11'6" three berth ply/foam/aly composite caravan

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Re: 11'6" three berth ply/foam/aly composite caravan

Postby Nobes » Fri Apr 07, 2017 8:59 am

I think you said you plan on using treated wood for the floor and inside--please look into that and be sure. I've heard that can be dangerous to the occupants.
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby rruff » Fri Apr 07, 2017 1:54 pm

hossesdad wrote:Yes, I am planning to do the walls in ¼” treated ply inside, 2” foamular 250 and 0.036 inch aluminum outside (edited to add a zero!), the whole lot held together by epoxy thickened with West microfibers adhesive blend made up to a putty consistency, spread with a slotted trowel and then a spreader and pressed together by a vacuum bag at about 2psi of vacuum (That is 288 lbs per square foot, of course). Did a test (without vacuum, using weights because my plumbing isn’t ready) held well although needed more weight round the edges...it is for that uniformity that I am using the vacuum. One has to pull off the face of the foam to release the ply or aluminium.


I'm using frame/stringers because I don't entirely trust the foam, plus I like having plenty of attachment points. A note on bonding, I'm pretty fond of using both the carpet seam roller and wallpaper perforator to texture the foam. The foam *will* pull apart first regardless, but the texturing makes it stronger by bonding deeper into the foam. I also like making the filling paste with micro-balloons. They save some weight and cost, and are plenty strong for this application.

I'm guessing you are good with your vacuum setup and that is surely ideal, but for someone who doesn't want to vacuum bag I think you can get a pretty good bond to thin skins by putting weights on the foam side rather than the skin. The foam is relatively thick and spreads the pressure out. Pressure isn't really need for the bond, you just need everything touching (ie flat).

I'm going to have a curve on top of mine also, but it will be gradual. I'm just going to build it flat except for the final FG layer, then cut a few kerfs and fill gaps as needed and do the FG layup with the top attached.

I'll have a very curved nose that goes over the cab (truck camper). This will be built as a separate piece from the rest of the box, and then glued on later.

A note for anyone doing FG work in the US, I've found a ridiculously cheap epoxy supplier. E-Bond bought direct from the company. Shipping is high because they are in FL and I'm in NM, but even then I can get 6 gal for <$250 shipped.
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Fri Apr 07, 2017 10:50 pm

I think you said you plan on using treated wood for the floor and inside--please look into that and be sure. I've heard that can be dangerous to the occupants.


Thanks, nobes, it's an interesting point. You deserve a beer for raising it; I will drink one for you. :beer: :lol: No, I will drink two! :lol: Is Chromated Copper Arsenate ("CCA") treated timber appropriate for use inside a caravan or teardrop? I think it is. :thumbsup:

Firstly, why use it? Because the caravan might leak in ten years time. Untreated pine ply starts to rot soon after it is wetted and is composted in three to nine months in some environments. Treated plywood (with waterproof glue, an important point) will last for years, five to twenty years, in the same environments. Less frequent replacement means more environmental friendliness. It hardly costs any more (about 10% on thick sheet of ply). The delay in damage being done gives one longer in which to uncover the problems and rectify it without facing a complete rebuild. :thumbsup: I wouldn't look forward to replacing my floor, for example :thumbdown:

Secondly, how safe is CCA? It's not a matter of opinion, it depends solely on whether the copper, chromium and arsenic can get out. This is what the NZ Department of Conservation says:
"A unique feature of CCA is that by a complex series of chemical reactions, the preservative components become fixed in the wood. .....Without any doubt the key issue is that the preservative has had time to completely fix in the wood before it leaves the treatment facility and that it is free of deposits....current research at NZ FRI and elsewhere is in the development of treatment processes which guarantee that these conditions are consistently met”. (NZ Environmental Risk Agency holds a similar opinion)

There are no reported cases of harm to an adult or child in NZ through handling or inhalation of CCA treated timber. It has been in constant use since 1960. :applause: Why no known harm? :thinking:

Firstly, arsenic is a natural element. It is common in volcanic soils in NZ for example. All bodies are familiar with it, in minute quantities.
Secondly, the arsenic fixes into a compound with the chromium, the cellulose and the lignin in the wood. It leaches only reluctantly.
Thirdly, sealing with paint or varnish takes care of the small possibility of leaching.

