Choosing a deck core

Discussion in 'Materials' started by Charly, Nov 23, 2010.

  1. CatBuilder

    CatBuilder Previous Member


    And the poor guy (Kurt) is under serious assault He has had more than his fair share of questions from me. :D
     
  2. War Whoop
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    War Whoop Senior Member

    The group I was with built a Airex plant Torin here in Jersey, then later on started corecell which was sold to SP.

    Airex goes way back. I have put this up before do not try this with balsa or Plyrot.

    [​IMG]
     
  3. CatBuilder

    CatBuilder Previous Member

    Sounds like a plan, Charly. I drive by from time to time since I live in New England, but I'm building (or trying to??) in FL.

    One thing about doing the whole deck in foam, that I'm not looking forward to if I can get my foam/glass build off the ground is this:

    If you mount a piece of hardware, you MUST replace the foam core in that mounting location with special, high density foam or go straight glass with a backing plate. In other words, it's a complete PITA compared to a balsa deck where you can just put a good aluminum backing plate on and forget about it. The reason? Foam doesn't have much strength in compression, but balsa does.
     
  4. War Whoop
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    War Whoop Senior Member

    The high density foam is very common and used for transoms among other things ,hardware mounting should be planed and even with wood some extra laminate in that area.
     
  5. Steve W
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    Steve W Senior Member

    LOL, that drop test reminds me of my sons stitch and glue 3mm okume,dynel/epoxy sheathed kayak that was stored up by the ceiling at his cabinet shop with a block and tackle system for each end,both lines running to the same cleat on the wall,well,when uncleating it i let one line go by accident and it dropped about 16ft to the concrete,one end first and then the other end slid out of its sling and dropped, we fully expected serious damage but nothing at all,this was 5yrs ago before we had painted them, then last summer im talking to him on the phone as he is driving down the freeway with the kayak in the roof and he makes a comment that the boat is moving around a bit then oh **** gotta go,the boat flys off the roof at 60mph,he gets back on the phone a few minutes later to report that the thing survived again,the only damage was to the double bulkhead behind the cockpit where it was bolted together,puled out some 1/4" Tee nuts,but no damage to the hull skin, tough stuff that dynel.
    Steve.
     
  6. Steve W
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    Steve W Senior Member

    Catbuilder, despite the compressive strength of balsa you DO NOT just bolt through it with a backing plate,this thinking is exactly why balsa has such a bad rep,fortunatly there are good core substitute materials available that you should use whether you are using foam or balsa,buy a couple of sheets of Coosa board,(Jamestown carries it )and use it anywhere you can pre plan your hardware locations and use the drill out and pot method anywhere you didnt think of REGARDLESS OF CORE MATERIAL.There are other materials such as Pensky board or G10 that are just as good but maybe less available,but they are less of a PITA than removing core and laying in extra glass.
    Steve.
     
  7. CatBuilder

    CatBuilder Previous Member

    Steve, I posted after a couple beers to relieve build anxiety last night. It is way easier to do unplanned hardware on balsa decks and you do use a backing plate. I guess I failed to write in above that you also remove the core and fill with epoxy a la Gougeon's book. Drinking and posting... just say no! :)


     
  8. Steve W
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    Steve W Senior Member

    Right,the process for preparing a deck for unplanned hardware is EXACTLY THE SAME for foam or balsa but for different reasons,for the foam you pot the holes for crush resistance,for balsa it is to keep water from rotting out the core, one is no more of a pita than the other, as potting the holes after the fact is a poor substitute for proper pre planning and replacing the core in the hardware areas during the original build.
    Another thing i dont think i have seen mentioned on any forum is that it is sometimes necessary to add external pads under highly loaded hardware to compliment large backing plates under the deck even if you have potted the holes, Why? because the top skin on a deck simply does not need to be very heavy to meet its structual purpose and often is not,the trouble is if the core is not replaced during the build over a decent area with solid replacement material a highly loaded piece of hardware such as a stanchion(lots of leverage,small footprint) can break through the top skin. I have on many occasions had to add external pads on top of the deck under stanchons on C&Cs and Pearsons and others to increase the footprint, 3/16" aluminum or G10 work well,this is after i have potted the holes, it also allows you to pot the holes larger.
    Steve.
     
  9. CatBuilder

    CatBuilder Previous Member

    Good post, Steve.

    This thread really got me thinking last night. I had priced out my entire boat for Core Cell and I'm choking a bit on price.

    How about doing the interior bulkheads and cross beams, deck and deckhouse in balsa to save a few bucks over 100% Core Cell? I would have to do the hulls in Corecell because that's part of the build technique.

    Does anyone have input on this?

    If I have, say, 1", 5lb Core Cell, how do I convert that to the appropriate balsa?

    I know from a basic understanding that the balsa is heavier and stronger, so I assume you don't need as much volume of balsa compared to foam. However, I also know that the distance between the sandwich layers is what determines stiffness in the continuous I beam. So... how do you convert?


