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Old 08-20-2013, 02:10 PM   #1
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Plywood floor alternatives - engineering opinion

Hi Guys,
I'm new to the board, having just bought a '73 31' Land Yacht.

I haven't even touched it yet (it's pretty much all original), but I'm anticipating getting in to address a rusty frame and probably floor rot.

I have been reading up on this site and there was an extensive discussion (seems never ending) about floor materials, replacement processes, etc. I even saw a guy do a fantastic job replacing all the wood with aluminum sheet.

I'm an aerospace engineer and have worked in the aircraft (actually helicopter) industry for 15 years. From a structural perspective, i had a couple of thoughts / questions:

1. Has anyone attempted to replace the plywood layer at the perimeter (under the "C" or "U" channel) with a structural member, like 3/4" square steel or aluminum tubing? You would effectively create a perimeter frame that matched the channel planform, and connected to primary frame either by branching across each outrigger, or having perpendicular sections that bolted to the major frame rails?

2. By doing that (and choosing the correct materials / insulators to avoid galvanic corrosion issues), you would decouple the floor from the frame/shell, at less weight than a full sheet of aluminum...

3. You could then fit your plywood (or other non-rotting) material inside that frame (tabs on the perimeter frame could be made to support the edge of the floor), at whatever thickness was needed to give you sufficient stiffness for floor loading.

What am I missing?

Thanks
Simon
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Old 08-20-2013, 03:03 PM   #2
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You can do it!

Hi Simon and welcome to the forums.
I spent 20 or so years maintaining the machines that came from Statford. I even spent some time there working on a project back in 1999-2000.

This discussion comes up pretty often here. I'm not an engineer. I do know that without a sound sub-floor firmly attached to the frame, the frame flexes a great deal more than you'd think it would. When you do get down to the bones of your Sovereign, you'll see that none of the stringers are attached to the ribs and most of the ribs are barely attached to the U/J channel. It appears to be built like an aircraft, but when you get down to the bones, it isn't. It is a travel trailer. The best travel trailer being built today. It would be great to have an Airstream that didn't suffer from rotten floors. I'm sure it could be done. I like the idea of some type of composite marine panel in the place of plywood, but haven't found what I think would work well yet.
To see an Airstream that IS built like an aircraft, see Kip's GT thread.

I believe that Avion used this type of construction you are talking about.
I'm interested to hear the replies.
Good luck!
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Old 08-20-2013, 04:24 PM   #3
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Yes a perimeter frame would be the way to go. Here is a thread I started a while back about this. I am an aerospace engineer as well.

http://www.airforums.com/forums/f36/...gn-100154.html

Perry
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Old 08-20-2013, 05:40 PM   #4
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My thought would be to use 3/4"x2 1/2"aluminum bar stock around the perimeter of the shell. Using clips between the edge of the bar stock and the plywood. But also leaving a 1/4" gap between the edge of the bar stock and the plywood.
Leaving a gap would allow any water from exterior leaks to drain into the belly pan. Without edge contact on the plywood, the water would not wick into the wood.
With a proper insulation in the belly pan that would not retain the water and a system of vents and drains. The corrosion of the frame would be significantly reduced.
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Old 08-20-2013, 10:07 PM   #5
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Quote:
Originally Posted by TG Twinkie View Post
My thought would be to use 3/4"x2 1/2"aluminum bar stock around the perimeter of the shell. Using clips between the edge of the bar stock and the plywood. But also leaving a 1/4" gap between the edge of the bar stock and the plywood.
Leaving a gap would allow any water from exterior leaks to drain into the belly pan. Without edge contact on the plywood, the water would not wick into the wood.
With a proper insulation in the belly pan that would not retain the water and a system of vents and drains. The corrosion of the frame would be significantly reduced.
Why not just build a new brand of trailer instead of trying to reengineer the time tested airstream. You can have a better frame, better axles, better body and in 75years people will be going remember those airstreams they sure were cool untill these guys came up with a better trailer.
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Old 08-20-2013, 10:41 PM   #6
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plywood floor alternatives

I think the point of these discussions is how to improve upon the design, not start over...

I'm sure the airstream engineers would change things if they could, but like all engineering challenges, you are faced with the requirement to compromise and balance issues like cost, manufacturability, weight, etc.

As one off modifiers of existing trailers, we do have the luxury of perhaps not having to worry about it taking too long to make, eroding profit margins, or requiring too many different materials.
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Old 08-20-2013, 11:50 PM   #7
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This is good stuff.