A number of governments, under pressure, have made precautionary rulings against CCA treated timber indoors, or in kids playgrounds, or even generally. And there are reasons, including problems with disposal, burning and the manufacturing process that justify those rulings. But there aren’t vouched cases of harm, other than from people drinking the preservative liquid either accidentally or with the intention of suicide. These rulings are not fact-based, they are opinion based. The Australian Minster, introducing a partial ban, said “I cannot rule out the possibility of harm”. No, he could not. But, then, we cannot rule out the possibility that he is a fifth columnist from the planet Zog :twisted: whose strategy is to weaken the earth with confusing and ultimately pointless rulings. I'm pretty sure he isn't, but I cannot rule it out. It doesn’t take genius to see that his position begs the question whether a sensible man would use it in his own affairs if there were advantages to doing so.

In the meantime, it is the far superior material and overcomes one of the fundamental objections to plywood as a building material; rot.

Incidentally, there are probably twenty-five elements and compounds in cars that are toxic to human beings. There are Chromium, Bromine, Chlorine and Antimony in abundance in the cabin of some new cars, and some of them outgas at up to 8000ppm. That new car smell! It's Antimony and Chlorine mixing! Ahhhh. (For comparision, the measured levels for arsenic leaching from CCA are about 3ppm) Fortunately, like the arsenic in CCA, these elements in our cars are generally compounded, bound, or impounded and cannot do us harm and, if they do, the harm is so minor we never, ever notice it.

To sum up, it is my opinion that far more people will be far more harmed falling down caravan steps :lol: than will ever be harmed by CCA timber in caravans, even if we all used it, provided we used it properly and followed the sensible precautions and that. And our vans might last twice as long. I myself think that is significant but I understand that some might not care. :thumbsup:

I support everyones right to hold the opposite opinion :thumbsup: or any or no opinion at all. This is just my 2cents worth. :wine: Thanks for reading.
Last edited by hossesdad on Sat Apr 08, 2017 5:05 am, edited 3 times in total.
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Sat Apr 08, 2017 1:54 am

I'm using frame/stringers because I don't entirely trust the foam, plus I like having plenty of attachment points. A note on bonding, I'm pretty fond of using both the carpet seam roller and wallpaper perforator to texture the foam. The foam *will* pull apart first regardless, but the texturing makes it stronger by bonding deeper into the foam. I also like making the filling paste with micro-balloons. They save some weight and cost, and are plenty strong for this application.


rruff, looking forward to your first photos. Take a photo of the space the camper will occupy when built to get our taste buds going :lol: . Are you going to glue your frames and stringers to the foam? The difference between of composite and framed (if any at all) is in the ability to spread the load. Framed probably exceeds composite if it is all glued together lets call that version "framed composite". :applause:
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby rruff » Mon Apr 10, 2017 11:51 am

hossesdad wrote:rruff, looking forward to your first photos.


Don't hold your breath! I finally bought some redwood 2x6s and cedar 1x8s, the 2" foam (for the floor), and borrowed a table saw. Plywood (3mm Okoume, and 4mm Meranti) and fiberglass supplies should arrive next week. The cedar looks great, but the redwood has a lot of knots and cracks. And I picked out the good ones (only one lumber yard)! I'll probably need to make the long drive for better ones.

I have some diagrams in the "More durable foamie?" thread: viewtopic.php?f=55&t=67779&start=15 Changed to a curved roof, but might change back. Solar panels and vents prefer flat surfaces.

Yes, the whole mess will be glued together. The primary motivation for the frame (stringers) is a lack of trust in the XPS foam. Not the bond so much, but the foam itself. The foam isn't very strong and marine builders seem to never use it. Rather they buy much denser and expensive PVC primarily. Surfboard builders and a couple camper builders have had issues with FG delaminating from XPS. Speculation is that inadequate texturing leads to a weak bond, and then heat and outgassing pushes the FG loose. I hope my bonding method is good enough to eliminate that. At least I haven't had an issue in testing.

Anyway, if the foam or bond fails for any reason the camper would simply fall apart. With the stringers I know I at least have some support for the skins if that happens. I expect it to be a good deal more work though, and weight of course. When I envision the project with broad strokes it seems like it should be an easy and quick build. Then I ponder the details and realize it's going to take a lot of time. :o
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Mon Apr 10, 2017 5:02 pm

Rruff, I like your thinking. I like a lot of what I read here. That's no surprise. I could stand on the roof of my house with binoculars and never see a soul who thought that building their own caravan was both possible and a good idea. TNTTT has a filter, people only post here who have already lost their marbles :lol: .

