Why use plywood?

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Re: Why use plywood?

Postby QueticoBill » Thu Mar 02, 2017 4:35 pm

This is a great thread. It has convinced me that I'll be fine building with a plywood floor and stressed skin ply-foam- ply walls and roof. Still weighing (not literally) 1/8" v 1/4" ply - I know 1/8" is strong enough - but 1/4" affords more veneer and glue options.

But convinced I'll be happy with plywood. Thanks all for thoughtful posts.
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Re: Why use plywood?

Postby Tomterrific » Fri Mar 03, 2017 8:39 am

If you have to ask... Plywood is fast and easy to work with, takes paint and glue, it can be nailed and screwed, readily available. Punch a sheet of foam and punch a sheet of 1/4", which hurts? I used underpayment on the sides for light weight. I stiffened it with vertical battens. Wood is pretty inside. :-)

I am intrigued by the idea of a foamie but wood is intuitive to work with and naturally strong, so I used plywood.

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Re: Why use plywood?

Postby BelchFire » Fri Mar 03, 2017 12:39 pm

IndyTom wrote:[<snip> Wood is relatively light, strong and very easy to work with, and with proper construction techniques, virtually immortal. I hold a Master's Degree in Wood Science and Technology, and I can assure you that delamination and rotting are problems with construction, not problems with the material. With the proper protection, wood will last indefinitely.<snip>
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Hear hear! :applause: USS Constitution, anyone? What -- 200+ years afloat?
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Re: Why use plywood?

Postby KCStudly » Fri Mar 03, 2017 3:17 pm

BelchFire wrote:USS Constitution, anyone? What -- 200+ years afloat?


:lol: :lol: :lol: ...and literally built like a battleship! From oak. Solid oak. Thick solid oak. Heavy thick solid oak. Just saying.

I guess TPCE walks on both sides of the discussion; nice wood finished (semi-) exotic plywood on the inside, composite plastic on the outside. :thumbsup:

Cheers!
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Re: Why use plywood?

Postby rruff » Sat Mar 04, 2017 11:41 am

KCStudly wrote:nice wood finished (semi-) exotic plywood on the inside


Small segue, but what method did you use to glue your interior plywood to the foam panels? Also how did you seal/waterproof/finish them?
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Re: Why use plywood?

Postby JaggedEdges » Sun Mar 05, 2017 2:14 pm

Tackling the weight question, even though the plywood is "heaviest material possible" line has been proven utter BS,

Taking the following premises...

Average smaller vehicle and TnTTT combo around 4500lb
Most miles done on faster roads, 50mph plus, not much city stop/start.
20 mpg towing average.

Effect of weigh on consumption at highway speeds is through the rolling resistance which is one of the lower components of total load on the vehicle at approx 15%

If we assume that in a combo, trailer contributes 5 of the 15 total, this, and varying the weight +/- 500lb about a median of 1000lb is plus or minus 50% then we're playing with about plus or minus two and a half percent of the gas mileage. this is 12-13 extra gallons per 10,000 miles, or about 40 bucks worth of gas.

What do we think the lifetime mileage might be for an averagely frequent camper, 50,000 miles??? That's $200..

Anyway, for the spitball "average" situation, a super light drop not going to save you all that much in gas, so it better not cost you $$$ more to build than an average one. Be sure though that it's apples to apples, you can look at the bad mileage some of the big "crouchy" drops get and say "Weight!" but remember that at 6 wide and 5 high they'll have practically double the frontal area of a 4x4, and aero drag is almost all the rest over that 15% rolling resistance drag.

I'm not saying though that this is definitively the way to go for you or "May as well make it heavy." Because optimal use of material will be lighter, and save money in construction. Then there are not easy to quantify costs, like strain on the tow vehicle. Now larger ones with 3000lb and up towing capacities, there's probably going to be no noticeable difference. However, when you get lower tow capacities, where usually the transmission is the fuse, then the last couple of hundred pounds up to and over the manufacturer rating is probably quite non-linear for transmission wear and eventual replacement or repair cost. So you are not necessarily looking at gas mileage savings there. Towing with something like a Caliber, Vibe or an Element, you're definitely going to notice the difference between 500lb and 1500lb. Most people with V6 minivans, midsize SUVs however will say it tows a small trailer or popup "Like it's not there", so more room to play.

However, stuck with your average commonly available trailer frame at about 300lb and your average load of kitchen and equipment at 100-200lb by the time you add stove, cooler/fridge propane tank and all the trimmings, what your structure/skin choice is, is only around half the weight you're playing with for the all up weight on a smaller one, as shown in previous post, box made of thick slabs is gonna be about 400lb, so yeah, not going to be able to influence the outcome by all that many pounds on that score. I think that $40 per 10k would be optimistic amount of saving for a sensible 1/4" skinned foam and stick sandwich vs the moutaineering tent nailed to a flatbed.
(Probably at the same weight as the 3/4 walled benroy anyway, but you have R5-6 instead of R1 if that walls.)
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Re: Why use plywood?

Postby linuxmanxxx » Mon Mar 06, 2017 10:48 am

I had 2 identical size builds on 4x8. One was all 3/4 plywood solid frp skin and weighed 1100 and the other was 1x2 framed 3/4 foam filled torsion box build Luan skins and aluminum outer skin complete water based contact cement glue up of skins and 480 total weight so yeah plywood is heavy.

I could walk on the roof of the torsion box build at walls only an inch thick total. The glue spreads the load and creates massive strength at such narrow thickness and light weights.

I stood the 1x2 up for the roof and used double layer of the 3/4 foam so roof wouldn't sag across the open span of the roof.

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Re: Why use plywood?

Postby KCStudly » Mon Mar 06, 2017 4:51 pm

rruff wrote:...what method did you use to glue your interior plywood to the foam panels? Also how did you seal/waterproof/finish them?

