Polycore for flat sheet catamaran

Discussion in 'Multihulls' started by b_rodwell, Feb 14, 2011.

  1. b_rodwell
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    b_rodwell Junior Member

  2. catsketcher
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    catsketcher Senior Member

    This is a very personal observation.

    I tried using it and wasn't very happy with the result. I wasn't able to do the same things I could do with foam. I am not sure how expensive it is but I would be very wary of using it on the basis of cost.

    I didn't like the way it performed when I tried to dig core out and it seemed that the laminate was more flexible than foam. We had a supplier down here who would sell 10mm for about $100 a sheet. I could get foam for $150 so went this road.


  3. Triroo
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    Triroo Junior Member

    We are building a 38' river boat using Polycore with great success . Would certainly use it again. Northern NSW 02 66828557 give me a ring if you want to have a look. Paul
  4. rattus
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    rattus Señor Member

    This is a quote from Derek Kelsall via Rob Denney on the Harryproa mailing list:

    "This from Derek Kelsall
    Two small samples were made. 2x460g
    biax in one skin and one biax in the other, on honeycomb and on PVC
    foam. Visibly, the laminates would pass any inspection. I held a hair
    dryer to the honeycomb for half a minute and then held it under the cold
    tap – the conditions on a deck, in a shower on a sunny day. 15% of the
    cells had water in them.

    end quote"

    Ouch. 1% of the cells would be 100% too many.

    Another concern would be regarding thermal insulation; the cells allow vigorous thermal transfer and thus would be far less effective than foam in keeping things cool in hot weather, and vice versa.

  5. brian eiland
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    brian eiland Senior Member

    I'd like to know more about this subject as well. I've written directly to Derek to ask about his alleged quote on this material.

    If this is at all true, then these builders have something to worry about

    ...but I doubt they would have embarked on this ambitious project knowing that such a possibility existed
    Last edited: May 30, 2012
  6. Alik
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    Alik Senior Member

    We are widely using this type of material; major concern is shear strength is lower compared to foam of equivalent weight. Usually for boats below 10m in length shear strength of structure is satisfied for topsides, decks, cabins and bulkheads without any problems; for bigger boats one needs to reduce spacing between stiffeners to get structural compliance. I would not use this material on bottom of planing craft. Generally with this PP honeycomb material as core we use somewhat thicker FRP skins. The structure of equivalent strength win PP honeycombs is heavier compared to one with foam, but the cost is big advantage. It is also noted that normal compression strength and resistance to peel for PP honeycombs is higher.

    To foresee this question, for small pleasure craft under RCD/ISO 'Small Craft' no certification of cores is required, and there are no minimal properties requirements for cores other than foam and balsa.

    Besides some surveyors and 'industry experts' (most of those having no experience in design and operation of such craft) would tell horrible stories on this materials, we have monitored such structures on our boats after few years of use and no sign of delamination or buckling has been ever seen. As to water penetration, nothing excessive was observed and core is basically dry. I have few samples of such panels in my office some of them cut out of boats after years on water, and a lot of photos.
  7. upchurchmr
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    upchurchmr Senior Member


    Sorry for the ignorance but what is a DUFLEX panel?

    Is there something special used to bond the core to the skin? My understanding is that polypropelene is very difficult to bond to anything.

  8. Corley
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    Corley epoxy coated

  9. brian eiland
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    brian eiland Senior Member

    Where as PolyCore utilizea polypropylene honeycomb core with epoxy glass skins.

    I was a bit confused by this 'Duflex' wording at one time myself, as I thought it was just confined to balsa cored product. But rather those folks that produce it allow to use the same name to refer to both balsa or foam cored product.
  10. brian eiland
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    brian eiland Senior Member

    For years I rejected the use of a polypropylene myself due to this 'bonding' question. But it does appear to have stood up to years of good service due to the thermal bond that is made between the scrim material and the core, and that is subsequently bonded to by conventional boat building resins.
  11. brian eiland
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    brian eiland Senior Member

    And you are utilizing this material in a VERY hot and humid environment that surely could present moisture intrusion into the core cells if that were occurring on any regular basis.

    That is one of the reasons I questioned this quote that was supposedly made by Derek Kelsall.

    Is it also possible that you are using heavier skin laminates due to their being polyester resin ones rather than epoxy ones?

    I also noticed that in some cases the layups are more 'conventionally applied' rather than prefabricated in flat-panel form on tables under vacuum conditions
  12. brian eiland
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    brian eiland Senior Member

    Here is a reply I got back from Derek:
    Foam core is ideal for all kinds of reasons - a few -
    PVC does not rot or deteriorate and is monocellular. Water does not permiate through the foam.
    The cut cells fill with resin and provide both a complete contact with the skin as well as backing up the thin skins.
    Where two sheets come together, the join will fill with resin if not already glued.
    In the 47 years I have been specifying foam it has been 100% reliable. The most damage I have seen or been told of has been through the outer skin but not the inner skin.
    Easy to cut, sand, bevel, join, bend, taper thickness etc. Cost of the basic sheets may be slightly higher but the very low wastage due to being able to put offcuts together more than offsets this. There are lots of things that simply cannot be done with any other materials.

    Honeycomb is claimed to save weight (a minor amount) but it does not have the record of PVC foam. Some cells are likely to get filled with water. A shower following a hot sun on the deck will act like a powerful pump. Some people say - we would not use below the wl but that is where by my theory there is less likely to be a problem. 8mm diam cells, unbacked thin skin is more vulnerable to damage and water ingress.

