Spar size and ply dynamics ?

[RogHodge]

The background, building a 5x8 foam/ply hybrid. Walls are 1/4 ply with 1" foam exterior. Roof 1/8 ply with 1" foam. All encapsulated by canvas/glue/paint shell. The shape is basically the same as the 'ultralight'

The question, spar size- if any. The roof ply will be glued and stapled to the wall ply, preliminary tests seem to indicate good holding power. The foam ply combo is quite stiff, so if the roof ply is bent to the curve of the profile- do I even need spars? My thought is to go without, thinking the curved ply is the real source of strength. The interior 'spars' serve mainly as a cosmetic cover for the joints.

Any thoughts?

[RogHodge]

I know you are out there...
I can tell you have been looking...
Why no responses?

[mikeschn]

Maybe there are no responses because we can't visualize your construction method.

For example, if there are no spars, what are you stapling to what?

In a perfect world, with jigs and fixtures I can see how you can make that work.

But tell me how that works in the real world...

Mike...

[RogHodge]

Or maybe it is because you all hate me and I should just throw my lollipop in the sand and run home and cry

So here we have the interior wall, don't judge me by the mess, or water damage, or etc or etc.


And the close up of the wall / foam joint



And the proposed placement of the roof, which would be glued and stapled, and then would then be covered by foam



In other news, thank you for this forum. It was improved tremendously since I lost contact. Uploading pics is a breeze and as always the information is wonderful. Mike, I know you probably here this a lot but you are greatly appreciated.

Roger

[48Rob]

Roger,

When you ask a very specific technical question like that, it leaves those of us who are not experts, waiting to see what the "experts" say...

I could offer an opinion, but since I'm not an engineer, I hesitate...

Well, okay, I'll offer my opinion anyway...
I would want some sort of blocking in the corner between the two thin sheets, to strengthen the corner.
As for spars/no spars, the curve is very strong, to a point.
The structure will be self supporting until the span, from side to side, becomes too great.
I just don't know how to calculate that distance using math.
Just guessing, I'd say about a three foot span would be the maximum I'd trust.

Rob

[mezmo]

Hi Roger,

Don't forget to let a bit of time to pass to allow your post to
have some exposure. I don't think a day is enough time to
get enough exposure for multiple responses to your query.
Plus, as Mike said, it may have been/be hard for others to
visualize your question just by written description. Your later
posted pic really brings your question out into the open.

Your construction method is close to one of the building approaches
I've been thinking of - using the plywood boat building method of
stitch-n-glue for the interior walls, then covering those in foam.
[I like natural birch paneling and want that as the interior finish.]
What you are asking about is very close to that. The one caveat
or consideration I'd have is, if the glued plywood continuous butt
joint between the wall panel and roof panel would be strong enough
to hold them together. I don't know what kind of glue you're using,
but the stitch-n-glue method [from what I've read about it] uses
thickened epoxy as the glue and makes a continuous fillet/cove of
that between the joining plywood panels that in effect makes them
into one unit. [Edit: 6-7-13, Forgot to mention they also fiberglass
tape and epoxy all the continuous joins to achieve the 'one unit'
effect] If you're not gluing with epoxy, maybe you could glue
some kind of extra, thin pieces of wood or molding that could follow
the curving continuous butt joint at the interior panel's join that'd
act similar to the thickened epoxy fillet. The curve should help give
the plywood width span strength. Just Google 'stitch-n-glue' or that
along with 'plywood boat building' and you should get multiple search
responses that you could check out to further inform you more on that.
They do have small flat-bottomed punt-type designs that use some
epoxied-in spar-like bottom stiffeners, as well as other designs that
rely on the 'intersecting curves or compound curves' of the boat's
shape as stiffeners.

One other question I'd have is, how are you going to bend the 1/4 inch
plywood along the curve of the sidewalls ? Kerfing the back of it some ?
As it seems that the 1/4 inch ply sometimes needs persuasion to get into
the curve one wants, you need to come up with a method to do so. Maybe
using a few spars [1x on edge] on the back/outside surface of the interior
1/4 inch plywood roof panel would give you something to hold on to
while trying to curve the roof panel to the sides, and in the meanwhile
they'd add some additional hidden strength for the width span of the roof.
[A couple builders on the forum here have done that, both for hidden
(within the roof) and exposed (visible under the roof/ceiling surface)
spars.

