Epoxy over XPS method

Discussion in 'Boatbuilding' started by mvoltin, Nov 16, 2018.

  1. Dejay
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    Dejay Senior Newbie

    Oh you already did some experiments with this?

    You could also make the tow columns thicker if you use 12k or multiple strands. And use carbon tow for increased stiffness in the push direction. I think tow is the cheapest form of carbon or fiberglass since it's the raw material.
    I think you could build a machine for 4x2 foam sheets that first makes all the holes using a hot 2mm needle, then switches to the sewing tool and threads tow though those holes. That way you wouldn't need much force and get precise cuts.

    And yeah this is just a rather complicated and crazy idea. The theoretical advantage would be that you could prepare a bunch of tow stitched XPS panels automatically and then place them in the mold. Less manual labor.

    But you're still stuck with the same problem of point loads. And once you fix the point loads for impact strength you probably don't need a fine webbing anymore to support the XPS foam.

    I've also considered making wood veneer strips and vacuum infusing your own plywood. Wood strips could also work as "webbing" around the XPS and would solve the impact resistance.
     
  2. rxcomposite
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    rxcomposite Senior Member

    Yes you can. Boston Whaler used expansible foam which has very low shear. When you look at the cross section, the average depth is thick and there is little to no longitudinal stiffeners. Low shear foam is designed to be thicker. If the thickness by design constraint is not sufficient, a stiffener is added to add stiffness. Foam adds stiffness.

    Early development of the experimental aircraft used solid styrofoam carved to shape and several layers of outside skin was added. Engineering will show that there is a limit to thickness of foam versus the thickness of skin to attain certain stiffness to weight ratio. There is a point where adding more thickness to gain more stiffness or compensate for the low shear of the core will result in a panel that is heavier than intended. There is a point where they cross.
     
  3. Eric ruttan
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    Eric ruttan Senior Member

    How is this not a great attitude?

    I already have...
    Local loads are things like masts and beam and waves and wind acting upon them with a fully loaded boat. Those loads tell us how strong the laminate and beams and other parts need to be. That very different than a point load. but yes, in general 25psi XPS is a bad core for a sea going vessel. Corrugated fiberglass however seems excellent. But I do not know.
    I see them as very different. For one, the XPS in a corrugated glass core is irrelevant to the core structure. It is just a former, with excellent properties that we exploit. Any load that would delaminate the XPS, that does not damage the corrugated glass or the skin glass, does no damage to the corrugated glass laminate, because, again, the XPS is just a former/filler. For two, it is possible to make a very strong core, in both shear and compressive strength with corrugated Glass. Way stronger than any PVC foam. This is just not possible with XPS core laminate. for three, you are not going to delaminate a corrugated glass core, as the glass to glass bond is crazy strong. One may crush the core, or split the skin, but you wont delaminate it. and for damage that may crack the corrugated core but not split the glass, a repair can be to inject resin under vac.
    I think we are talking about two things. when you say point loads, do you mean dropping an anchor on the deck? Or do you mean local loads like where a mast fits into a boat?
    A composite laminate, as I understand it, has a risk of a blow de-laminating the skin from the foam. Some cores, like wood, have excellent core skin adhesion Others, like XPS and EPS, have poor skin adhesion. even the high end PVC foams they add groves that fill with resin to to enhance the skin to foam adhesion. The possibility of delamination is remote with wood. However they suffer from decaying if an impact event breaches the skins environmental protection. That very same impact would have nearly no effect on a foam core. Yes you can make the skin thicker to add more protection, but generally that is not how you protect a core. You specify a better core. Unless that core rots, then, perhaps, you specify a thicker skin. Or you use a core that has excellent skin adhesion, and can be very light. Like corrugated glass.
    I disagree. Fiberglass is a stronger core than wood. or anything else that is stronger than wood. Wood is a heavy core, that rots. For a core, we only really care about shear and compressive strength. And weight. and cost. and not rotting.
    The same laminate thickness as a not foam wood core boat? I am confused here.
    But I certainly would not use 25PSI XPS as a foam core for a boat. However it makes an excellent former for a corrugated glass core.
    You could ask him. But, afaik, nothing different than the rest of the boat. He adds skin in the higher stressed area between the rudders. As I sail in the Great lakes, with LOTS of very rocky shores, he has suggested nothing additional when asked, although, perhaps a layer of kevlar on the upper hull bottom skin to stop penetrations. He would rather have it easy to fix, as damage proof is impossible. His suggestion is to hang an old tire under the HULL as a bumper. I love that guy.
    Do you have a reference for this happening? Because I have never heard of it on glass epoxy boats. perhaps wood or ester glue I guess. I would really like a reference.
    Meh. I suggest if your boat is made of lead and wood, then yes. If your boat is light and made of floating things, then the risk is very small. And you just need a plan. I expect to hole the bottom of my boat hulls on purpose at some point to prove that I can fix it before I need to. Because it is easy to fix.
    BTW robs boats are designed to collide with crap in the ocean. I do not know of any other designer who does this.
    I have talked to Rob about doing this. Rob has an even cheaper hull he is doing for his 24m. I am excited to hear about that. He posted the other day the 24m build is 30k$AU in materials for the shell!

