I always step gingerly onto the deck of a strange boat. I developed that habit after watching a 250-pound crewman make a 3-foot jump from the dock to the deck of a sailboat he was boarding for the first time. The sound that followed was not the resounding thud of solid deck but the sickening crash of ripping fiberglass. A nanosecond later he was hollering, ensnared up to his waist in a Japanese finger trap of busted deck. It took a saber saw to get him out, and we still laugh about it over beers.

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. When Joe Boater steps aboard at a boat show and feels the deck yield, it usually spells disaster for a sale. One more plus: Cored parts are dramatically cheaper (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 piecescut into squares of about 4 by 4 inches. (There isn���t necessarily anything wrong with this coring material; I���m just giving you a peek into the sausage factory.) 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.

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 is left to absorb moisture, and the rotting process begins. (I���ve even seen poorly sealed cores with gaping holes attacked by termites.)

A properly encapsulated core can do a good job for decades. No such luck for the average boat. Before it leaves the factory, that sacred 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. Dealership guys 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. This madman with a screw gun 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 (a topic for another time), so you should at least research the reputation of the builder and 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. Next month we���ll talk about wood-core options and core-repair techniques. ���Til later, smooth seas.

Ken Fickett is founder and president of Mirage Manufacturing in Gainesville, Fla. Since 1971, the company has built about 1,000 sailboats, sportfish boats and trawler yachts.