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Core Materials

For years, builders have made parts of their boats by laying fiberglass into a mold, then using a "core" material to thicken the part. Finally, layers of fiberglass are added to the top, creating a sandwich effect. Doubling the thickness of a part in this way can make it up to eight times stiffer, which provides many benefits: Not only do nearly all hulls offer better performance when they do not flex, but almost all stiffened fiberglass parts enjoy a considerably longer life span. Cored parts are also dramatically less expensive and lighter than parts of solid fiberglass.

Boat dealers talk about "hull thumpers." This is not the nautical equivalent of tire-kicking, because the "solidness" of the sound - that is, the soundness - can indeed be a measure of quality. Basically, hull-thumping is what a surveyor does when he inspects a hull for purchase; he taps hundreds of times with a hammer and listens with a critical ear to search out hints of deterioration, most often in cored areas.

The deterioration the surveyor finds can be the result of many things. Let’s look at the most common scenario. Thousands of boats are built every year with wood core in the decks. Usually the wood is fir plywood - often scrap from larger interior pieces cut into squares of about 4" x 4".  The blocks are arranged on the part in a continuous sheet, and with a little help from resin, they usually bond quite well - the first test of a core.
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Problems with wood cores begin just before the last skin is laminated. At this stage, it is imperative that the wood blocks are completely sealed; otherwise, the wood will absorb moisture, and the rotting process begins. We've even seen poorly sealed cores with gaping holes attacked by termites. A properly encapsulated core can last decades. Unfortunately, this is rarely the case for the average boat. Before it leaves the factory, that sealed core stands a good chance of suffering holes for the installation of windows, rails, deck hardware and a zillion other things. These wounds have great potential to spread the infection of rot unless they are meticulously sealed and cared for by the builder.

Even when the builder does the job perfectly, the vessel continues to run the gauntlet. It has to safely make it through surgery at the dealership, where non-factory-supplied equipment is added. Dealerships rarely take as much care as the factory, because they are not responsible for the core’s warranty. But let’s say a reputable, knowledgeable dealer ensures the core remains perfectly sealed, with no exposure to moisture. The boat still has one last hurdle to clear: the owner.

The owner is often a core’s worst enemy. Armed with a screw gun, he attaches doodads everywhere, with little regard for the health of his core. Down the road, this can lead to a game of finger-pointing: Who’s responsible for the core problem? Most of the time, it is impossible to tell, and repairs can be incredibly difficult to effect.

The bottom line: Most people do not have new boats surveyed, so you should at least research the reputation of the builder and determine whether that reputation includes a high incidence of core problems. Learn what to look for. See if the insides of the boat, the "raw areas," seem well laminated and free from obvious exposed wood core and holes in the laminate. Look for good sealing practices on installed hardware, and ask the builder or dealer just what the heck they do to prevent core problems. Any cracking or creaking sounds you hear when you walk on the deck could be early signs of trouble.

The good news is that most builders and dealers are conscientious about the work they do, and core issues have diminished over the years.