(with excerpts from "Professional Boatbuilder" Magazine)
Definition: The use of a structural, lightweight core material sandwiched between two skins of fiberglass, to improve structural properties while reducing weight.
Coring has been used since the 1970s in composite yacht construction. The range of specialized materials designed for this purpose include closed cell Linear PVC foam, cross-linked PVC foam, end grain Balsa wood, polypropylene honeycomb, etc. The core material is bonded into the over all laminate as part of the laminating process in the initial construction of the part.
Theory. The stiffness of any panel is a cube function of its thickness. If you double the thickness of a given panel, you increase its stiffness, or resistance to bending, by a factor of eight. If you were to achieve this doubling of thickness with additional layers of fiberglass, the weight of the panel would also double. Since it is the outer layers of fiberglass in a laminate that are subjected to the highest tensile and compressive loading in bending, it’s possible to dramatically increase the stiffness of a given panel with little or no increase in weight by adding a light weight core material between two load carrying layers of fiberglass. Compare this to an "I" beam where the wide, thin horizontal flanges of the beam take the tensile and compressive loads in bending, and the narrow vertical web ties these load-bearing horizontal components together in shear. In a laminate, the skins represent the flanges, and the core represents the web of the "I" beam.
(The laminates ability to hold together under an extreme momentary overload condition.)
There is a common misconception that a cored bottom panel cannot absorb the same level of local impact loading as a solid, single skin laminate. The reasoning is that the relatively thin outside skin is easily punctured under impact. This is borne out neither in theory nor in practice, simply because the outside skin does not work in isolation from the rest of the laminate. When a cored laminate undergoes an impact, the outside skin and core deflect locally, absorbing considerable energy and dissipating it into a large area of the laminate as a whole. It is precisely this ability to dissipate impact energy that allows a cored laminate to withstand greater impact loads than a single skin laminate.
A solid skin laminate, on the other hand, absorbs impact energy in a relatively small area, and delaminates locally, immediately under the loading.
The skin and relatively flexible core working together can be thought of as changing the properties of the other wise stiff, brittle solid laminate to that of a more durable energy absorbent material. This is especially so with vacuum resin infusion and vinyl ester resin due to the high quality consolidation of these materials.
Additional benefits, are the suppression of engine vibrations and noise, including wave impact noise. Coring gives the boat superior thermal qualities too, reducing condensation, and the effects of extreme temperatures on passengers and contents.
The bottom line is coring can make a remarkably strong, stiff, light weight, boat that is more nimble, quieter, comfortable to ride in and more efficient.