Plywood composite panels?

I'm building a small trailer ("land-boat" if you will) that involves a bunch of interior benches & cabinets and I've been pondering making some home-made plywood composite panels to keep it light and strong.

It's not a boat, but I'm wondering if anyone here's ever used/made anything like what I'm thinking about. It seems to me like a similar problem must have been solved in a boat cabin before.

I'm imagining that I'll build a thin-ish wood frame (3/4"), cut out 3/4" sheet foam to fill the gaps (probably XPS for $ reasons), then vacuum bag 1/8" birch ply skins over the whole shebang. Maybe a thin layer of glass over the top if I feel like it needs extra dent resistance.

Use case:
- Bench tops & removable panels (panels convert benches into a bed) for a small travel trailer
- Panels are all interior, so UV isn't a concern, but occasional water spillage, rain, leaks, etc to be expected
- Light weight is important - trying to keep the whole trailer weight down
- Widest unsupported span is about 27"

Not sure if anyones whipped up something similar for the inside of their boat, but if they have I'd be interested to hear about their experience/materials/techniques.

 

I imagine that the bonding of the foam to the skins is one of the bigger issues. What to use for that ? Contact adhesives might melt the foam. Maybe cheap builders adhesives might not.

 

http://www.joewoodworker.com/

 

@Jim I ordered a bunch of vacuum stuff from them! I have an adjustable vacuum switch so I can keep the vacuum low enough not to crush the foam.

@Mr Efficiency I think titebond may work - some folks laminate canvas directly to XPS to make foam teardrop trailers. There's also some contact adhesives designed for foam (3m 77), I think there are also some water-based contact cements that won't eat the foam. I'll probably try all of the above and see what works best.

 

http://theplywood.com/weight
The above site has a weight calculator for various thicknesses of ply and material

Quite a few aluminum boat builders in the Pacific Northwest will use poplar plywood to keep weight to a minimum.

3/4 inch softwood SP ply weighs about 61 lbs per 4 x 8 sheet

3/4 inch poplar plywood, weights about 43 lbs per 4 x 8 sheet

What would your composite weigh?

If you take your two layers of 1/8th birch, you are starting with 27 pounds of birch, adding in some 1/2 inch foam maybe a couple of pounds.


Then you have to add in two layers of cloth and resin, and maybe a quart of resin, another 4 pounds? and then some weight due to the glue that you need to bond the foam to the ply, say a pound.

So now your composite panel is up to 27+2+ 4 + 1 = 34 pounds compared to 43 pounds for a difference of 9 pounds per 4 x 8 sheet of poplar ply.

But you might not be finished yet.

If you want to make a box, ie a 90 degree corner, you could glue your composite ply but would not be able to put screws in the corners, ie a screw into the foam or 1/8 birch so you would have to glue in a blocking in each corner. If you want to be able to screw something onto the composite, you would need to have added in a blocking/backing plate to add holding strength for the fastener.

So depending on your application, you might save 8 pounds per 4 x 8 sheet of plywood.

So moving from SP ply to poplar saves you 18 pounds per sheet, but your composite saves you only 8 pounds per sheet compared to poplar.

This is not a lot. As it is a trailer that you are building, you could use the poplar ply on the floor base, whereas maybe not your composite, and the rest of the bench structural components and save big weight over using regular softwood ply construction, ie 18 pounds per sheet.

If you want to save some weight in your trailer, use aluminum for the subframe, a torsional spring set up for suspension, aluminum wheels.

 

Using plywood in general, isn't the lightest way to make your panels. Veneers over foam or honeycomb would be lighter and you could arrange the grain to load paths. Another way to make light panels is to skin foam or honeycomb with fabric set in goo. These could be done flat on a table, insuring smooth finishes, then cut up and assembled into cabinets, countertops, etc.

In the end, the method and techniques you elect to employ will be based on what you're most comfortable with. Most folks will have few issues with "stick built" methods, whereas you use 1x2's to form a frame and skin these with 1/8" plywood. This is relatively light, the 1x2's form a structure and "nailer" for attachments plus it's cheap. Fabric skinned foam or honeycomb can be lighter, though attachments and the "goo factor" will be more convoluted and more difficult for some. If you really want o go light, composite panels, maybe bagged down on a bench initially, is a method. Of course, your budget and experence will come to play with each. So ask yourself what tools and experence do you have for each process and does you budget permit the various options.