    What? go look at Steinar's 66' harryproa build. you infuse it silly. make it part of the structure.
    You cant do that, afaik. You can add structure on top of the hull structure that you infuse, like stringers or rings, to stiffen the hull as needed. But foam core needs a continuous core along the span. If the core is not continuous, stresses will concentrate where the difference is, localizing damage. All the math in how foam core is designed and works is based on a relatively continuous core. If you want to chase this you are much better with layers of different strength foams, but again, they have to be adhesed and cannot float free. I listed above how to do different layers of foam. I could not see a way to make that work.

    but you can laminate the foam layers and hole and groove them for infusion, just fine.

    If the embedded cheaper core is not up to the expected compressive strength and shear (detailed prior), it should not be in the core.
    the stiffness is a function of thickness.

    I suggest you go buy Robs E25, and build it. Its cheap, simple, and fast to build. You will learn a lot and you can sail it while you build your bigger boat.
     
  4. Eric ruttan
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    Eric ruttan Senior Member

    Actually the outside skin could be very light. Aircraft engineering is such that the skin could be so light as to be punctured by a pencil point under it's own weight. this was impractical, as just touching such skin could damage the form, so they made it thicker, for durability in handling reasons.

    And Rutans cores were 25PSI XPS dock foam.

    Dear rxcomposite;
    Can thickness of a hull alone compensate for low compressive strength and shear of a core?
    Do you not need to worry about skin delamination?
    How Boston Whaler get sufficient skin adhesion?
    EDITED FOR CLARITY
     
    Last edited: May 27, 2020
  5. Eric ruttan
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    Eric ruttan Senior Member

    thicker tow columns don't help, as your needle still needs to go in and out the hole with twice the string plus the needle. like a sewing machine does.
    If you stitch one way, like a weave, then its ok, but how do you automate that? and you constantly pull string through your prior holes, which gets ripout.
    If a roll of foam sticks, one for top and the other for bottom and a sheet of glass between are too much work, I cannot imagine stitching up 4x2 sheets at any density that would make a good core.
     
  6. Dejay
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    Dejay Senior Newbie

    I can't find it again but I think it was a surveyors opinion piece about foam core below the waterline. Not sure if it was called hydraulic corrosion but basically you get a pump action from waves. Here is a little bit: Structural Issues : Core Materials https://yachtsurvey.com/core_materials.htm

    Maybe this is less of an issue. So you think 600gsm or 800gsm is enough impact resistance without a real backing core?

    Do you have a link for the 24m build 30k$AU build?
     
  7. rxcomposite
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    rxcomposite Senior Member

    I did not ask that question. You are misquoting me.
     
  8. Eric ruttan
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    Eric ruttan Senior Member

    um, ya. don't use chopped glass or ester resin. or crap foam.
    But none of that applies to fiberglass cloth, and epoxy boats. and it does not apply to any PVC core a modern designer would suggest. It does not even apply to XPS.
    perhaps it could happen if you don't use decent materials, but it wont happen on a not **** materials boat.
    no. I am assuming a 'real backing core' is PVC?
    are you looking at the C50? or something else?
     
