A more durable Foamie?

[rruff]

Ruff, had that been allowed to cure completely ( weeks ) ....

Yes, that was one of the first pieces I made, so at least a couple weeks.

[rruff]

I'm just wondering if adding stringers, like wing spars, to the 2" Owens Corning Foam, glued with GG would add strength and stability to it or actually decrease the strength?

....The foam has break points at 16" and or at 24" running the long way down the foam. 2" wide 1/4 inch plywood stringers could be glued between those snap points to facilitate a stronger hold down technique. They would become embedded stringers, just like the foam plugs made by Clark.

I'll let someone else answer your GG question since I haven't used it. I'm pretty sure people like it for gluing foam, though. I've been using PLP, and now that I've found cheap epoxy and thickener I think I'll use that when it's warm enough. IME (experimenting), either is more than sufficient to rip the foam apart.

Adding wood stringers gives me some piece of mind, mostly because the cheap XPS foam is pretty weak and not rated for structural use. The foam isn't the source of panel strength and stiffness though, rather it's the skins. The foam's job is to keep the skins apart, take shear loads in bending, and support the skin when it's subjected to denting or puncturing forces.

Regarding your question about the hard points for solar panels, I've seen people just make a cut in the foam and glue an aluminum L in place, one leg straight down into the foam and the other nearly flush with the top. That way you have a glue joint on both sides of the embedded leg, and another under the one that lays on top. It's an alternative to using wood that might rot.

For your 1/4" plywood spars are you using marine ply?

[GPW]

We used stringer and former (aircraft) construction with XPS over that ... almost 3 years living outside ...

In case any of you live in a WET climate like here , it is really a good idea to put some kind of Drip Edge on the outside so the water falls straight to the ground and does not wander off under the floor ... That added a whole new level of Durability to my trailer , and will surely be required on my future builds ...

[gizmotron]

Regarding your question about the hard points for solar panels, I've seen people just make a cut in the foam and glue an aluminum L in place, one leg straight down into the foam and the other nearly flush with the top. That way you have a glue joint on both sides of the embedded leg, and another under the one that lays on top. It's an alternative to using wood that might rot.

For your 1/4" plywood spars are you using marine ply?

I'm putting the hard points for the angle aluminum rails on the inside so that when I bolt the rails to the roof the leg of the rail sticks up to bolt the solar panels to it. I need a rail so that I can tilt the panels into the sun and also bolt them down when driving. Three 180 watt panels only weigh a little less than 80 lbs. But it's the wind that can get under them at 60 miles per hour that bothers me. I guess an aluminum flat rail across the inside of the roof directly under the outside rail, having it cut across the roof stringers perpendicularly would be almost bomb proof. The surface bonding and the foam would be a way to keep the stringers from changing shape and bending. With that in mind it could not fly apart. So this all comes down to the surface bonded form of structural integrity. The World Trade Center was a form of surface bonded engineering. There was a fad building surfaced bonded dry cement blocks a few decades ago too. They used a vinyl concrete mixed with fiberglass fibers as a surface bonding method. They just stacked dry blocks the way they wanted them with no cement mortar to set them with. Those composite built experimental airplanes are all based on surface bonded strength too. Last year a very light aircraft was designed as a foamie.

That 1/4" plywood is "Sande Plywood" available only at the home depot. It's lighter than Douglas Fir plywood. I have proved that it stands up to bad weather without delaminating. All I used was Behr Waterproofing Wood-Protector.

So the big question is, should I use one, two, or three layers of 10 oz canvas with TB2 ? I was thinking of using one layer of 1/4" Sande Plywood and one layer pf PMF for the front and two layers of PMF for the sides, top, and back. The front plywood would add to impact strength and provide better lateral shift movement support. I would use one layer of PMF on the inside. That method of plywood and PMF would be like a Box Beam for shear strength as a simple engineering trick to the rest of the structure. If the leading edge is tied down solid then the rest of the junk following it has a tendency to stay in formation. It would not surprise me that three layers of PMF are stronger in shear strength than by using 1/4" plywood. I was reading your findings on different forms and thicknesses of PMF. Thanks for all that research.