AS has brand recognition.
AS has been doing it (mostly right) for a while.
AS has reduced overhead (compares to a startup) and retains key people in their processes.
AS has lots of dealers, not the most, some elite.
AS has "the club"

Most of this can be said of HD...but HD vastly improved the product, for a while. There are plenty of used HDs in dealers inventory today.

All this to say, "why not?" Many here have cumulative business and life experiences to make a NEW product... To give AS something to chase. Now the dollar is all they appear to chase.
But ask yourself, if I could, would I?

Simple changes.(improvements ).. Conduit for wiring, built in "moisture" sensors at strategic points, access panels for inspections/repairs, mobile teams of experts to GO TO customer and resolve problems as they travel if necessary. Facelifts/retro options with new feature function could be added.....

Simple, cost effective as it targets long term ownership.

It may happen "one day"... After all, we rebuild them here...

Others have restoration services...why not strive to become the leader there?
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Old 08-21-2013, 08:21 AM   #8
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I too had thought of fabricating a perimeter frame and removing the plywood interface. Instead I just decided that using the age old method of ply only would work best for me if I sealed everything up. I'm still not sure that I made the right choice though, time will tell. I have gone overboard in sealing up the trailer with PRC. I do believe that with proper sealing that the plwood rot problem can be reduced to something manageable. Condensation is my only concern at this point, and I plan on some type of condensation drains, just not sure what type yet. Thanks Top
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Old 08-21-2013, 08:55 AM   #9
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If Ford ran their business the way A$ has been run the last 60 years. We would be driving a '54 Ford.
No seat belts, no air bags, no computer controlled fuel system. But it would have a digital stereo.
About the only thing I can see in the basic A$ design that has changed, is the frame has been beefed up. Mostly because it had to be done to stay together when the larger trailers with holding tanks came into play.
The only reason that A$ is still considered the top of the line. There is no competition, like Ford had with Toyota, Honda and many others.
A$ lacks the vision required to correct problems such as floor rot, leaks and frame failures, access to devices such as converters, furnaces etc. The market doesn't demand improved quality because the customer usually is enamored by the shiny looks. It is only after the fact that they discover the problems.
There is a saying. "Beauty is only skin deep".
There is a very expensive car built in Northern Germany. They compete with Ferrari and others in the limited elite car market. The company started in '87. They have state of the art design and facilities. As a matter of fact the car is riveted together, using state of the art adhesives and sealants to prevent it from leaking.
The Weismann Company builds each car by hand. Much the same way A$ claims to do.
They have been in business for over 26 years competing with the "Big Boys". Using high tech materials and innovative ideas.
Many will say that these coaches are built to last. They will only last when buckets of money have been poured into them to restore them to at least their original condition.
To me, those who love A$'s are like Classic Car buffs. Money is no object.
Even classic cars have been upgraded with disk brakes, rack and pinion steering etc.
So! Why can't A$ resolve the problems like floor rot and other problems that have plagued them for years?
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Old 08-22-2013, 02:59 PM   #10
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Overly stiff frame?

The concept of increasing the frame stiffness too much is an interesting one... theoretically you're redistributing the relative deflections and could actually cause issues where there weren't any in the past.

We've see this on aircraft - inserting for example a steel frame into a spot where aluminum was used originally did solve your cracking issue on that frame, but drove all sorts of fatigue issues into the surrounding structure...

That being said, as someone pointed out earlier, it's a camper, not a fuselage... I'm intrigued to see how much (or little) structure there really is in the aluminum shell...

I think the bottomline is that one may not want to mess too much with something that's already working - so if you're going to replace the U channel interface with something other than plywood, you don't want to overdo it and cause problems elsewhere.

I'm encouraged by Aerowood's work - proper application of proven aerospace corrosion resistant surface treatments (primers, epoxy paints, that zinc chromate stuff that really works) and sealant (Proseal is your friend) is probably the pragmatic choice if you don't want to turn this into a decade long project...

I would also think that liberal application of non-curing sealants would be helpful too - mastinox anyone? Just don't let your kids touch it...