  9. Eric ruttan
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    Eric ruttan Senior Member

    I edited the post to make it more clear. sorry for the confusion.
    I asked you, directly, some questions, namely;
    Can thickness of a hull alone compensate for low compressive strength and shear of a core?
    Do you not need to worry about skin delamination?
    How did the Boston Whaler get sufficient skin adhesion?
     
  10. Dejay
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    Dejay Senior Newbie

    Yes I mean PVC or PET or SAN structural foam core instead of air or XPS. I'm considering the costs and weight gains. If you need to add another 800gsm to your hull on the outside and the floor area you add something like 2.6kg/m² and about 30€/m². That is roughly the price of 10mm structural foam which only weights 0.8kg/m².

    Thanks, didn't know there was a mailing list!
     
  11. fallguy
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    fallguy Senior Member

    My concern with xps is getting a better shear rating and tolerating a bump on the dock with better compressibility. San is 6 times the shear rating of foamular 250, or was it compressibility? Touche'. If we step to foamular 1000, we get 100 psi compressibility (a good enough number for sure for a dinghy). I am unable to find the shear rating of foamular 1000, mostly because Dow touts its use for thermal breaks mainly. However, it is also likely that a higher compressibility would result in a more difficult shear (a stretch perhaps).

    Only trouble is, there is then another problem it appears.

    I wish to build ultralight dinghies. And Foamular 1000 does not appear to be available in 1/2". So, my core thicknesses would be double the weight at the sizes their sell info suggest as minimum which is 1".

    Now, if I wanted to, I could purchase a sheet of 3" thick foam and cut it all sideways and lay the pieces together, creating an epoxy thixo bond in between each 3" strip and build all my panels up, but the cost in epoxy and time for me is just too great. An xps strip boat building up a simple dev panel hull. It could be done and the bonds between the pieces would likely work as shear webs. But the job is so tedious I can't find it fun. Then any deviation in the surfaces must be dealt with during subsequent laminations, etc. And the epoxy pushes up the panel weight, so I would probably need to build them a bit thinner to make things equal.

    I think if they had foamular 1000 in 1/2"; it would work fine for the dinghies; the 250 is available at 1/2", but no. It will delam.
     
  12. fallguy
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    fallguy Senior Member

    it should be noted Eric, I wish you were right I would really like to be able to order a sheet of boat building foam for $50 bux from the local HD...
     
  13. Eric ruttan
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    Eric ruttan Senior Member

    I do not know why you arr saying this.
    What reason would one need an extra 800gsm on both sides?
    Rob's planning numbers is 5$ a kg for glass and 12$ a kg for epoxy. So adding 800gsm per side should be like ~13$. Why do you say 30€?
     
  14. Dejay
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    Dejay Senior Newbie

    Yeah I roughly calculated with 10€ each and and 800gsm + 60:40 split is 540g Epoxy. So yeah 13€ each side. Twice that for both - at least for the floor? I guess not for the walls and roof inside?

    I do not know what you'd really need for laminate schedule and have no experience so I'm guessing here. But assuming that 800gsm was with structural foam, you would need to add something if you only use XPS to reach similar impact strength. I mean 800gsm is only like 0.7mm thick.

    PS: Actually Rob wrote somewhere that Harryproa C60 only has 600 gsm. But still replacing structural foam adds weight and cost in other ways.
     

  15. Eric ruttan
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    Eric ruttan Senior Member

    you can. There has to be a building supply within resonable distance. I found one near me wanted twice the price Menards wanted on a special order, but I had to order 20 sheets from Menards.
    And why would you not cut them along the flat, with a hotwire? Then drill and groove like normal? Or just get a thickness you can live with, like 1½ inches?
    The 1000 is only 2' x 8', so one could throw a web of uni in easy every 2 feet. And its a resin path if you infuse.
    I think I stole the shear moduls from finnfoam F700. Ya, its cheating but they are identical products with identical stats.
    Strength https://www.finnfoam.com/products/finnfoam/properties/strength/
     
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