In regards to stiffening, I have found, as Alen hass, that the box stiffeners really hold up excellently. I think this is because allthe endges are glued and you are forming this solid little cubes and rectables with hardened edges. What I don't like about fom board, is that the foam is not "Structural foam". it is open celled usually, and over time, I have seem any foam panels bend. This is because the foam is not stiff enough and as the paper loosines it tensile strength due to moisture, the sheet bends. Now, if structural foat were used, you could just fill up the hull with it in liquid form, paint the whole ship with water proof paint, and it would float in water.
