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The Hard Core Facts

Craftsmen apply core-bonding putty to the inner surface of the laminate.

It's probably just human nature. Whether the controversy is Ford versus Chevy, Yankees versus Red Sox, or simply "Tastes Great" versus "Less Filling," people love to take sides. Among yacht builders, one of the more impassioned topics of debate is which core material is best for composite construction. Some builders stand by the old standby, end-grain balsa, while others embrace one or more of a plethora of foam-core materials. And still others extol the virtues of the various honeycomb core options.

In my roles as a consulting naval architect, journalist, and professor, I have fielded that question countless times, and my customary reply probably sounds like a diplomatic cop-out: There is no one best material, although some materials are more suitable for certain applications. But that's not a cowardly dodge; it's a fact. And by considering facts rather than clinging to popular perceptions, you'll have a better understanding as to why different builders might rightfully adopt different solutions to what may seem like the same design requirement.

But before we get into the controversy, let's establish some facts with which everyone can agree. First of all, what is a "core material," and what role does it play in a fiberglass structure like a yacht? Solid fiberglass laminates, i.e. those without a "core," are fabricated from multiple layers of glass cloth and resin. To achieve a given stiffness, laminates must be built up to a certain thickness, typically one half to one inch or more. But only the outer layers of a laminate effectively contribute to flexural stiffness; the inner layers just add weight. For boats smaller than say, 30 feet, that's not such a big deal, but for larger vessels the weight associated with using enough solid laminate to achieve a given stiffness can significantly diminish speed and/or increase fuel consumption.

Cored-composite construction, on the other hand, uses two relatively thin skins of fiberglass laminate separated by a thick, light core material. It's a sandwich; and the thicker the core is, the more effective the skins are in resisting flexural load. All the core has to do is keep the two skins working in unison. If the core lacks adequate shear strength, then the thin skins will flex independently; think of how easy it is to bend a pack of playing cards. But if a core material has sufficient shear strength, it makes the skins bend in unison with vastly greater stiffness; imagine trying to bend the same pack of cards if they've all been glued together.

So while everyone agrees that cored-composite construction is inherently lighter, stiffer, and stronger than solid, single-skin laminates, the fur flies when advocates espouse their opinions about the different options. I'll shed some light on the subject, so you can be more objective in your assessment—or maybe just argue on behalf of your favorite with a bit more authority.

Balsa. The old standby, end-grain balsa is still a top choice among many production builders. Viking Yachts, for example, has been using the material successfully for more than two decades, arguing that it offers greater shear strength, pound for pound, than any of the available PVC foam materials. Since its grain is perpendicular to the fiberglass skins, it offers excellent compressive strength and it bonds well with the inner and outer skins. It is also less costly than the PVC foam corings.

Detractors claim that it's vulnerable to water absorption if one of the skins is breached; the grain of the balsa can act like a wick, infusing large areas of the core with water from even a minor breach, such as a poorly sealed through-hull fitting. And if water enters the core, the balsa will eventually soften and rot, necessitating a generally costly repair job. But advocates point to years of success boatbuilders report with the material, noting that if work is done correctly, there is no problem with water infusion.

One application where most would agree that balsa shines is as a core material for interior decks and bulkheads. Its superior shear strength and stiffness make it ideal for supporting a wide expanse of open deck without need for intermediate supporting pillars or columns. And within the yacht's interior, the issue of water absorption is clearly moot.

Closed-cell foam. Poly-vinyl-chloride (PVC) is a closed-cell foam popular with many builders of larger, upscale yachts. But even within this category, there are differences of opinion because not all PVC closed-cell foams are the same even though some properties are common to all. For starters, because they have a closed-cell structure, they don't tend to absorb water, and even if water-soaked, they aren't susceptible to rot. So as a category, the PVC foams overcome the major issues associated with balsa. Secondly, they inherently offer thermal and acoustic insulation properties, combating condensation on the inside of the hull and reducing irritating noises, such as wave slap while at anchor.

One of the more common varieties of closed-cell PVC foam is a so-called cross-linked foam, of which several commercial brands are available. The name is suggestive of its molecular structure; i.e. molecules interlock in a way that gives the foam relatively high shear strength, though still not equal to that of a comparable density end-grain balsa. But advocates feel its other advantages outweigh its somewhat lower shear strength, and so it is widely used for hulls, exterior decks, and superstructures.

There are some, however, who feel that cross-linked foam is less tolerant of impact loads, and results of controlled testing on sample panels suggest that the argument is not without merit. Not surprising, those who are most vocal about such tests are the manufacturers of competing linear foam, another variety of closed-cell PVC. Having a different molecular structure, linear foam has more "give" under impact and is therefore less likely to suffer a core shear failure after a hard impact or grounding. On the other hand, the "give" comes with a penalty; its shear strength is somewhat lower than that of either balsa or any of the cross-linked PVC foams. And it is more costly. Because the main advantage of linear foam is its impact tolerance, it is most effectively used as a core material in the hull bottom, with other core materials used in the hull sides, decks, and superstructure. Nonetheless, some premium builders adopt a spare-no-expense posture, using the marquis brand of linear PVC foams (Airex) throughout the structure.

One more player in the crowded pond of foam-core varieties is a material called CoreCell; not a PVC material, it is a Styrene AcrloNitrile (SAN) polymer. It offers all the advantages of the cross-linked foams plus the impact tolerance of linear foam. A relative newcomer to the marine market, it has gained favor among many high-end builders.

Honeycomb. As the name implies, this core material is formed from hexagonal shapes arranged in a matrix. Two common materials used in this type of core are Nomex and Nidacore. The former is relatively expensive and is mainly used on competitive sailing yachts. The latter is a relatively inexpensive polypropylene well-suited as a core for decks and superstructures.

The last word. Unlike many of the great controversies, this one is multidimensional; balsa versus foam versus honeycomb and with several categories of PVC foam thrown into the fray. But as I said at the outset, there is no single right answer; just solutions that are best for any given application. Wherever there are boaters, builders, and a couple of beers, you can be sure the debate will rage on.

This article originally appeared in the September 2007 issue of Power & Motoryacht magazine.

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