Need Advice: Expanding Design and FRP

[TomsTwin]

Hi… I have become strangely obsessed with the idea of building a teardrop to accommodate my photo journeys. I want to build something that is both roomy and light. And, since I am a complete germaphobe, I want it to have its own bathroom and shower. I will be towing it with a Honda Element which has a 1500 lb limit.

My design pics are attached below, but here are a few questions. Any opinions and guidance will be much appreciated.

1) I am planning on using a 5’ x 8’ Carry-On Trailer with a rim and with the ramp gate removed. The rim will act as a place to bolt the sides to for added support. Supposedly, the 5’ x 8’ weighs about 300 lbs without the ramp. http://www.carry-ontrailer.com/product/4x6WM_OT.htm

a. Is there any reason his trailer won’t work for me?

2) The images illustrate the side design. I will be using ½” plywood (sealed and stained red) glued and screwed to yellow FRP (Fiberglass Reinforced Paneling). This will be the outer shell. Behind this will be 1” insulation and then paneling for the interior.

a. Is ½” plywood okay?

b. What glue would be best for attaching FRP to wood, Titebond III, PL Polyurethane, Gorilla etc.?

c. Should I glue the pieces before sealing the wood, or seal the wood first?

d. Should I attach any inner beams and crossbars directly to the wood or to the inner FRP?

3) I plan to have the roof raise about 30”, totaling 6.5’ in the interior.

a. Will 1” aluminum tubes suffice for the tracks that will guide the roof to its upper position? Aluminum C-bars fit perfectly inside of these.

4) The front dome will tilt to create the ceiling of the shower/toilet area.

a. How do I collect waste water from the shower?

b. Does anyone have a recommendation for a great port-toilet?

5) I understand the general idea for the back hatch construction, except for one thing…

a. How do I protect the curved side seems from leaking water when closed?

I’ll probably have a bunch more questions, but for now, I look forward to any help and advice anyone can offer.

Thanks so much… Tommy

[mikeschn]

Hi Tommy,

Welcome to the forum.

I can't see the shower in your design. Or the porta potty for that matter. Where do you have them hidden?

Mike...

P.S. I stuffed the porta potty in the corner of mine...

[TomsTwin]

Hi,

The shower and potty are located in the room created under the front dome when it is lifted up with the roof. The panels that surround it are placed after the roof is raised.

Thanks... Tommy

[ZendoDeb]

b. What glue would be best for attaching FRP to wood, Titebond III, PL Polyurethane, Gorilla etc.?

Hands down the answer is epoxy thickened with colloidal silica - usually sold as "Cabosil" Mix the epoxy per instructions, when really really mixed add cabosil until you get mayonnaise to peanut butter consistency. If you add too much, the epoxy will go off immediately. (Depends on hardener/ratio/temperature/etc.)

Gorilla glue is a good - but distant - second.

c. Should I glue the pieces before sealing the wood, or seal the wood first?

epoxy the wood together first, put fillets of mixed epoxy and wood flour to round out inside corners. Round over outside corners (1/2 inch or better router bit) Then cover with epoxy saturated mat - or cloth if you must. Mat is stronger. Cloth will cure clear - or nearly so.

Epoxy is strongest if it is done in as close to one step as possible. Depends on the size of what you are working on. Anything that will be glued with epoxy/covered with mat should be bare wood. Seal any wood that is left exposed later. Epoxy to cured epoxy forms a mechanical bond that is only capable of supporting about 200 to 600 pounds per square inch - I think that is shear, but I don't have the figures in front of me. Epoxy to bare wood forms a chemical bond that is more on the order of 2300 pounds per square inch (Something like West System, or good laminating epoxy. NOT 5 minute epoxy.) It is usually stronger than the wood itself - failure with often occur in the wood member itself before the GRP covering or the interface.

The things that impact strength are cure time, (slower is stronger) ratio of resin to hardener (more resin/less hardener is stronger)

Also beware of amine blush. Depends on which epoxy you select.

Epoxy won't adhere to everything. Most notably, epoxy won't adhere to gelcoat.

