Hull Core Materials

I always assume that everybody is fully up to speed on the different hull core materials. Then I come across some choices that I see being made. I ask “why would they do that?” Then I realize maybe a reminder is needed.
I saw where cedar was recently chosen for a 50′ catamaran. I assume western red or similar.

I don’t understand why someone would choose that over Core-Cell foam for example. True, cedar is a “live core” in that it does usually satisfy global loading of a hull, with the off axis glass fabric binding the strips together and protecting them.
I consider it to be far too heavy however. While it does have slightly more compression strength perpendicular to grain (240 psi in the Wood handbook) than Core-Cell (165 psi) it is far heavier at 22 lbs/cubic foot, compared to the 6# foam at, well 6 lbs. For a cat with two 1000 sqft hulls lets assume 3/4″ core. The cedar core will weigh 2750 lbs. compared to 750 lbs for the foam core. Actually the foam will be a bit less as 6# is only needed in high load areas.  But dude, that a ton.
And the cedar is not stronger than the core in everything. The NFPA gives the cedar a horizontal shear strength of 70 psi. The 6# foam has a shear strength of 191 psi.

Cedar does have a bit more compression strength perpendicular to grain compared to 6# Core-Cell.  240 psi compared to 165 psi.
But, for me the biggest advantage of the Core-Cell is the hystereisis; it bounces back when impacted.
In panel bending tests to destruction for the USCG, the laminates would practically explode (and I recall that Al had some great numbers, like 70,000 psi bending strength) and the Core-Cell was intact. It could and would stretch up to about 30% as I recall, and bounce back. On impact the cedar does not bounce back. And since the cedar has less laminate covering it; remember the 0 degree laminate is not needed as the cedar takes care of the global loads. The cedar is thus less protected from impact but it needs the most protection.

Finally throw in the foam’s resistance to water damage, and up in the high lattitudes, insulation values.  (Isn’t it snowing in south Australia this week?)

Why would someone build catamaran hulls out of cedar?

 

15 thoughts on “Hull Core Materials”

  1. Having seen the results of both cores in serious damage conditions, and adding the water vs wood factor, I would only consider cedar for my canoe stored in the garage.

  2. Are several kind of foam cores, there are a guide to select them
    can we make our own panels out of polyurethane foam? were i can read about foam ?

  3. great variety of cores. my construction manual has a lot on that choice. usually polyurethane foam is friable. That is it crumbles under repeated loading. this blog has earlier postings on poly foam as I recall.

  4. 1- Money saved in core cost, glass cost, and epoxy
    2- The builder likes the smell of cedar dust to make the time pass while longboarding the hulls.
    3- Beauty: Tell me that the interior of Cris White’s 36 year old Juniper isn’t more attractive than any one covered by white paint or fake dog hair.

    Still doesn’t mean I wouldn’t choose CoreCell for all the reasons you state!

  5. Good info. KH at his best! How do you feel about using cedar-Corecell in foam sandwich application with carbon or glass skins? I heard Schooner Creek BW calls this method “cove”.
    Is it worth the savings? Thank you.

  6. Kurt,
    How would the strength and weight of core cell compare to a stringer reinforced layer of vacuum laminated plywood common to CM hulls?

    Cheers
    Andrew

  7. against global loads like forestay stress, they are about the same. for out-of-plane loads like water pressure, core is much stronger.

  8. Cost is the obvious factor, and looks would appeal to some. You can acquire softwoods in some parts of the world, the PNW being one of them, for free. I have a Stihl 090 saw and mill, and procure my hardwoods for free, but we don’t have softwoods of sufficient size, for the most part. The work is heavy, and you have to store and dry the wood. But given how long owner build cycles last, that is a manageable problem.

    For large projects there is always a commercial issue, and the overall cost of core diminishes in importance, and the importance of resale increases. So no question core is king once a certain scale is reached.

    The enviro impact of corecell both at a theoretical greenie level and at an “I have to live with this crap as I build and so do my neighbours” level, is horrendous. With wood core, you put in a boat, that is carbon capture, as you grow more wood where it was taken from.

    I think to get higher performance you still need to use a structural approach where the cedar is working the longi loads. That was well described by Meade in his article on Adrenalin, where he showed that 3/8 of cedar with carbon uni skins was equal to 1″ of d-fir ply in hydromat loading.

    One has to make sure one isn’t comparing apples to the universe. The nature of composite structures is they can be anything therefore once you load the deck with any one product, the available universe of every other product is sure to exceed the capacity of that one product. It is fundamentally a dishonest argument.

  9. Also, there is the issue of availability. It is a big world out there and Seattle is or was a really bad place to base your impressions of the global availability of products on. Since I built my KHSD 30 years ago matters have to some extent got worse. Seattle with it’s Fishery Supply, Boeing surplus, System 3, etcetera, was a gold mine compared to what exists elsewhere, when I built. And these days, anything outside of the US, beyond same day free delivery, and so forth is like building in a glider in a POW camp, in comparison.

    The old thing about what happens to wooden boats when the water gets in is also bogus. After 50 years of boat ownership, and 40 of epoxy wood boats it is never the problem I thought is would be. My only really big repair happened when I was injured, and out of it for 3 years, and a pihi, I had rough installed in a deck with drywal screws let water into a few unsealed sections of the boat, where it ran up a rear deck and part of the cockpit. There were some easy fixed, and much of the difficulty was just replenishing the stock of my materials, which had fallen a bit at the time. I never had to fair any exterior surfaces as the paint and glass were always intact, and the last layer of wood still adhered (the 1 mm of ply skin. That was enough to rebuild from in a few hours here and there. Not the big deal I had thought it would be. But obviously these are approaches that appeal in smaller boats, sub 30. So the idea one would not want those problems at 50 foot scale is for sure.

  10. all true, but get the foundation right. Yes the foam looks expensive, but wait until you start buying the stirling paint and the harken bits.

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