I'm not an engineer either, but then if the world waited for only engineers
to try out things, there'd be extremely slow progress in everything. An
educated guess/approach, or even some WAG-ing along that vein, by anyone,
often results in as many successes and advancements as failed attempts,
in my view.

Just some thoughts on your topic for your consideration...

Cheers,
Norm/mezmo

[KCStudly]

I am building using a similar method. The difference is that my ceiling ply will go on top of the wall giving much more glue and compression contact area on 1-1/2 inch thick foam walls. My cabinet face frames and bulkhead will be installed before the ceiling skin goes on and the main part of the roof has a very gentle 520 inch radius. The tops of the cabinets and bulkhead will be planed to match the roof arch and will give me spar like features to screw and glue the ceiling skin to. For the tight front radius and hatch I have made ribs. The front ones will be fully contained in the front over pillow cabinet.

On the side walls I intend to rely solely on the glue and canvas sock.

My original plan was to build temporary forms or props, but then I decided that the cost of the material, time to construct, install, remove and remedy any damage to finish work for something temporary would be more than what it would take to just make something permanent.

Ultimate light weight is not necessarily my goal (my TV's are more than capable), but I did want to save on gas and still haul too much gear.

I did a small scale test of my planned vacuum bagging technique this evening to see if TB2 will work for laminating larger ply panels to blue foam board. I used a piece of 5 mm (1/4 nominal) luan underlay about 18 inches wide by 2 ft long (to simulate the more dear 5 mm Okoume marine ply) sucked to an equal sized piece of 1-1/2 thk foam. After 1-1/2 hours under the bag (the TB2 calls for 1/2 hr clamp time and 24 hr cure), my initial prodding and tentative flexing show great promise for a very rigid panel. Not sure how that would be with 1 inch thk foam , but I suspect that it would still be impressive.

I would not be comfortable trying to staple into the end grain of such a thin inner skin, and would almost certainly split several through the finish face. However, I wonder if just toeing the nails or brads through the ceiling and into the foam would be enough to hold until the glue dries.

Did your test simulate the geometry of the ply to foam ledge, or just gluing and stapling face to edge of the ply? I suppose you could make the foam above the inner skin thinner, giving you more surface area to lay a wider ceiling panel onto, if that is an option and you decide it is better or necessary.

Just some of my experience and $.02.

[atahoekid]

Since you've been on the foamie site I know you know about TLAR engineering. For my eye, and for what it's worth I think I would put some widely spaced spars in there. I would maybe put in 3 or 4 spars, you might be able to get away with good quality 1 x 2's. I put in 1 x 3 spars about 24" apart. I put in a Fantastic fan/vent in the roof so I really thought I should and my trailer is about 7' wide. Hope that helps

[Strop]

As a person just in the planning stage I have no experience but was wondering:- could you cut out a bit of the foam so that you have the roof lining say 1/4" into the foam as well as over the 1/4" interior wall. That should give you a neat join inside the tears well. I would like to know if it is possible to use a router on foam BTW.

If you wanted to go away from the traditional method how about a 1/4" dado into the foam on both sides and sliding the ply roof into the foam. That would give you a total of 1/2" overlap

I am not sure how screws go into the foam and hold but surely you will need a few spars to assist in the mounting of the top vent, lights and anything else you want on the inside roof.

Just a thought

Laurie

[KCStudly]

I would like to know if it is possible to use a router on foam...

Oh yes indeed, cuts like "budda". I have more time cutting recesses for hard point blocks in my TPCE build thread than I care to remember.

...how about a 1/4" dado into the foam on both sides and sliding the ply roof into the foam... ?

The problem with dadoes or grooves then becomes difficulty with assembly. With the rabbet along the top edge of the walls the ceiling and roof panels can be dropped in from the top and formed to suit after the walls are already standing.

I am not sure how screws go into the foam and hold but surely you will need a few spars to assist in the mounting of the top vent, lights and anything else you want on the inside roof.

Screws go into foam real easy... they strip, wallow, and come out easy, too. The only place I used screws directly into foam were temporaries just to hold the router templates into place... hand driven. Drywall screws are nice and thin, and do okay when acting as locating pins... and that is about it. After removing the router template the screw holes fill easily with spackle.