Life has made me a student of fear :worship: . I have only two responses to fear; give into it :( and get it out of the way by limiting my range of action and the choices available to me. The other, which I find harder, is to make a decision as if I had no fear and then execute that decision in the face of the fear. I fear the foam will let me down. Logic says it won't.

For myself, I am going frameless because it seems to me quicker, lighter, probably stronger (frames can be seen as "cuts in the foam") and more modern. But feel for me, I have loads of fear but I am too stubborn to give in to it once I have made my mind up. I may end up looking stupid. That's not unfair; sometimes I have ended up looking clever and there wasn't any good reason why that happened, either. I moved to NZ because I loved sunshine and mountains; I have wasted my life climbing mountains and sunbathing! People say I am lucky. I have never seen it that way.

Again, I am explaining my decisions. I don't wish to force my logic on anyone else. :wine:
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby rruff » Mon Apr 10, 2017 9:57 pm

hossesdad wrote:I fear the foam will let me down. Logic says it won't.


It's very funny, before I even saw your reply I was rethinking my whole design. Came on here to tell you about it! You must be sending me subliminal messages...

Do I really need to worry about the foam failing? What if I assume it's fine? Forget the wood framing/stringers. I can use FG to join the edges together too. Do I need a plywood interior? I like the look of wood but there is the rot concern if water gets in. Now that I've found some inexpensive suppliers, epoxy FG is cheaper than marine ply. Imagine just cutting the foam to size and laying up the FG on both sides. Seems a lot easier than cutting pieces of foam to fit perfectly in the wood framing, then trying to fair the two materials to make the surface perfectly flat. Basically it won't happen, you'd be able to see the discontinuity of the stringers. I'll definitely get a better finish with foam only. I'll still need some hard points, and some frame around windows and doors, but that is a lot less wood than I was planning.

Certainly something to think about. I'm going to mull this over a couple days before I buy plywood. Maybe I won't need to get any.
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Mon Apr 10, 2017 11:04 pm

rruff, you will find some help on the australian Styromax website, they make motorhomes, caravans and campers (without frames) from Dow corning blue extruded polystyrene with two layers of fibreglass. :thumbsup: Neat as a new pin. Here is one of their truck campers. (Lots of their other videos on youtube, too.)

[youtube]https://www.youtube.com/watch?v=pEtc3zelA9c[/youtube]
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby rruff » Tue Apr 11, 2017 6:20 pm

hossesdad wrote:I am going to rout and epoxy 4” strips of ply flat to the outside walls where the foam pieces join (the foam pieces are 600mm wide). That strengthens the joins and gives me a furring strip to screw or rivet the aly to in the future if it delaminates anywhere.

Inside, I mean to have ply furring strips routed and epoxied into the foam beneath the wall ply, one at bed height, one at shoulder height. I will probably put framing at the door and windows and access hatches when I make the wall panels but I am toying with adding it after, although that is not so easy to do because of the aluminium. I am also considering other material i can use, eg, plastic...don't like wood near the windows and doors.

The shape of the caravan will be determined by building the walls oversize and cutting them to a profile I like, then putting them up. The roof will be wooden stringers, foam and thin ply covered in fibreglass. I may make the roof in place, I may make it elsewhere, in two pieces and epoxy it on and together.


Still don't know what I'm doing! :? I haven't thought of a good reason *not* to ditch most of the wood and all the plywood though, so I'm still leaning that direction.

Thanks for that link! I knew about that company but didn't know they had a bunch of videos.

Actually looking at your build plan, you don't seem to be trusting your foam (or bond) either! Routing the foam and bonding in ply strips will be a fair amount of work. Will the roof stringers be inside the foam or under, and why are they needed? Why ply with FG on top rather than FG directly on foam? Why Al on the walls and FG on the roof? Why plywood as the interior skin?

This is my current thinking: The bottom (which mounts to the truck frame at 8 points) will still have some wood frame/stringers (cedar and redwood) because loads will be high in some spots. It will also be 2" thick because there is a couple ft overhang on each side and it's holding up the rest of the camper. But I was planning to use 4mm Meranti with a 6oz FG on top for the floor surface, and I could just as easily use FG alone. I like the look of wood, but FG can be painted or gelcoated, and should look fine. Less wood, less chance of water sneaking in and rot.

