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?