[gizmotron]

We used stringer and former (aircraft) construction with XPS over that ...

Interesting design. I like the stringers buried in the foam. The foam prevents them from rolling over and losing their vertical loading strength. That's why light weight 1/4" stringers can work.

[GPW]

The real question is .... How much strength and durability do you NEED ??? ... Just saying , our simple Foamie #1 was just foam and a light canvas, bedsheets in the inside , and it held up Great for the four years I had it ... Now Bonnie has it , and is adding a Galley ( I’ve said this before ... I know )
Thinking if you want “ Durable “ , make it the usual 3/4” plywood sides ( sealed with the mix ) , and just cover it with a canvas , like we do with the foamies to protect the plywood. Then paint it with some real tough oil based paint ( no foam to melt) like the Old Timers did ... you can add the foam inside that for insulation ... Sure it would be Heavy , but it’d be Durable ... ...
We all have special needs , so build what you need !!! ... no rules here .... but there are a heck of a lot of Suggestions ...

[QueticoBill]

Great point GPW. Stronger versus strong enough. Just a variant of more is better. I wish it were so when it came to desserts. Everybody designing and building anything struggles with this. I'm trying to come to grips 1/8" ply is OK instead of 1/4", while you prove canvas glue is enough.

[gizmotron]

The real question is .... How much strength and durability do you NEED ??? ...

The trailer is only as strong as its weakest point. If the foam can't split then the surface bonding is all that is needed. This kind of foam with surface bonding is good enough for airplane wings. I've seen several windsurf boards split apart under the extreme loading of a 200 lbs man getting hammered in 45 mile per hour winds and 4 ft wind chop. The thing that prevents them from flexing is the stringers. But sometimes the continual pounding fatigues the rigid design and it fails.

I want a super ultra-light. I also want to thank you and others that have pioneered these foamie experiments and builds. I'm building a stand up trailer, without a sunken floor, that comes in at about 900 lbs, with a real stationary built in 31 gallon black water tank and a common RV toilet in it's own closed in bathroom. It has a 5' 3" wide by 24" deep kitchen counter for cooking, and a fold up and down single bed that increases floor space. It has a sink and drain that also goes into the black water tank and the water is gravity feed only. When I add the solar it will add 150 lbs to it.

These foamie experiments and builds made it possible. I plan on bonding the vertical and horizontal stringers together and down to the floor. I'll take plenty of pictures and will have the results shown when I do a build thread. Prevention of movement prevents lamination from failing. This thing will stick up into the air stream behind the tow car. I can't have it rip away from the floor. ... unless somebody can think of a better way to hold it down. I just need to know the tear strength of PMF. I might be able to put a secured plywood edge below the floor that the PMF can bond solidly to. Then I would only add stringers to the top only. That would be the best.

I've read the data on the screwdriver puncture tests. I wonder how that relates to a tear test.

[rruff]

I want a super ultra-light.

In that case I'd look very closely at the better Foamie examples and how well they've weathered. The 2" XPS where I live is 25psi and weighs ~.25lb/sq ft. 10oz PMF skins are ~.15lb/sq ft, for ~.55lb/sq ft total. That's a very light panel. If you want something heavier duty I'd look at 18oz cloth. Doubling and tripling layers of PMF isn't that much fun.

I've read the data on the screwdriver puncture tests. I wonder how that relates to a tear test.

If you make sure the canvas is secured to the floor, I don't think you need to worry about the camper flying off as you drive down the road. But it's best to buy materials and play around with them to satisfy yourself. A mere 1" strip of canvas will hold a lot of weight.

One thing I've learned today is that an equivalent weight of PMF over foam makes a lot less stiff panel than FG over foam. That surprised me a little, because I expected them to be close, but I guess the modulus of PMF is relatively low. That isn't a bad thing, just different. Might even be better. You really just need it to be "stiff enough", and you will get that with 2" foam. I made long skinny pieces on 1" foam just so I could see the relative difference.

[ghcoe]

You might also consider that hardening for doors and windows also strengthens the walls. Weather it just be wood hardening for screws or a clamp ring style windows/doors, it builds in a lot of strength and shortens surface distances that are not reinforced in the initial build.