Epoxy resin is stronger than polyester or vinylester resins (more traditional fiberglass.) Epoxy is more waterproof than polyester resin. Polyester and vinylester don't adhere to plywood near as well as does epoxy.

Although the smell is better with epoxy than with polyester resin, epoxy is much more toxic. Wear gloves when mixing and a respirator - not a cloth mask - when sanding. If you are working on a large section, or in an enclosed space - ventilation is important, and I would wear gloves, a respirator and a Tyvex suit. (I would do that with polyester too)

For a smooth finish use epoxy thickened with micro-balloons, or other fairing compound. Makes the top layer easy to sand so the paint gets a mirror-like finish. (use a 2-part primer)

Epoxy has NO tolerance for UV. It must be painted.

d. Should I attach any inner beams and crossbars directly to the wood or to the inner FRP?


See above - you will get a stronger bond between the unsealed wood pieces using epoxy.

You might want to look at a book (or a website) on stitch and glue boatbuilding. Your library may be able to get you a copy of "Devlin's Boat Building." You are not building anything that will take near the stress as a boat, but what you are describing is very close to stitch and glue. A monoque structure is light and strong. Dinghies and small power boats are built out of 1/4 inch (6mm) marine grade plywood. The finished products - which contain a few wood ribs/stiffeners - are quite strong for their weight.

Incidentally - if weight is really an issue and cost is not so much - you can substitute a foam core (special foam) for the plywood in places that won't take a lot of stress - it can handle a fair amount of stress. And the foam planks for stiffeners. (see previous link or search for KLEGECELL) KLEGECELL can be used to build decks - so it is fairly strong. (some foam can be used to build hulls - really strong when impregnated with epoxy.)

[ZendoDeb]

If you are doing anything in the galley that will come into direct contact with food, you had better use food-grade epoxy. It is out there.

Or cover the epoxy with something like laminate.

[kennyrayandersen]

Is it going to pop up like that and be canvas or something?

[Miriam C.]

Tommy, about your diet! I see enough wood in your design to almost make 2 teardrops.

1/2" is too heavy for what you are doing, if you are framing and insulating. You are very close to Gary's Winter Warrior and it is 1700lbs.

You are also talking about carrying water, or you could fill and empty at the campground. A bottle of bleach is lighter and cheaper... Just joking!

If you look at Steve's Puffin he managed a shower inside. Mike's (see above) would work for a shower too. Just depends on how you finish it.

I am waiting for Scott to weigh his Winter Warrior and to see the weight on Steve Coxes Gypsy Caravan. You might look them up just to get an idea of how much wood you can put in a build and keep it light.

Scotts
http://tnttt.com/viewtopic.php?t=27592

Steve's Gypsy Caravan
http://tnttt.com/viewtopic.php?t=26296

Good luck

[TomsTwin]

Hi, thanks for your quick responses.

I am confused though. I was worrying that the 1/2" ply might be too light. The yellow parts of the drawing are FRP (which isn't light).

The layout of the side walls are as follows:

The 1/2" Ply frame -> FRP backing -> 1" foam insulatin -> light-weight paneling.

The yellow walls of the expaned view are just FRP -> 1/2" insulation -> light-weight paneling.

Does this still sound too heavy?

Thanks... Tommy

[Miriam C.]

Hi, thanks for your quick responses.

I am confused though. I was worrying that the 1/2" ply might be too light. The yellow parts of the drawing are FRP (which isn't light).

The layout of the side walls are as follows:

The 1/2" Ply frame -> FRP backing -> 1" foam insulatin -> light-weight paneling.

The yellow walls of the expaned view are just FRP -> 1/2" insulation -> light-weight paneling.

Does this still sound too heavy?

Thanks... Tommy

Great diet! Sorry for the mix up. So you need to glue the FRP to foam and foam to paneling. I need to research that one. Dunno

[kennyrayandersen]

1/2 inch still sounds like too much

go at least to 1/4 and glue the whole panel to gether for the strength

[TomsTwin]

Wow! I really am surprised. I was afraid that I should go with 3/4" ply for the sides. If I use 1/4". what would keep the whole thing from folding at high speeds?

My original thought was to cut the shape of the sides and cut out the knockouts (where you see the yellow FRP showing through). I figured this would remove some weight, but still keep the strength. If I went with 1/4" ply for the sides, would that still support the trailer?