    I do have a serious problem with Nida. Their consumer style comparison puts nida top and balsa second and PVC foam way down the list. Ie a material can be unsuitable on one count, which should rule it out entirely but it can still be top on the numbers count. Repeated emails to their MD and to their chief designer does not get answered.

    Balsa should never ever be used in any boat. I learned this from own experience 40 years ago and there have been numerous instances since. My question, what is there to gain? Better shear strength but the PVC does what you need so what is gained. Balsa is heavier than the foam we use and is less resilient and more likely to break the skin to core connection.

    Water will get into both honeycomb and balsa. it is only a question of time - ie a time bomb as there is little or no indication of the problem till it is too late. A very nice looking 55 cat, built in Brazil, was slipped alongside a 36 power cat of mine in New Caledonia, to have a soft spot at a chain plate investigated. Two days later, the whole hull side skin had been removed and no one had looked at the other side. A new owner as well.

    The nice part of KSS is that most parts lose the flat panel look. This particularly includes the hulls. We would never call the KSS workshops, flat panel clinics - though all parts do start as flat panels.

    Why go for steel? I have designed for steel and ally but hate the stuff. Noisy and dirty while building, taking a lot more than twice the time, lots of finishing and then build another boat inside.

    Why go for polycore? How will you ensure that the water that will get in does not run between the polycore sheets. Resin infusion is the magic, but not for any system I have seen to use honeycomb.

    We built lots of foam monos. Can easily do hard chine monos.

    Here is a reply I sent back to Derek:

    Kelsall wrote:
    Foam core is ideal for all kinds of reasons

    Brian replied:
    So what foams do you recommend for deck constructions...particularly for tropical climates?? If I remember correctly there are not that many that come recommended for use as deck cores?

    I believe there are also a number of these foam cores that are NOT recommended for use with dark colored hulls?

    And aren't there a number of these foams that don't take kindly to being bonded to with polyester resins? Isn't this questionable bonding in some instances, combined with the brittle nature of many of these foams the reason that some of these sandwich structures delaminate. Upon delamination can't the water migrate along the skin-core boundary?

    Perhaps you have experienced far fewer problems with these foam core structures as you have learned what best foams to utilize, and you have built your sandwich structures in more ideal controlled conditions (on flat tables with infusion and bag pressure), than traditional one-off hand-layup conditions. But isn't that the same premise that the kit-boat guys are touting with their Duraflex and Polycore prefab panels.?...regardless of the core type.

    Balsa appears to hung in there as a core material because of its superior ability to stand up to deck temps of the tropics. But too many post-construction penetrations without proper sealing have led to many problems with balsa cored structures. And a considerable number of new construction proper skin adhesion problems turn up. That why I recommended against the use of balsa-cored Duraflex construction upon a recent inquiry about same. As I read even more forum submissions on various type of cored constructions I was finding some problems with foam cores as well.

    Concurrently I am finding fewer problems with polypropylene cores, those cores that I had for years ignored as inferior. I've had to do some reconsidering, and thus far I'm liking this material much more than I would have ever thought possible. I also like its 'toughness' and elastic ability to give without brittle failure..

    Kelsall wrote:
    Water will get into both honeycomb and balsa. It is.....nice looking 55 cat.....

    Brian replied:
    Sure the cells PP honeycomb are 'hollow' and can hold water, but that appears to be restricted in a very local manner....and absolutely no rot. Subsequent water migration should be stopped by the moisture barrier scrim layer that is termo-fused (not glued) to the cell edges.

    You did not mention whether that cat was a balsa cored or PP cored vessel??

    I'm looking at redesigning a couple of monohull motorsailers, and my preference for building these is steel hulls with prefab poly-honeycomb decks and superstructures.

    Kelsall wrote:
    Why go for steel?

    Brian replied:
    Steel hulls are perceived as low simply technology, low cost, very high durability construction....you can bounce off of those stray floating containers, or rocky outcroppings without fear of slicing or puncturing thru that outer fiberglass skin of a cored vessel.

    And since we are not that overly concerned with weight in the monohull construction as we are with multihulls, why not utilize this very marketable material for that worldwide cruising owner. Besides steel hull maintenance has become much easier with modern coating materials.

    I would consider an all sandwich construction per this gentleman when he was building his 65' schooner of 3” Nide-core (WWW)

    ….but I would still be willing to bet that most clients will opt for steel in the hull itself.
  13. brian eiland
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    brian eiland Senior Member

    I wanted to bump this subject thread back into prominence, as I am once again looking at flat panel construction materials for several projects. i want to reconsider the alternatives to wood-epoxy (plywood) materials for these 'flat panels.
  14. brian eiland
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    brian eiland Senior Member

    Can you recall where you saw this quote? I'm wondering if the wording got changed any, in translation/hand-me-down fashion?
    That is really tough to believe, and if true would have probably have removed this material from the market in any body's consideration LONG ago.

    I wonder how 'vigorous' the thermal transfer is inside these individual honeycomb cells,...still air??

  15. rob denney
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    rob denney Senior Member

    That is, or should be, a partial quote. I did the same, and many other tests, many with lighter laminates and could not replicate what Derek claimed. I suspect you would have trouble replicating it with sponge as the core, due to the waterproof characteristics of the skins.

    Polycore is great stuff, but a bit of a worry for infusion as the resin proof layer was (may not be now) easily damaged. The chance of getting resin in the core was too high to justify the benefits.

    For wet laminating and non infusion bagging, it is a versatile material, although some practice with a heating iron is required to get the bevels right. Removing core is hard work, bevelling is easy and there is no backfilling required.

    rob denney
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