After I posted construction of Giulietta, I received a lot of PM from a lot of people asking me what was the Divinycell.
I replied once, twice, then on the fifth PM I decided to post here what it is and the web page of the brand that was used for the construction of my boat.
They have here all the different densities and purposes.
The Diviniycell is like a lighter stronger styrofoam, tha comes in sheets and can be cut with a knife and sanded just like foam on a surfboard.does not rott, and makes the boat unsinkable (I hope!!)
Basically you make it to whatever shape you want, fiberglass one side, turn piece around, remove wood structure used to help it keep its shape, then fiberglass the other side. The result is a 25mm thick chunk of very light very strong fiber glass, wity minimal structure required to keep its strenght.
I believe was around $90 each sheet (here)and a lot was used, over 150 sheets if I am not mistaken. Don't know exactly.
For the tech freaks here is some more stuff