[KCStudly]

Typically the outer skin wraps under the floor. Not only does it get glued, but also typically gets mechanically fastened in the pinch between the floor and trailer frame. The walls will not "blow off". Again, most all of the strength and attachment comes through the skins and hard floor, not the foam cores. Gluing the wall foam to the floor is a way to keep it aligned and in place until the skin goes on (better yet, skin the walls on a flat work table then just wrap the corners to attach them to the floor). Yes, the bond between skin and foam adds a lot of strength, and yes this is a weak bond due to the weakness of the foam, but it is spread out over a large area so the cumulative strength is great. Don't allow the skin to start peeling up at an edge and you will not have a problem.

Typical blue foam has a compression strength of 25psi, so a simple 1x wood block 3-1/2 inch square (like I used) let into the foam should be able to take about 300lbs of compression. Four of those should be more than adequate to support a solar panel with no concern. I would prefer to see blocking underneath the skin with support rails spaced up a little on top so that there are just a few small single points (fasteners) penetrating the skin, and plenty of air to keep moisture from creeping under the rails. Much easier to seal well and less likely to cause weakness in the outer skin than would occur with a long alum angle penetrating thru or resting on top along its length.

[ghcoe]

If I was going to do the angled aluminum I would install it before the canvas covering. Otherwise you are compromising the canvas at the cut line. Also, I suppose, you could lay the roof canvas, install the rails and then canvas over the rails. I would do that before I painted the roof so that you have a good canvas to canvas glue up.

[rruff]

A note regarding the stiffness of PMF skinned foam: It's very low compared to fiberglass. Qualitatively I knew it was a lot lower, and when I actually measured it, the difference was more than a factor of 5.

Both samples are 6"x48"x1" thick foam with skins on both side. Weight is a little over 1 lb/sq ft in both cases. The PMF is 3 layers of 10oz canvas and TB2. The FG is 1708 biax and 10oz cloth.

I supported the panels on the ends and loaded the center with another concrete block and measured deflection. The PMF sample deflected 17/16". The FG only 3/16". The FG panel seemed so strong I thought about standing on it, but chickened out as I wasn't ready to risk breaking it just yet (concrete block is ~28 lb and I'm ~180 lb). Anyway, there is a huge difference in the stiffness.

A more normal configuration for Foamies is one 10oz PMF skin per side and 2" thick foam, and this has been shown to be plenty stiff for a small camper. You gain a lot of stiffness and strength using the thicker foam.

[gizmotron]

rruff, your work is great.

So, I want rich man's fiberglass. Heck with poor man's anything. It will be lighter and stronger, issue settled. So somebody, and it might be you said that they found a cheaper outlet for the fiberglass and the resins. I need to find the glass and epoxy at a good price too. Was that ever discussed?

[rruff]

So, I want rich man's fiberglass. Heck with poor man's anything. It will be lighter and stronger, issue settled. So somebody, and it might be you said that they found a cheaper outlet for the fiberglass and the resins. I need to find the glass and epoxy at a good price too. Was that ever discussed?

I wish it was that clear cut! There are tradeoffs to everything. You might want to look at this thread: http://tnttt.com/viewtopic.php?f=6&t=64484 And KCStudly's build: viewtopic.php?f=55&t=48630&start=3495 Does anyone know of other builds that have used FG over foam?

US Composites is the cheapest epoxy I've found. http://www.shopmaninc.com/epoxy.html If you buy say 6.6 gal with medium hardener it comes to $44/gal but shipping bumps that to ~$53/gal.
Their cloth prices aren't bad, But this place has some killer deals on 1700 x30" ($2/yd) and 6oz x 50" ($3/yd): http://www.fiberglasssite.com/blowout-sale-save/
Silica thickener here: https://www.amazon.com/System-Three-310 ... B00VN4GWP4 Use it in ~50/50 ratio with resin create a paste for filling the textured foam before laying on cloth and resin. At least that's what I've been doing.
You'll need a ridged "bubble busting" roller, acetone, gloves, and a respirator.