My main 2x2 roof spars will go on top of the ceiling skin to frame the cutout for the roof vent. Between these two, the front and rear cabinet face frames, the bulkhead, and the hatch hinge spar, I will have wood spanning wall to wall every 18 inches or so. My concern was early fall or late spring record snow fall loads that might catch me off guard.

Atahoekid, 24 o/c and seven foot span is sparser than mine, but then again you used glass reinforced epoxy for your outer skin (right?), so I wonder if you even needed the spars at all.

I'm going to cut my test piece in half tonight and try to assess how well the TB2 dried on the areas away from the edges of the foam, maybe do a weight load/deflection test on the panel. Not sure if I will have time to report back this evening as it is my 14th wedding anniversary.

[RogHodge]

Happy anniversary!

[Corwin C]

What you are proposing is called a monocoque structure. It is commonly used for specific components in the aircraft industry where the curved skin of a surface acts as a structural member. Like an egg shell, it can both be incredibly strong and light and yet fragile to specific forces at the same time.

The best visualization that I can think of is the metal tape measures that we commonly use. The curve in the blade makes it difficult to bend in one direction and yet it is very easy to bend in the opposite direction. If you place one of these tape measures across a span with the sides up, it will bend to practically any pressure in the middle. If you place it with the sides down (as the skin would be in a teardrop roof) it will hold quite a bit until it collapses dramatically. The secret to this is the curve. If your design has a constantly curving roof (aka cub) and all of the edges are securely fastened, this type of structure will work well. However, if you have a flat spot, (say for a roof vent) it will be very weak in that area.

I am planning using this type of structure in my build. I've never built a teardrop before, but I do have some experience with monocoque structures. I'm looking at doing a hybrid foamie with wood interior walls (about 1/2" to support the galley and internal cabinets) and a fabric covered 1/8" masonite headliner. The only spars will be on the hinge for the hatch, the framing for the hatch itself, and in front and behind the roof vent (because I'll have 14" of flat surface). After applying the masonite (firmly glued/fastened on all four edges), I will layer and build up the exterior walls and roof with foam sheets (thickness to be determined by how well I can get it to bend (planning on ending up with close to 1 1/2" of foam). The final exterior surface will be either fiberglass/epoxy or canvas/paint. When I'm done, I'll essentially have a layered stressed skin structure that has been formed in place. I believe that it will be plenty strong for a teardrop.

[KCStudly]

Rog, Thank you!

Corwin, I think you will find that the 1/2 inch inner skin is excessive, unless you are planning to avoid a lot of the "hybrid" framing and recessed blocking. It may not be lighter, but you may find it much faster to build. The pockets and blocking glue ups, and all of the subsequent "tuning" are real time killers.

Maybe the drill and glue dowel method would be quicker, and still adequate structurally, but there is not nearly as much "fudge factor" on what ever might be being screwed to later.

All of this cutting edge design talk about breaking ground building techniques has me all 'a twitter'. Thank you GPW for kicking this all off in the modern era. I know you meant for this to be simple and thrifty, but those of us with "the knack" can't help but try to figure things out and make them work, perhaps better.

[Corwin C]

Corwin, I think you will find that the 1/2 inch inner skin is excessive, unless you are planning to avoid a lot of the "hybrid" framing and recessed blocking. It may not be lighter, but you may find it much faster to build. The pockets and blocking glue ups, and all of the subsequent "tuning" are real time killers.

KC ... that's the plan. I just don't have the patience to put in the blocking like you're doing. I'm afraid it would drive me bonkers. The only framing will be around the doors, windows, hatch, and roof vent. Even the 3/4" ply floor will be attached directly to the metal frame with foam pieces placed between the metal members for insulation. I figure that 1/2" will hold a screw well enough for the planned coat hooks, light fixtures, cabinets, etc. I debated on 3/8", but the interior wood will be holding my cabinets in both the cabin and galley and I'm not real concerned about weight so at this point I've decided to error on the beefy side. I'm still a distance from saving up the required funding, so "the plan" may change yet again before I get there.

[KCStudly]

Not to highjack this thread, but I have posted results of my vacuum bag test panel over on TPCE thread, intermixed with this massive post.