I had actually figured that in all the build photos I'm looking at, people are using 3/4" ply for the main structure. Am I wrong?

[kennyrayandersen]

the key to making it light is to get everything working together. If you go with 1/4 on the outside, 3/4 foam frames with some 3/4 lumber, and 1/8 paneling or Luan on the inside and glue the whole shebang together (rough up the foam and suck a vacuum on it or get you a bunch of sandbags or whatnot). The foam reacts the shear loads and the 1/4 and 1/8 react the in-plane loads. You'll frame it out in hardwood which you can then use to screw it all together (glue and screw). You probably want to use epoxy, though lately I've been reading about some guys using Titebond to fiberglass stuff (who would have thunk it). Still the conventional way is to use epoxy -- lots of epoxy info on this site and others. One the bottom is attached to the sides plus any internal walls, like the galley) it will become pretty strong and the loads will spread out through the structure.

Wood weighs more than you think when you get a bunch of sheets together. I think your current concept will actually be heavier than you might think -- start adding up the sheets and the weight per sheet, plus the frame and see where you are at.

You might consider building a standie like the Puffin (long thread, worth reading). If I remember correctly it weighed in at about 700 Lbs, which ain't bad. It had accommodations for a porta potty and a simple shower. Or you could consider a canned ham. The thing about foldouts is that none of the stuff folding out is structural, do your structure and frame and whatnot have to be more robust. If you build everything joined together its much stronger so it can be built much lighter -- food for thought.

[TomsTwin]

Thanks... This is very helpful. I think you are right. In trying to make it lighter, I may be doing the opposite.

Question though... you wrote, "go with 1/4 on the outside, 3/4 foam frames with some 3/4 lumber, and..."; what did you mean by "3/4 foam frames with some 3/4 lumber"? Is the 3/4 lumber the framework?

I was exploring the lumber at Lowes this morning and the Contractor Rep suggested 3/8 ply.

What about for the galley back wall connecting the 2 sides. Should that be thicker since it is structural?

Thanks... Tommy

[jay]

[quote="TomsTwin"]...what would keep the whole thing from folding at high speeds?....[/quote]

are you going to travel with it collapsed or set up?

would the sides for the front domed section be carried inside? anything i have ever built gets the hardware selected prior to construction & the construction may need to be planned around the hardware.


seems like a good design & construction exercise, but it's way too complicated to be put into field service.

don't mean to harsh your buzz, but could you set it up after driving 300 miles in the rain - in the dark?

[kennyrayandersen]

Thanks... This is very helpful. I think you are right. In trying to make it lighter, I may be doing the opposite.

Question though... you wrote, "go with 1/4 on the outside, 3/4 foam frames with some 3/4 lumber, and..."; what did you mean by "3/4 foam frames with some 3/4 lumber"? Is the 3/4 lumber the framework?

I was exploring the lumber at Lowes this morning and the Contractor Rep suggested 3/8 ply.

What about for the galley back wall connecting the 2 sides. Should that be thicker since it is structural?

Thanks... Tommy

oops -- I'm on Korea time so, though for you it was early morning in USA -- it was really late for my brain here in Korea

What I meant was you should frame out the 3/4 foam in 3/4 lumber -- so that the foam fills in all of the spaces and the wood goes around the perimeter and door and any cutouts. You should put glue along the edges of the foam wood frame interface and then when you glue the outside and inside sheets to the foam 'panel' you will end up with a nice strong composite panel with wood on both sides and foam in the middle -- that way everything becomes structural. The wall across the back could be 1/2 as well, it really only reacts shear load and plywood is pretty strong for that. 3/8 would work too.

You really should read about the Puffin though to make really sure you don't want to build a little standie like Mike was showing pictures of earlier. There are several of those designs on the sight -- also check the design library (link at top of page). Mike, Andrew and others have put a lot of thought into it and there are a lot of good ideas out there on how to do it. I only bring that up because your design seems to little ole me to be getting kind of complicated, when you could build a standie for less money, lighter weight, and would require less time (maybe MUCH less time). Read. Think. Decide.