Floor, bulkhead and one wall I used 3M 30NF water based contact adhesive. IIRC, the other wall I used TB2. On the ceiling I used PL300 spread with a notched trowel and kerfed the foam to ventilate. If I had it to do over I think I would have used epoxy.

All interior and galley paneling was stained with Minwax oil based chestnut color, and top coated with several coats of high build polyurethane (indoor grade, but testing found it to resist standing water well; can't vouch for UV resistance).
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Re: Why use plywood?

Postby QueticoBill » Mon Mar 06, 2017 8:35 pm

Have you tried moisture cured polyurethanes other than the PL300 and PL Premium for foam to ply bonds? A lot of liquid and some trowelable options in bulk.
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Re: Why use plywood?

Postby rruff » Tue Mar 07, 2017 12:22 pm

QueticoBill wrote:Have you tried moisture cured polyurethanes other than the PL300 and PL Premium for foam to ply bonds? A lot of liquid and some trowelable options in bulk.


What are some examples?
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Re: Why use plywood?

Postby Aguyfromohio » Tue Mar 07, 2017 1:59 pm

I suppose a discussion of the weight and strength of plywood ought to mention one of the great military aircraft of WWII, the de Havilland Mosquito. It proves you can make light and strong things with plywood. From Wikipedia:

The de Havilland DH.98 Mosquito is a British multi-role combat aircraft with a two-man crew which served during and after the Second World War. It was one of few operational front-line aircraft of the era constructed almost entirely of wood and was nicknamed "The Wooden Wonder"...When production of the Mosquito began in 1941, it was one of the fastest operational aircraft in the world...


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Re: Why use plywood?

Postby QueticoBill » Tue Mar 07, 2017 2:10 pm

Gorrilla Glue and Titebond Polyurethane are two familiar ones, as is of course PL Premium. A lot are sold as FRP adhesive and as flooring adhesive - especially wood floors. And I believe it cures or chemically changes with moisture as a catalyst rather than dries like PVA adhesives - Titebond II for example. Low priced - in the $30-40/gallon range - one part - and both trowelable and liquid for rolling.

It is what is used for stressed skin panels for stage platforms and acoustic shele products, waterproof, and just trying to understand if its a but for some reason for teardrops or just not explored yet.
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Re: Why use plywood?

Postby Aguyfromohio » Tue Mar 07, 2017 2:43 pm

QueticoBill wrote:Gorrilla Glue and Titebond Polyurethane are two familiar ones, as is of course PL Premium. A lot are sold as FRP adhesive and as flooring adhesive - especially wood floors. And I believe it cures or chemically changes with moisture as a catalyst rather than dries like PVA adhesives - Titebond II for example. Low priced - in the $30-40/gallon range - one part - and both trowelable and liquid for rolling.

It is what is used for stressed skin panels for stage platforms and acoustic shele products, waterproof, and just trying to understand if its a but for some reason for teardrops or just not explored yet.


I have seen polyurethane glue recommended by many here for gluing plywood skins onto foam for wall panels and floor panels using structural insulated panel approach.
We are on our first build, so I have no personal experience with this.

We decided to use epoxy instead, because we worry about the foaming of polyurethane. We made a couple 24 inch x 24 inch test panels using Loctite PL Premium polyurethane construction adhesive. We got great adhesion, but the foaming pushed a bit of adhesive out the sides and lifted the skins just a wee bit off the pink foam and edge wood. We worry that on the larger panels for wall and floor we might get bulging in the middle, ending up with panels not flat or straight. A second factor is the very long working time we can get with epoxy, hours of time before it sets up, as much as 10 or 15 hours with West Marine system epoxies.

Better advice on this topic from people with real experience is most welcome.
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Re: Why use plywood?

Postby rruff » Tue Mar 07, 2017 9:53 pm

Aguyfromohio wrote:We decided to use epoxy instead, because we worry about the foaming of polyurethane.


I don't have any more experience than you, but PLP seemed to work very well. I did not notice any foaming, but that could be because it was cold and I live in a dry climate. Did you texture or perforate the foam? One big advantage of PLP is that it cures in sub freezing temperatures.

Epoxy will usually gel in a half hour or less. I would not consider the working time to extend beyond this, though it may take hours for it to get solid.

PLP in 28oz tubes @ $7 is ~$32/gal. $55/gal epoxy mixed with $15/gal silica powder is ~$40/gal, so not much more really.
Last edited by rruff on Tue Mar 07, 2017 10:41 pm, edited 1 time in total.
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Re: Why use plywood?

Postby Aguyfromohio » Tue Mar 07, 2017 10:34 pm

rruff wrote:
Aguyfromohio wrote:We decided to use epoxy instead, because we worry about the foaming of polyurethane.



...Epoxy will usually gel in a half hour or less. I wouldn't not consider the working time to extend beyond this, though it may take hours for it to get solid.

PLP in 28oz tubes @ $7 is ~$32/gal. $55/gal epoxy mixed with $15/gal silica powder is ~$40/gal, so not much more really.

Thanks, gruff. You are correct, working time is quite a bit less than cure time.

The epoxy we plan to use is boatbuilders stuff from West Marine.
https://www.westmarine.com/buy/west-sys ... er--318386
They have a system of epoxies that use a set of ingredients.

An adhesive family with one ( of a few) basic epoxy resin, then a few different hardeners and fillers to match. Each hardener gives a different cure time.
The 105 resin with the 209 extra slow hardener gives 3 to 4 hours working time, with full cure in 20 hours.
The 206 slow hardener is probably slow enough for us, 90 to 110 minutes of working time, full cure in 10 to 15 hours.
Plenty of time to roll out the epoxy on the sheets and position them; find a couple mistakes and correct them.

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