Mandan, an aerodynamic 6x10 foamie

[skyl4rk]

I would like to design a foamie that has good interior room but is also aerodynamic. My current car is not very powerful and any improvement in wind resistance would be appreciated.

I started out with a Super Simple Box concept, as stated in the Thrifty Thread:

Here are my notes on a super simple box foamie:

Foamie Build

Super Simple Box, 48x96x48

Old boat trailer, 48" wide frame area

Cut 2” foam panels to size (2 – 44x48 (front, rear), 3 – 48x96 (sides and roof))
Apply kraft paper over TB III, both sides of foam to all foam parts
Make floor using 1x2 frame, openings filled with 3/4” foam, skinned with 5mm underlayment top and bottom
Glue sides, front and rear panels to floor and each other with Gorilla Glue
Glue roof to walls with Gorilla Glue
Round all corners a bit
Glue Fiberglass Drywall Tape to all corners, seams and joins inside and out with TB III
Cover FDT with kraft paper and TB III
Paint with exterior acrylic latex paint
Cut in door in rear, sailboat washboards with hatch on top? Or side door with canvas hinge?
Cut in and build 2 dorade vents
Cut a few small windows on sides, rabbet, glue in plexiglass
Drop onto trailer, bolt down

I still need to work out a cheap and simple door.

I am not sure I will build this, because I am getting ideas how to make a more aerodynamic version. I need to wait till spring anyway, so I have some time to design.

The Super Simple Box would be fast, but still a lot of work and then you are stuck with a box in your driveway.

So using the same construction techniques and materials, I would like to shape the trailer in an aerodynamic way:

https://drive.google.com/file/d/0B6-YuMVYi0doNWMtWmxjM0RWX3N1RWdNYmVRWGpwUDZoanV3/view?usp=sharing

The wide end is sized to fit behind my car (Nissan Versa) and the door is in the aft end which is the narrow end. The floor dimensions are 6' x 10' at the widest and longest points. The height is about 56" at the highest point, and that would sit on a boat trailer that is about 16" above the pavement.

I am wondering if 2" foam will bend to a curve like this.

[KCStudly]

2 inch won't bend too easily, but you have options.

You could do a radial kerfing profile to get those conical bends, or you could miter the edges and fair the outside corners in by sanding them to suit.

2 inch is very thick, and with that shape, being structurally sound by itself, you might even consider using 1-1/2 thk for the walls.

[skyl4rk]

I plan to shape the edges so that they match the angle and curve of the joining panel.

I am also thinking about building a nose that reduces the gap between the car and the trailer body, but that is for the future, after the trailer is built but before finishing.

[Pmullen503]

I think you could use 3/4" XPS to achieve those curves without kerfing. Use a water based contact cement or to add a second layer for a total of 1.5". Would you use a strongback and forms like you were building a small boat? It resembles an upside down punt.

[GPW]

That upside down boat design has a lot of promise ...

[skyl4rk]

I would probably make a removable jig out of 2x4s or something, and remove it after the glue cures.

It is a pram. The design software is Carlson Hull Designer. It will produce dxf CAD files showing panel shapes.

http://carlsondesign.com/software/add-ons/shareware/hull-designer

This is the .hul file that will work on the Hull Designer program:

https://drive.google.com/file/d/0B6-YuMVYi0doSmd1ZE13eXRjeWhJdDFKXzVLRnlHb01xU3pz/view?usp=sharing

[Pmullen503]

Would you put the door on the end? A boat style companionway would be pretty slick! Not the most aerodynamic, however.

[KCStudly]

Not sure if I was clear. What I meant was, you could glue mitered segments of foam together to get the approximate shape of the side walls, then knock down the high spots to get a more uniform shape.

[skyl4rk]

I am thinking of the door on the rear of the trailer, although I am still open to other ideas.

A sailboat style companionway would require a hatch on the roof that covered washboards in a slot. That would be pretty cool, but I'm not sure that washboards are the best solution. They work on a boat but a hinged door would be more convenient. A hatch on the roof would make it easier to get into the trailer, and would allow standing room in the hatch area, which would be nice. It could take the place of vents if screening was added.

[Pmullen503]

I was thinking that the washboards would in fact, be some kind of door on a hinge. The roof hatch could be sliding or maybe just hinged. I like the idea of using it as a roof vent.

Would you do a galley or some compartment in the front? Maybe side access rather than straddling the trailer tongue?

[skyl4rk]

I want to push the trailer body as far towards the car as possible, so I am thinking of a lightweight bed up front, with no storage underneath. Perhaps a bed platform with some light framing to stiffen the floor.

I don't have the interior worked out beyond that.

[Pmullen503]

If you did a sliding hatch, so you would have standing room in the back, then an indoor galley near the back might work. Might want to mock it up with cardboard to see how big of an opening you would want.

I was thinking about an alternative to kerfs for doing curves. I wonder it you could make a hot wire cutter to make bead and cove strips.

[KCStudly]

Having work with foam for a bit now, the cove work would be a lot more effort than other methods.

Thinner sheets of foam will flex quite readily, and it would be far easier to laminate 2 or 3 layers, even if you had to make a simple form to weigh them down on, than it would be to glue tens of strips together, then fair back.

Sanding the foam is easy, but you have to be careful which glue you use because most are harder than the foam when cured. That makes it hard to sand consistently on the joints, tending to leave the joints and take away adjacent foam.

That's why, for the 2 inch foam and gentle curve, I was envisioning wide vertical slats more like barrel staves. Less pieces and less joints.

2 layers of 3/4 inch, are 3 layers of 1/2 inch would get it, but you'd have to work a bit to get good contact while laminating... and GG tends to expand, thus the idea of a horizontal form that could be loaded up with weights.

One thought I had for an alternative to vacuum bagging was to use an old waterbed mattress as a press. Lay a sheet of foam on the form, spread glue, add another sheet, put empty mattress on top and fill with water, allow to cure, then trim to final size. Just a thought.

[skyl4rk]

I would like to use 2" foam for the insulation properties, strength and surface area to glue on the ends. I would like to use panels cut to size rather than piecing it together. That means I may need to reduce the curves in my design.

Does anyone know how much an 8 foot section of 2" pink foam will bend? I may head out to the store later this week to do a quick test.

[KCStudly]

Does anyone know how much an 8 foot section of 2" pink foam will bend?

Not much at all. I have been using 1-1/2 thk blue and it doesn't flex very much, certainly not to what your sketch shows. For example, I would not have been able to do my 520 inch radius ceiling w/o kerfing. Again, that is with 1-1/2 thk. The 3/4 inch will do it with a little effort.

There are some techniques that might help (using a flexible backing as a stretcher, or, perhaps, gluing thin fabric or chip board to the outside of the bend first). To be honest, I never really took a big piece to failure, just hefted it to a point where I wasn't willing to risk snapping.

On my window cutout, 5 mm marine ply laminated to one side of 1-1/2 thk foam, I was able to stand on it and bend it quite a bit.