[slowmover]

As above, the trailer does need tandem axles. The loss of a single tire on a trailer is hairy (to be polite) on a single axle. A tandem just isn't going to create that much more drag or problems. They tow FAR better, and the braking surface increase alone demands their inclusion.

As nicely as the sketches are going, think about systems. Water, alone, needs a fresh water tank, a grey water tank, and a black water tank. Plumbing runs are preferably short, so this is the reason bathrooms were conventionally located at the rear (along with the water heater). The fresh water tank ought to be just barely ahead of, and mainly atop of, the tandem axles. A trailer will also have to account for several batteries, 30A service panel, air conditioning, etc. Propane will also have to be accounted for by two tanks, underbody (exposed) runs to refrigerator, stove, furnace and water heater. The floor of the trailer will be at least 6" to account for tankage/frame alone. I would not assume a frame of less than 5" channel plus outriggers. Other walls should be at least 1.5" and preferably 2" or a little better depending on framing, so wall/ceiling thickness of 4" plus. An 8'6" wide trailer is down to 94" interior width. Etc.

Generally the kitchen, furnace, water heater and toilet are on the port side of the trailer, and the refrigerator to the starboard (same as entry door). Propane lines need to be short and straight.

Storage is compromised by anything other than a roof air conditioner, etc. And the roof is "busy" what with the need for A/C, ventilation ports, antennas (TV, radio, satellite, WiFi, GPS, etc), solar panels, and the need for service. A flat roof.

Windows are even more important. The lack thereof (a low glass-to-floor-area ratio) is a killer for comfort and live-ability. They need to sited to maximize outward visibility from a seated position, to induce cross-flow ventilation and lighting. Sticking a few here and there isn't to the point. Either I feel I am a part of the surrounding landscape, or not. Cheap trailers (plenty of "aero" ones) lack this.

Am stating the obvious that "classic" layouts on trailers still tend to work the best in re space vs accommodations. Check owners manuals or specs from late 1960's AVION and AIRSTREAM as well as STREAMLINE trailers for tank capacities, weight, etc, as your guide. The factory never did add in weight for options like A/C back then, but as a basis for sizing things, it ought to be of help.

Twin beds are 39x80, for instance. The aisle does not need to be 27". If you want folks to balk at a trailer, cut the beds down. The older trailers used to have fold-down bunk beds in place of (or integrated with) storage cabinets above conventional twins.

It's easy to lose space to all of this. Maintaining a 6' 3" head clearance is never as simple as with tube and caps (to describe the early clipper designs).

There seems to be 2 lines of thought regarding the aero; Saftey/towability vs aerodynamics. Finding the perfect mix probably isn't possible so comprises need to be made and I'll have to figure out where to make them.

There is no either/or happening. Most of the FE is in the tractor spec, not the trailer. A trailer needs aero more than weight reduction, but both are important. The order of importance is: livability (for a given size) and then packaged to promote best towing characteristics. Otherwise we'd just load a pickup with a wall tent and all the goodies. But no one wants to set up and break down a camp that takes 3-4 hours for each to have similar amenities.

So the question is about packaging. Unused space is undesirable (especially if it contributes to sail area) as FE gains are far too marginal. The bulbous shape would be better on a motorhome, IMO, where forces acting against the vehicle can be offset by a dual wheel drive drive axle for instance. And a motorhome that is in constant motion, not parked for long periods is where it would pay.

If I can get 16-mpg with a 3.75-ton 34' travel trailer behind a 4.5-ton pickup truck, what gain would a non-conventional shape yield? The same trailer as mine pulled behind a circa 1980 big gasser truck or car would have been 7-8 mpg. Were I to use Andy Thomson at Can Am RV (London, Ontario) to set me up with a Mercedes AWD fully independent suspension turbodiesel SUV I might see 18-19 mpg. With a smaller trailer (25-28'), maybe above 20 mpg.

But not with a cramped trailer . . . let's remember that 20,000 miles per year is quite high for TT use, whether for business or pleasure or a mix of both. I've rarely seen higher than that. The spread between solo driving and towing is classically 30% (or a bit higher). If I can reduce fuel burn by another 10% on a 30-mpg TV I have only gone from 21 to 24 mpg. At the usual 5k annual use, this is (@$3/gl; 208 vs. 238 gals) a savings of $90 per year. Barely a tank of fuel is all my 10% gain accomplishes.

Closing the margin for the TV (tow vehicle) for fuel mileage when solo and when towing is more easily achieved. I've wondered what an aerolid would do for my truck as THAT would increase FE without compromising trailer specification. As the TV and TT need to treated separately for aerodynamics it is the more promising course is the way I read it today.

I think that for a cargo trailer a basic teardrop shape is do-able. And as these are business vehicles by any definition, it is an excellent subject once capacity, balance, handling and clearance issues are defined. The gains would be substantial, are there are many, many very high annual mileage trailers of this sort in use.

But not so much for a travel trailer. Advances there are best left to the TV as the aero basics of TT design were hammered out by 1952 or so. Materials and cost of construction are the real constraints, not design per se.

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