I have to agree with what srchr/gerald said. In larger boats, the foam sandwich construction works well because any additional weight is handled by increasing the motor size a bit. Also, if you look around, this method is predominantly used only on the larger boats (which I'll define as larger than a canoe in total volume). If you look at production kayaks and canoes, I doubt that anyone makes their boat using this manner because it does not work well in this application (i.e. small, light hand paddled boats).

However, at the same time, I do not want to discourage you from trying this method. If you have some ideas, by all means, go ahead and build scale models using the foam sandwhich core and a plywood or strip core and run some tests on them both. If you are able to get results that you feel comfortable with, then build a full sized 'yak to test.

Paul