Skin over frame sandwich, low cost high performance plywood carbon xps

Once that XPS starts to squeak total insanity isn’t far away.

 

.... Hahaha, well it that's probably true.... Only I can't imagine bonded XPS actually squeaking. Hmm... coz it is fixed, and can't rub against another piece.... unless it has failed (sheard or debonded), in which case you have an acoustic structural integrity alarm.... hmm. And would that be a really exclusive and handy feature? ;-)
Anyhow, sanity might just be over rated... my parents told me I wasn't like other kids... and still, I'm not like most other adults .

 

Yellowjacket, thank you for your valuable feedback! I hear your words. Only I still would like to dig a little deeper. Even if found not viable, the process still probably lifts a lot of knowledge to the surface. And it might just give insight into optimizing a traditional ply on frame construction. And the safety aspect is actually one of the main driving forces for this line of questioning.
1) A lighter boat make for less energy at impact. Just like many erroneous believe that a large SUV is safer than a mid size car. Well in a head on collision between the two, bigger "wins". But in a collision with a stationary object, the lighter car is often (but not always) safer.
2) The lighter boat can make do with a thinner hull.
3) The large amount of foam in the thick sandwich makes it float really high even if flooded. Actually retaining at least 4-8" of bridgedeck clearance even with big holes i both hulls.
4) If flipped, it would actually float on the hull decks. Barely letting water into the hulls.
5) The large distance between outer and inner skin makes a total hull breach less likely (and gives the hull much better energy absorption properties).
I'll come back to that last one later.
All in all, a fast, light, high flotation containing, thick walled cat, might just be the safest thing... since sliced bread ;-) Especially with a manual self righting system :-O

As for taking the traditional ply scantling, fill with XPS and glassing on the inside. That is actually the starting point of the conundrum. Only problem, you actually need decent inner skin to make the foam taking up forces. But in that case, the construction has already changed totally. Just like an open C beam is structurally completely different to a box beam. And that is where the challenge starts Also if weight reduction is a goal, leaving all the ply, and adding a lot of plastic wouldn't make sense.

 

The links I posted earlier give some very interesting conclusions.

 

The sandwich + 4mm ply on 19mm EPS gives the highes specific energy absorption for it's core thickness. Only just beaten by the 40mm thick H80 Divinycell. And it is way better than any single skin or glassed ply. One of the papers is actually meant as a evaluation of testing methods (281), more than an actual test. But still giving valuable results on sandwich constructions as well as a good discussion on test parameters. Allthough after carefull considderation I find the ISO 6603 very well suited for my suggested problem.
A few observations.
1) Single glassing and double glassing the ply make huge differences (improvements). Maybe the epoxy instead of polyester also contributes to the good results.
2) In FRP panels it looks as if continuous fibers and fiber content, make a big difference.
3) In sandwich, more flexible and isotropic CSM mat seams to spread and distribute the forces better into the core!
4) Sandwich thickness is obviously a factor.
5) The effect of a middle density middle layer (4mm ply) is astonishing!
My guess is it is very effective at spreading the impact loads over a larger foam area. And having a quite "soft" foam behind it, could actually cushion the impact better, and absorb more energy.

 

Is this a school project again?

Expanded polystyrene is an open cell foam and once water gets to it; it is useless.

Why on earth would you include it instead of xps?

 

No, it's a "middle age" project ;-). And no, I am not contemplating EPS. I only referenced it because it's the only polystyrene I could find data on (see the earlier links). And from an impact study perspective, it should have quite similar properties a the XPS. I know I have already written far to much, but I have been very clear on the point of EPS vs XPS.

 

Btw fallguy, seriously... school project!!! Be nice!...... And also, where did you go to school? MIT?

 

This forum is a place where a lot of school projects land.

The idea you would include eps means to me you haven't done your homework; school or no.

Look, I mean no disrespect. I even watched an airplane builder video with ply n carbon.

I am an independent self builder with a business degree and I would never consider eps, except for underseat flotation in an open skiff where the eps is not in contact with water unless the vessel sinks. And that is even not preferred! I don't care if someone else did it before or if it can be done, etc. Open cell foam and boats -> junk.

The trouble with including it is noone takes you seriously and you don't get engaged responses.

Take the advice. It is friendly and while a bit chippy, I am saying that which others may be thinking.

