Stiffness and strength are linked together in a fiberglass boat. To have a hull of proper strength you must have stiffness. Stiffness keeps the hull from deforming. Stiffness can be achieved by building up a thicker laminate (adding a lot of weight), a relatively thin laminate with an extensive stringer system (also adding a lot of weight), or by spitting a relatively thin laminate and moving the now two laminates apart with a structural core material. You can take a given laminate (for discussion purposes say 1/4" thick, split it into two 1/8" laminates and put a 1/4" core in between it and by doing so increase the stiffness of the panel by a factor of 7 times (700%) and increased the panel strength by a factor of 3.5 (350%) yet only increased the weight of the panel by a factor of .03 (3. %).
A cored hull is much stiffer than a solid glass hull of equal or slightly thicker skin. There are engineering guide lines that dictate how large a panel can be without being supported by some type of internal stringer support. Cored hulls, have a higher panel stiffness, and therefore can have internal bracing members farther apart compared to what would normally be used in a solid skinned hull.
If a hull has insufficient stiffness there is a possibility of cyclic loading that is similar to bending a paper clip back and forth...over time it is weakened and the panel fails.
For more on how cores work to increase stiffness in a composite laminate copy and paste this link and go to page 18 out of 24 of the PDF file (marked page 79 figure 2-13).
http://www.marinecomposites.com/PDF_Files/G_Composite_Materials.pdfThe fact that you would be hard pressed to find a single skinned performance catamaran from any builder ought to speak volumes as to what direction you should go all other things being equal. A single skin may not dent as easily as a cored hull, but that doesn’t mean that it is better when all factors are considered.