    Do you need the same thickness balsa with more weight? Not sure how to proceed and I think this might be helpful for the OP as well, since he's comparing the two.
     
  10. War Whoop
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    War Whoop Senior Member

    Balsa is not stronger, the fact is you need more glass with it as this material is unforgiving,that nasty moment of inertia thing again! simple impact testing shows that. I would chose something that follows Hook's Law.
     
  11. CatBuilder

    CatBuilder Previous Member

    I meant stronger as in sheer strength right? Isn't that all that really matters for the substitutions I'm looking at doing?

    I don't care if my deck is a little more susceptible to dropping a bowling ball on it from the masthead. Impact resistance isn't a concern for the deck or the deckhouse. The hulls need the impact resistance.

    I need to make sure I can afford the project, or I won't have a boat. Right now, it's looking too tight for comfort. I have to reduce the insane cost of the Core Cell.
     
  12. Steve W
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    Steve W Senior Member

    These are questions for the designer to answer,many other factors come into play,for instance,interior fitout,eg, if there is cabinetry that helps keep a bulkhead in column that can effect how stiff the panel needs to be,the components making up a lightweight structure are not designed alone. I know of a 43ft racing cat that has 4mm plywood bulkheads supporting the crossbeams,not a lot of stiffness there unless kept in column but clearly enough. I had a Mac 36 cat with 1/4" 3py b/heads under the crossbeams and well supported by berth tops at mid height so they could not buckle ,more than adequate. Just remember,the designer has a vested interest in the outcome so he will work with you on material substitutions to keep costs at bay.
    Steve.
     
  13. CatBuilder

    CatBuilder Previous Member

    My designer has a very complete opinion on cores and here it is, from the following link to his Apocalyptic Boat Building Article (next link). Clearly, he says you can use balsa and I know he prefers it in a lot of instances where it makes sense (right tools for the right job). I have the plans for my boat in both foam/glass and in plywood/epoxy/balsa.

    I have asked Kurt probably a hundred questions and I probably have another hundred to go while building. I don't want to over tax him, so I'm trying to ask some of the more basic questions here - like how to substitute a 1", 5lb Corecell core for a balsa one and have the same resulting panel strength (in sheer strength, not in impact strength).

    I have the plans to make the plywood/epoxy/balsa cross beams and bulkheads already for this boat. My idea is to use the materials that make the most sense where they make the most sense. :)

    Core Cell for hulls (building male mold and hand layup). This is good for impact resistance too. Balsa for deck. Still much easier than foam. Because you don't take out ALL the balsa, but only waterproof the area that you are drilling, it's easier. I will be working sloppy because I haven't determined exact locations of deck hardware yet. I want to be able to drill and fill to mount my sailing hardware. Personal preference.

    Balsa for the deck will make that easier. Also, balsa for the deckhouse is fine. Core matters very little way up there. Could be anything, really. Lastly, I'll do the bulkheads just like the plywood/epoxy/balsa design I already have. This should save the most $$$ and result in the same strength boat.


    http://www.multihulldesigns.com/post_apocalyptic_boatbuilding.htm


    "Core Materials:

    Core materials like CoreCell http://www.atlcomposites.com/products/cores/end grain balsa/index.htm (SEN) or Divinycell (crosslinked PVC) http://www.diabgroup.com/europe/products/e_prods_2.html are state of the art. They are the rock stars of core and consequently are shockingly expensive. The last time I checked, a cubic foot (0.3 cubic meter) of 100 kg foam core costs $65, bought in bulk. That’s about $5.50 per square foot of 1” thick core. Moreover, they might have long wait times to get. What about alternatives?

    One tool I use to compare core materials is a simple equation of shear strength in psi over weight in lbs./cubic foot. (Shear strength/weight=x) I call it shear specific. For example 100 kg Divinycell gives a number of 36. A heavier core with the same strength will give a smaller number. There are many other considerations, but this is a good first start. For example, honeycomb will give a higher number, but has other problems. Corecell will give a slightly lower number as the shear number is lower, but it behaves differently and will perform as well or better. In testing lab results with Corecell samples, the laminate always blew up before the core was significantly stressed. That is a good outcome.

    What are the alternatives to foam core?

    Balsa core is less expensive than the foams above, with slightly more weight and better shear strength. It must however be used with epoxy only and with best epoxy construction practice to keep moisture out. There may be problems with vessel value if built with balsa core. If properly built, that should not be a problem, but that is a market situation.

    Balsa does not absorb impact as well as the foams do.





    Urethane foam costs much less than the rock star foams. There are many manufacturers, but one is General Plastics http://www.generalplastics.com/products/index.php?PHPSESSID=6ac3fdb313fb070a2d6a8f4d0f5eb4a4. Several times a year I get asked if urethane foam is a good substitute for PVC or SEN. It is not. To get adequate shear strength, Scott Lewett at Structural Composites tells me that the urethane core needs to have about double the density of the PVC or SEN to do the same job. What’s more, it crumbles much more easily under repeat load. It is called being friable. Your boat could see a lot of repeat load. Friable core is thus not acceptable.