Another thing is mix n matching of various strengths n tensiles, etc. is generally not done because the weaker link fails. It is not a science of augmentation, but rather consistency(there is a better word) and unless I misread an earlier post; your tack to materials did not reflect this basic premise which also results in disengagement by people far more suited to discuss this than a casual boat lover and boat builder.

So your chart above is missing that as far as I can tell.

But, like I said, just a boat lover.

Be nice? You don't seem to understand that nice is not always what our own perspective believes. Nice can be rebuke.

All the best.

 

Niclas,

I've followed your thread and I don't really know what you want to do.
Is this polyester or epoxy construction you are considering here?
Modifying or new?
Either, on closed cell foam, is going to involve considerable load-design considerations.

I found several, good epoxy-on-foam airplane construction videos on YouTube.
Small, one-off performance-oriented builds.
I have an engineering background and can appreciate what efforts have been made in their design.

I'm not sure how to help you.

 

Xps and eps do not share properties.

Here is a tds for xps; and a lot of acronym fun to boot.

http://www.owenscorning.com/Network...LAR-250-XPS-Insulation-Product-Data-Sheet.pdf

Even the coefficients of thermal expansion are important here. If you build this beast and one thing expands twice as much as another; it will fail.

I also don't see that in the charts.

 

Dear fallguy! I have no idea as to why you think I even consider EPS. I am not. I promise you! I have only talked about XPS in this thread except when quoting results from an interesting experiment I linked to. It was only a mentioning and discussion of those results. And I still claim they are valid, since the compression strength of EPS35kg/m3 (a really heavy variant), is similar to XPS32kg/m3. I actually first thought they had made an error in the documentation of the test, and used the wrong acronym. But of that I can not be certain. Still. In this thread I have only ever talked about XPS, 14 times to be exact, not including it being in the title of the thread. And as to the thermal expansion of XPS vs Divinycell, it's only about 3 times, and I really could not see how that could be a problem in this application. XPS, 0,06mm/m/degC, vs Diviny, 0,02mm/m/degC. And of course some might argue that XPS if **** too. But so far, I have made a whole bunch of factual statements, as to why it really isn't for some lighter applications. Or rather I am just repeating what other more knowledgeable individuals have been saying. The topic has been brought up in this forum several times earlier. Only not in this angle. That is as structural insulation in a semi ply on frame, semi sandwich construction, pushing the envelope into the realm of mini cruising catamarans. Every one is entitled to their opinion. And since I am inexperienced in sandwich construction, I would appreciate arguments in helping me figuring out if it would be doable, backed by objective arguments. Best regards, N

 

BlueBell,
my biggest problem so far is to determine how to solve the problem with the low shear strength of the XPS. In the case of a very lightweight sub 33' cat, the 4 or 5cm XPS only handles about half of the shear forces it should (without making the sandwich so thick it becomes heave and intrusive). To double the shear strength, the thickness has to be quadrupled. The main alternatives would be to:
1) Try and adapt the reinforcements (stringers and ring frames) in the same way as with lightweight aluminum or plywood constructions. Meaning very tightly spaced, to take up the loads and stiffen the construction, and allowing for thinner plating. And in this case, allowing for XPS with inside skin, without shearing.
2) Start with considering it a pure sandwich from begin with only with some ply left in the outer skin, and rather find a way of taking up the shear forces in the core. One way could be layers of epoxy/fiberglas, positioned like a very coarse honeycomb, but not with hexagonal 5mm cells, but maybe quadratic 10cm wide grid, connecting outside and inside skins (filled with XPS)
And if that doesn't work, maybe fining another alternative. I can se problems with both above variants. 1) hard spots and difficult assembly. 2) difficult to predict behavior of the fiberglass grid, debonding aso.
The goal being to optimize most parameters. Affordability, toughness, durability, performance, weight and maybe even second hand value. If we wouldn't strive for development, this discussion might still be about the best way to carve out a log, to get a canoe. I really get the merits of the tried and tested. But most things can be improved on. And whether or not you succeed isn't always important. By trying, you'll mostly come out a whole lot wiser.
I'll post a summary of possible hull wall sections, their weight and material costs. That will probably shed some light on my line of thought.

 

The CTE of div vs xps is not the issue. God sakes!

The items within each of your sandwiches need to be relatively near each other in certain attributes; cte, tensile, elongation, etc.

Well beyond my pay grade. I only have a general knowledge.

But it seems you are disregarding these; this is my point to you.

 

Well, I toast to you and wish you every success.
Post pictures!