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

Discussion in 'Materials' started by Niclas Vestman, May 29, 2018.

  1. Niclas Vestman
    Joined: Aug 2017
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    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

    Hmm.. found some more interesting info. First, a paper about the different takes classification bodies have on the subject of cores. Quite informative.

    http://www.diva-portal.org/smash/get/diva2:549834/FULLTEXT01.pdf

    One of the main conclusions is that the core shear yield is the relevant measure. And that this mostly lies in the span of 55-60% of ultimate strength for the usual suspects. Divinycell, Corecell, Airex aso. That is, the shear strength that is elastic (flexible) and fully returns to it's original shape. If loaded over yield, the deformation becomes plastic, meaning it doesn't fully recover. (Permanent deformation resulting in compromised properties). But, if tested to ASTM C393, Diviny revealed it could handle slightly higher loads, over the yield limit, (i e slamming loads).
    So, time to figure out how XPS behaves. And wouldn't you know. Wonderful www came through again.

    http://jmes.ump.edu.my/images/Volume 10 Issue 2 Sept 2016/3_Fajrin et al.pdf

    Again, just like the earlier finnish reports, it talks about EPS. Only this time mentioning it is a closed cell foam, meaning XPS. I strongly believe that was also the case the last time. Only XPS doesn't seam to be a consensus terminology. Well anyway, this was a really interesting paper. Not only did it give some good approximation for yield vs ultimate strength for XPS, which seams to come in at the 40 to 50% mark depending om the skins of the sandwich. The article actually investigates a key feature proposed in this tread. A thicker intermediate layer (here ply, and in the article other fibers). In the article an inner skin of different properties ha a remarkable influence on the total mechanical properties. Check out for your selves.

    I also found a nice piece on slamming load on a VO60 ocean racer.
    https://higherlogicdownload.s3.amaz...-86fa-9ca63a85f102/UploadedImages/2010-01.pdf
    And one on a planning v hull.
    Experimental Study of the Hydrodynamic Loads on High Speed Planing Craft - ScienceDirect https://ac.els-cdn.com/S1877705814022048/1-s2.0-S1877705814022048-main.pdf?_tid=0d369d74-90ab-42a1-9875-64018731657f&acdnat=1528145300_8b6f361005545dfe49661fe4d124588b

    Giving a good starting point on empirical relevant data. Well, next step would be to run a few numbers as per Hexcel:

    http://www.hexcel.com/user_area/content_media/raw/Honeycomb_Sandwich_Design_Technology.pdf

    I'll be back! ;-) But so far, preliminary numbers look promising.
     
  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Great collection of studies Niclas.
    Do you have a project start date in mind?
     
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  3. dinoa
    Joined: Oct 2007
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    Location: florida

    dinoa Senior Member

    Very nice find on XPS shear strength. I have found it unnerving how easily skins unzip off XPS cores, but have never found a a measure to quantify it. At some point egg crating XPS panels should overcome this or perhaps indenting core faces with a spiked roller as a means of reducing cost. I doubt a weight saving can be acheived verses high performance cores.
     
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  4. Niclas Vestman
    Joined: Aug 2017
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    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

    It seams working temerature might be XPS's final weakness. Finnfoam state 70C as highest working temp. PVC has way past 100C. At 90C finnfoam are stating less than 10% of the mechanical properties remain. :eek: sail magazine claim syntetic teak decks get slightly warmer at 66C (150F) then real teak. That would imply, XPS should likely be ok in this regard, as long as you keep your boat strictly white, or chrome silver foil wrapped. :confused: if you leave a dark towl on deck and then walk over it, it might be like stepping on grilled marshmellow.
     
  5. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    IMO, white is the only way to go for a base colour on a build like this.
     
  6. Mike Inman
    Joined: Oct 2018
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    Location: Jacksonville, Florida

    Mike Inman Junior Member

    Hi Niclas,

    I, too, am thinking about a modestly sized Catamaran design, perhaps 12m waterline x 6m beam, and was researching XPS foam as an insulating layer. I'm wondering how bendable products like Foamular 1000 and 150 are? 1000 is tempting for its 100psi / 690kPa compressive strength at 48kg/m3, while the 150 is a more modest 20.8kg/m3 with 15psi / 103kPa compressive strength, both insulating at R5 per inch of thickness (0.029 W/mK): http://www.foamular.com/assets/0/144/172/174/1fb2fb08-5923-46de-b387-f4bdc3f68d50.pdf

    Originally, I was thinking of a composite sandwich build, but the more I look into things, the more attractive an aluminum hull becomes for a one-off build. I'm very much liking the idea of an aluminum skin with a bonded layer of foam insulation inside, and perhaps on the interior a thin wood veneer overtop of a layer of high strength foam.

