's focal length, frame rail width 4' back looks like a reasonable angle. Longer? Wider?

Does the plan include curved panels, pace Mair; or flat 'because it's easier'? Most importantly, please tell me you hope to use PolyMetal or equivalent.

[aerostealth]

The mount platform is about 3 ft long. The boat tail will terminate just beyond the spare tire which is not shown in the picture. This gives me three mounting points. The length of the panels are 4 ft. The run on the 10 degree angled panels will be around 46 inches. The 25 degree angle panel will have a section added to it to match that run. There will be a bottom panel about 10 inches long to complete the boat tail from the bottom of the shelf.

[freebeard]

I went looking and found a mention of foamie construction here.

What do you think would be the materials cost of 1" foam and fiberglass for say 32sq ft?

Because it you don't pay too much PolyMetal would be about $100. And it's pre-finished, no fumes or glass fibers in your skin.