The walls and roof will only have wood where I need to bolt or screw something "heavy" like a door. Otherwise it's just FG over foam, inside and out. ~2mm exterior (2 layers of 1708) and 1.2mm interior (1708+6oz). Seems like it should be easy to bond windows in place instead of screwing them, but I haven't seen that done. Thoughts? I've been looking at riveting through the FG skins too and that should be a viable option for modest loads (like solar panel mounts, strut mounts, latches, etc).

I'm definitely going to cut the foam to shape (with a knife) prior to building the sandwich. IME that seems like the best way to go by far. Foam cuts and shapes very nicely with a knife or other hand tools. Power tools make a mess of weightless foam dust that likes to get everywhere. Cutting FG with power tools makes a lot of itchy dust too. So I don't want to be cutting the panels after building them. Trying to keep cutting and sanding to a minimum, and hoping the only sanding I need to do is before the final gelcoat.

For interior hard points and wiring channels, why not wood trim boards bonded to the FG? Should work fine and be decorative, and easier than embedding wood in the walls, with the bonus of letting me place them when I build the interior instead of when I build the walls.
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Fri Apr 14, 2017 10:34 pm

Been busy for a fortnight on other stuff. Today finished the sash clamps and epoxied some foam together as an experiment. Decided to cut off the shiplaps and do a ply tongue instead. The fitup of the shiplaps is too terrible to work with ...nothing is straight, tongue thicknesses vary etc. :? Easier to start again. :roll:

Here is the gear.

Here are the sash clamps being used for a test epoxy run:

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Firstly, the sash clamps work with simple wedges:

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The bars have a moveable lump of pine screwed to them

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This is a shot with the wedges hammered (rubber mallet) into place:

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Here are the angle iron "hooks" at the other end of the bars:

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Here are the hooks on a rail (the same thickness as the foam) that is screwed to the table:

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Here, for those of you with good eye sight, :( is the reason why the shiplaps have to go; the impossible fit-up!:

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I am going to set up my saw and router and process all the foam at once, I think.

It's a fair bet that Dow Corning wouldn't be able to sell this foam in America, with such a poor finish. I had to drive one hundred miles to pick it up. No real problem, I can work around it.
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Sun Apr 23, 2017 4:03 am

Cut off shiplaps and squared and straightened the foam boards for all the walls. Here is one squared board.

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Trusty Makita router cut 8mm grooves to take 7mm plywood splines. Following previous experiences , rather than try and centre the groove in unreliably thick material, I set it slightly to one side and marked all the boards appropriately. All small variations in thickness are gathered on the inside.

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Board with splne in it, dry run.

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Splines were painted with neat epoxy, then thickened epoxy was made up with microfibres (West) and the grooves and board edges coated, then the spline was coated with thickened epoxy and the whole lot assembled and firmly (but not brutally) clamped.

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The finished boards while epoxy is curing. This is half of one side wall. The white/black plastic protecting the table is old sileage cover, about 250 micron I believe, washed, I have 200 square metres of it. Epoxy doesn't stick to it....or not very well.

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Re: 11'6" three berth ply/foam/aly composite caravan

Postby rruff » Mon Apr 24, 2017 11:29 am

Good looking joinery! :thumbsup:

Do you know what the material is for the silage cover? I need something for the same purpose.
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Mon Apr 24, 2017 6:15 pm

Hi, Rruff, it is polythene I believe, although the manufacturers (Donaghys) refer to "seven layer technology", heavens, life is complex if there are seven layers in a grass cover :o . Seems to be 127 microns, not 250.

I am on the third half-wall. :D
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby hossesdad » Tue Apr 25, 2017 6:31 pm

Made a small rack to help put the epoxy on the splines and not my fingers or the table (done with router)

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Here are two half-walls set up (not joined, of course) so that we can admire the size. We can start designing the doors, etc. The walls will actually be two inches higher, due to 2"x2" epoxied to the bottom, then trimmed to profile. There is about 8" to come off the length The broom is for scale.

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So we have the chassis built, the floor built and the walls half-finished. Looking forward to putting it all together.

Incidentally, I was quoted eight dollars each for high tensile bolts in small quantities! That was the list price at a big box shop :cry: . I went to the largest steel supplier and they sold them to me (well known manufacturer, 8.8 zinc etc) for ninety cents each. :)
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Re: 11'6" three berth ply/foam/aly composite caravan

Postby rruff » Wed Apr 26, 2017 10:43 pm

:thumbsup: I envy your progress!
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