Hull boat design practice poor man's composite idea

What you say is very true. Obviously one needs to add a structural reinforcement to the foam elements. I am in the late phase of building a 12 ft flat bottomed foam sailboat (34" beam). Between the bottom tow layers of foam I have placed two 8 foot long 3/8" plastic coated rebar coupled with 7" wide side hull/gunnel. I have stacked 6 layers of foam (12" tall - 8" deep cockpit and 4" thick bottom amidship). The bottom has been rasped to form a rocker (should have bent the foam from the start but paid the price for not fully thinking this thru in due time). For the weaker area of the aft cockpit (due to the rocker) I have placed a 1/4" plywood to distribute the weight. This space is under the tiller so it is unlikely that it would be stepped on. Before outer shell is applied hull appear to be very sturdy and it shows very little flexibility.
The point of the entire contraption was to deliver a very light (under 50 lbs) yet unsinkable boat (the buoyancy of the foam structure including enclosed air pockets is greater then the weight of the fully flooded cockpit).
An example of /XPS foam/PVA/cotton shell on a sea going kayak : Sawfish, an Unsinkable, Lightweight, Foam Kayak (23 Lbs). Free DIY Kayak Plans, the Hardware Store Boat http://www.instructables.com/id/Sawfish-foam-kayak-build-a-funtional-light-wieght-/