I'm excited to find such a DIY community. My last big project was building a replica Porsche Spyder (you can see my car here - www.spyderclub.com • View topic - Thunder Ranch), and a vast majority of those owners were far less technical...

cheers
Simon
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Old 08-22-2013, 06:44 PM   #11
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Well mine has been leak free for a while now. I can see the actual plywood surface and have not seen anything bad so far. I am hesitant to cover the plywood which will hold water and not allow me to see problems before they get really bad. Putting in gutters and drains and sealing penetrations through the bottom c-channel will help. Airstream has made some improvements. The bottom c-channel now cradels the floor all the way around the perimeter even on the curves. They have put in drains as well. They got rid of the double pane windows that were a leak waiting to happen. The seals would leak and the seam between the two halves of the frame would leak. I still think they need to deal with the bumper plate issue and seal the bottom c-channel so water won't go into the floor. They are also getting away from the rear bathrooms which caused problems with too much load back there and it covered up leaks under all that plastic. I also think they have gotten rid of the stinky pink crap that holds water against the frame and floor.

Perry
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Old 11-12-2013, 09:23 AM   #12
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This is a very intimidating discussion. I do not have anywhere near the engineering qualifications to suggest structural change. In fact, now that I can see and manipulate the exposed frame and have to replace some crossmembers, I have already changed my repair game plan due to fears I might add too much weight and reduce flexibility to the unit. This steel seems to have interesting properties, strong, yet lightweight while being flexible (mine is stamped "Roswell, NM 1947" & has some strange sort of alien characters on it, Ha! )

But here is a weatherproofing suggestion: Has the idea of an aluminum flashing been discussed between the outer skin and the C- channel/plywood? It could be bent to follow the outside contour right below the trim (at least that would seem to help with my '72). Perhaps it could act as a mini external gutter to follow below the trim, effectively incorporating with it in appearance. It would be sacrificial and sparing of the skin at the end plates. This could obviously affect aesthetics, but once I get down below the trim, I am thinking more about weather proofing than looks. But I am sure there are different ways of possibly incorporating it into the belly, also. In theory, I picture it as the inner lip that you have on a cookie tin. I don't know if it is original factory construction or not, but my belly skin overlaps the outer skin (under the trim). The inner skins are also the same in the bathroom/shower (although, I give the PO credit for incorrect reassembly here. But it seems that if your seams overlap in an underbite, you are inviting water seepage.

If you have a cookie tin and sprayed water on it, you would have some structural protection. But if you turn it upside down, the structure invites seepage. *NASA if you are reading this, please note I have had no formal training* Getting to the point... flashing may provide the inner "cookie tin" lip structural advantage without changing the time-honored c channel/plywood sacred cow. But it is possible that all of what I am seeing and basing my suggestions on are PO reassembly error. My rear bumper box and its union with the body I feel is, by far, the reason for my floor rot. The box flooring was continuous with the belly pan which made the belly pan a nice little drainage pool. The top of the box behind the hinge went below the outer skin and under the c channel, making the plywood flooring a nice little collecting pool as well, with nice little eventual waterfalls to the pools down below. I can't imagine this was factory construction. What was original? I think this box would be better of completely separate from everything else.

If anyone is still reading this, and in the interest of moving closer to initial thread topic, I am interested in ideas on pros/cons of plywood & all the potential treatment methods and any non-plywood alternatives. Some current thoughts: Coosa bluewater 20 (?), HD or LD (?) polyethylene sheet (kitchen cutting board material), Plywood with (1)water stain 2)epoxy primer/paint 3)fiberglass layer 4)linoleum or shower wall material (?) sheet covering 5)just plain Marine. But to me, more importantly, is what to do at the exposed ply ends: coatings, bumpers, or what?

Now...fire away!
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Old 11-12-2013, 12:14 PM   #13
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How to improve the time tested airstream floor rot problems due to design flaws. If one does a shell off restoration the 1st problem one will notice is that the belly pan overlaps the shell and the floor sits where it can soak up water. It's bad practice, over time it is sure to fail.
Personally I eliminated the c-channel and put in a 1.5 steel tubing perimeter band. I will then place stubs that bolt to the ribs then will weld them to the perimeter band. The flooring sits inside the band separate the shell and frame. That way the floor can placed or removed anytime. I'm also boxing the frame near the axles. After 35 yrs the frame has developed a curve after the axles. I'll jack it up to take the curve out then box it in. The belly pan will be riveted under the shell so it sheds water. I'm also spray foaming the shell for insulation. That will strengthen the so it should be able to handle a beefer frame.
These are the steps I'm doing to eliminate the problems I see. So far, it's great. Only time will tell. But one thing is for sure, I'm having blast😀. Maybe one day I can go camping.
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Old 11-12-2013, 12:20 PM   #14
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To answer the flooring question. I went with advantech subflooring. It's 3/4 inch thick. It's also water resistant . I left a sheet outside on the ground exposed to weather for 6 yrs. then used it for a project and it was still like it was when purchased.
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