The stiffness is closely related to the height of the sandwich construction since the height of the beam is elevated to the power of three. The stiffness is of course based also a product of how stiff the materials you are using are, however, since very low Vf's are reached with hand layup the most dominant factor is height.
YEP -- stiffness increases by the Heigh cubed !!
And This is where choosing a material like Carbon or spectra is a Must !
Place fiberglass on a thin core ; and Ya might want to stay close to shore
so the swim back in won't be so tiresome.
You can adjust the flex-rate by adding or subtracting the fibers on each side
of the core, after the thickness is determined. but unless your fibers can stand
up to the rigors of thousands of cycles, than it will probably just break in half.
for a strong flexible board you want to choose a thin core and use more reincforcement. eg fibreglas
He "Meant" Carbon or spectra !!
Ideally , a well constructed board that you want to Last
would NOT contain even (1) gram of that SH*T called fiberglass.
Fiberglass is Good for row-boats that you plan to use on a flat
windless lake. Carbon is GOBS stronger for the same weight,
and if purchased on the roll and un-woven , it can be had for about
$25 a pound.
Seriously -- Carbon is (5) times Stronger than Steel.