    Coretex, a fiber reinforced urethane foam is still weak, but it has relatively high weight. I see no use for it in multihulls or light monohulls.



    Nida Core http://www.nida-core.com/ has evolved in the last few years. Look into the latest offerings. The original Nida-Core is a polypropylene honeycomb used as a core in sandwich panels. It comes covered with a non-woven polyester scrimm that is thermofused to the cells. This covering helps bonding properties and keeps resin out of the cells. The density is 4.8 pounds per cubic foot. The shear strength ranges from 70 psi to 130 psi. That gives a shear specific of 14.6 to 27. It is available with foam filling. I see it as being most useful for interior furnishings, but the weight is a bit high for that. Corners cannot be easily faired into a radius. Nidacore has superior sound deadening properties to foams or balsa of similar densities. Again, lately they have greatly expanded their product line.

    Polycore http://www.polycore.com.au/ looks similar to Nida, but claims better mechanical properties. Density is 80 kg (4.8 lb/cf) and core shear strength is 0.8 MPA or 109 psi. It gives shear specific number of 21.8.

    Plascore http://www.plascore.com/index.htm has a variety of honeycomb core materials. The most useful one to boatbuilders has polypropylene core and seems to be like Nidacore or Polycore.

    Styrofoam has core shear strength that is so low it is measured in psf. (pounds per square foot) not psi as the usual foams are. Enough said.

    Norcore (Raven Industries) and http://www.pepcore.com/products.html#

    A plastic honeycomb made from a variety of plastics including PEI, PC, ABS, HIPS, and SURLYN. Available in thickness from .5 inch to 3 inches. Strength unfortunately goes down with increased thickness. One inch thick Norcore has a weight of 10# per cubic foot but a compressive strength of only 224 psi. It is a very rigid "live core" and cannot be bent. Corners cannot be easily faired into a radius. In a 1” thickness the polystyrene core weighs 9.7 lb/cf. and has shear strength of 105 psi. In polycarbonate the core weighs 13.3 lb/cf and gives shear strength of 142 psi. In polyprophelene the core weighs 10.68 lb/cf and gives 57 psi. The best shear specific number is 10.67.

    Nomex A hexagonal honeycomb made from DuPont aramid paper sheets. A thermosetting resin is used to bond these sheets at the nodes. After expanding, the block of paper is dipped in a phenolic resin. A variety of cell sizes, core and paper thicknesses, and densities can be produced. Flat panels can use the hexagonal cell, but doubly curved surfaces such as the side of a hull must use an over-expanded cell to conform to the curves. While the paper appears to be very waterproof, work I have seen by the Boeing company suggests that water will condense in the cells over time. I do not recommend honeycomb for the primary structural parts of a multihull. I do use honeycomb for bunk boards, soles, shelves and cabinetry. Corners cannot be easily faired into a radius. Some tricks are required to vacuum bag glass onto bare honeycomb.

    One nice product that uses honeycomb is Tricel. It is used for interior surfaces. It consists of thin plywood faces bonded to a honeycomb core. Weight is very good. See http://www.tricelcorp.com/.

    Core Craft FRP Honeycomb http://www.infracomp.com/index.html

    Custom fiber reinforced plastic honeycomb panels of 1" thickness to 30" thickness. Panels may be flat or slightly curved and up to 12' wide with unlimited length. Compression strength of core can range between 100 psi and 1000 psi. May be good for large charter boat bridgedecks. I don't know what price is like.

    Penske Marketed as a replacement for plywood. A glass reinforced urethane. Now owned by Alcan, it is called PXC. It seems to be too heavy for multihulls and light monos. Cost is about like pvc foam.

    One client is using Paulowina wood for core in place of balsa as it costs much less. It is heavier than balsa at 15 to 19 pounds per cubic foot ( balsa averages 11 lbs/cuft), but has better structural properties. The bending modulus is double that of balsa. I don’t find a shear strength printed, but I’m confident it is much better than balsa. It would need to be hot coated with a tie-coat like bare balsa must be. I have no personal experience with it yet, but it looks very interesting. See http://www.paulowniawood.com/as a starting place."
     
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  14. War Whoop
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    War Whoop Senior Member

    Sheer?? How is this being tested? ASTM ??? where Movement is considered failure !!! and this foam is designed to give and NOT Fail ,But there is one test the Hydromat that can be applied here and the balsa did not hold up well in Impact testing either and failed badly.
     

  15. fg1inc

    fg1inc Guest

    Back to backing plates for a moment, one of the most common failures we see in the repair business is the result of making the backing plate approximately the same size as the hardware base. This results in a "cookie cutter' effect, no matter the core type or even with solid laminate. Always make the plate bigger than the hardware.
     
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