    My ideas lean foremost to a 3 bedroom home on the water, but more old-school skinny hull catamaran (cabins up and central) with low wetted area, and the weight reduction with the aluminum hull helps with that cause - something with relatively low displacement, easy to push by propeller or sail. Have you gone so far as to investigate who might build your 32' bridgedeck cat concept?
     
  7. Niclas Vestman
    Joined: Aug 2017
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    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

    Hi, and sorry for late reply. For me at this stage it was more about the fun of designing and trying to learn as much as possible. Sure, still hoping to be able to build in the not to distant future. But in that case, i'll build myself.
    As for your thoughts on concept and design/construction. Pls understand that i'm not knowledgeable enough to give any advice. But my amateur thoughts would be something like this.
    Design would depend heavily on sailing grounds. If storm or hurricane season or rough weather is likely, I'd go for a heavier concept. Also much less motion in ideal conditions. Very light craft will pitch disturbingly even if just a little fishing skiff with 5hp outboard passes by. Also using a boat for accomodation more than a week, makes weight consiousnes a real hassle. And weight saving doesn't realy give any advantages exept for semi trailerable concepts. Not even as far as ease of propulsion. A small improvement in reducing wind resistance (which is probably the main parameter dictating power requirement on a habitable cat), would offset quite some added weight.
    I'm not sure what you considder a skinny design. Usually those are considdered to have higher wetted surface ratio. 30' sitze is on the absolute minimum end of having standing headroom hulls. And for my design making the space in the hulls attractive and usable, very much influenced the hull shape.
    As for materials, I realy couldn't say. For sandwich construction with composite or alu skins, core shear strength is what limits XPS. And also i haven't found a definate anwser to the possibility of using eg a traditional wood or alu construction with stringers and girders and just using the XPS and a possible thinner weaker inner skin like veneer or similar, as only a partial structural element. Say dimensioned for taking 30% of structural loads, and leaving the rest to a traditional alu construction. As for alu, i'm not convinced. It's primarily a cost effective material for industrial or large commercial builds. But has limitations. Popular for small John boats for durability and to some extent easy to repair. But, has lower specific strength compared to epoxy GF. Actually not all that different to wood. Also difficult to weld. And as far as I know, alu needs heat treatment to obtain high specific strength. Wood or foam help with insulation. Alu can corrode fast if in contact with other metalls, tricky when fastening fixtures. A general advice I've come across is to keep a panel design as symetrical as possible. Eg if using a cored construction, then to try and use similar material and dimension for inner and outer skins. Hope u find the anwser useful. Best wishes
     
  8. Niclas Vestman
    Joined: Aug 2017
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    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

    As for bendability, xps of upp to half inch seem to be flexible enough if heated with hairdryer. A swede did a project called yat yet another trinardo. If you Google that, he had a build website with lots of info on xps and even bending. He also wrote here on BD.net
     
  9. Mike Inman
    Joined: Oct 2018
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    Location: Jacksonville, Florida

    Mike Inman Junior Member

    Hey, no problem, and thanks for the reply. Same here - I think in my situation I will probably end up buying an existing old boat first (and with my budget probably a monohull), but I think there's a lot of value in learning about all the tradeoffs and simultaneously thinking about what's important for "our boat."

    One "unusual" value I have identified for us as a cruising family is the tendency to use a lot of water, particularly for washing, and as such it would seem attractive to design a catchment into the roof to collect rainwater into rather large tanks. I've been thinking that these tanks could be located near the 4 corners of the craft (baffled to prevent sloshing) and that they could provide not only stability with their high polar moment locations, but also a measure of trim control - not to mention an ample supply of washing water, plus the ability to dump the tanks and run light when the occasion calls for it. I've heard occasional mention (such as "Wild Thing") of large ballast tankage in monohulls, and of course there's the MacGregor, but I've never seen serious design for catchment nor large tanks in a catamaran.

    In that vein of affordability, I have come to the conclusion that a 40+' aluminum cat, while attractive, is not realistically within my reach - but, perhaps a 34' epoxy/glass/plywood cat could still meet our needs, if I can figure out how to get a layout we want and still afford the ~1000 hours of build labor that it seems to take to make such a beast. I can do some labor myself, but if the whole build is to be done by me, I'm looking at many years before it would complete (not unusual, I know.)
     

  10. mirkosebastian
    Joined: Feb 2012
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    Location: Hamburg

    mirkosebastian New Member

    ;);)


    Niklas, thanks for that topic and that link!

    Had the same idea to insulate and stiffen my 7m trailer sailer project:

    Will use thin air ply over frames to create a working surface, then PET foam, fibre, all bonded with EP.

    To be on the safe side for offshore and below WL the ply layer could be neglected in the scantling calculations and replaced or reinforced with a fibre layer.

    I have used XPS above WL for repairs on my cruising multihull. Bonding improves by poking holes in the foam with a spike roller before saturation.

    My two-cents from 40 years fiddling with boats: the interested DIY builder makes less short-cuts and uses better materials than the average professional, especially for small jobs and underneath the surface.
     
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