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Rotten to the Core

Nothing good happens when water seeps into cored laminate.
Nip this intrusion in the bud or pay the piper later.

01-transom recore[1]

Does anything put a damper on summer faster than discovering your boat’s plywood-cored transom has been soaking up water like a deck sponge, and now it’s about as flexible as a trampoline? Maybe you’ve got 1,500 pounds and 500 horses worth of outboards bolted to it, and the only structure keeping things together is a few layers of cracked fiberglass and force of habit.

Do you feel good about this? If the answer is, “No, not really,” you have two choices (three, if you’re a dedicated DIY’er): Get out the “For Sale” sign, write a fat check to your boatyard, or fire up the circular saw and start cutting. If you choose door number three, it helps to have a masochistic streak.

Until the early 21st century, boatbuilders typically cored the transoms of fiberglass outboard and sterndrive boats with plywood to stiffen the laminate and take the weight and thrust of the motors. (Nowadays most use a high-density synthetic material.) Sometimes they cored other components with plywood too—stringers and frames, maybe cockpit soles or swim platforms, and, occasionally, decks. Plywood is strong, relatively light, makes a good coring if it stays dry and much cheaper than solid fiberglass of equivalent stiffness. But if it gets wet, and eventually most plywood cores do, it rots, gradually metamorphosing into black gunk. Folks thinking of upgrading the outboards on older boats, or maybe buying and refurbishing a classic from the days of their youth, should pay special attention to any component with plywood coring. Repair is expensive, so hire an expert surveyor before throwing good money at bad plywood. Don’t be afraid to drill some test holes to ascertain the condition of the core. You may be surprised at what runs out.

Before the handful of plywood-lovers reading this column come after me with chop saws, let me tell you: I love plywood too, and if I were building a one-off boat for my rapidly approaching old age, I’d build it out of top-quality marine plywood, for the same reasons 20th-century boatbuilders used it for coring. Choosing a design that can be planked with sheets of plywood makes for a faster build and a tighter hull with minimal seams, and cold-molding the hull with narrow strips of plywood laid atop one another in epoxy resin creates the curves typical of plank-on-frame construction, but stronger, lighter and tougher. The problem with plywood comes from sealing it in fiberglass, and then letting in water.

Keep Your Sandwich Dry

Any cored laminate is susceptible to water intrusion; the core material doesn’t matter, although plywood and balsa have the added disadvantage of rotting when wet. Water can seep in through a myriad of paths—through holes drilled to mount outboard motors, stern drives or trim tabs on the transom, or to fasten deck fittings, or around hatches or ports, or through poorly sealed limber holes in stringers and frames. Any perforation in the skin can admit water. In a perfect world, boatbuilders would substitute solid laminate or a water-impervious material in and around cored areas where the fiberglass skin needs to be drilled or cut, especially below the waterline. And both hull and deck cores would end short of the hull-to-deck joint, so those fastenings would occur in solid glass. But that’s in the perfect world; it’s not always
that way.

Water that seeps into the core never seeps back out, but gradually meanders through the core and eventually causes trouble. A wet core that freezes and expands during the winter, or that gets super hot under the summer sun, can cause delamination of the fiberglass skins. That reduces the stiffness of the cored-fiberglass sandwich, making it more likely to flex under pressure and delaminate even more, eventually fracturing the core itself. More delamination makes it easier for the water, which is still seeping in from somewhere, to migrate even farther though the laminate. Even if the sandwich isn’t yet delaminated, years of water ingress adds excess weight to the boat, sometimes enough to deter performance. Bottom line is, whatever the core material, letting it get wet and stay wet is never a good idea.

A diligent boat owner has plenty of opportunity to prevent this. How? By hiring a surveyor every few years to check for wet areas and delamination in the hull and deck—with luck, this will reveal problem areas before any significant damage has occurred, and before the core and skins have parted company. You’ll be able to repair the laminate by drying it out and filling any voids with resin, without having to do any serious cutting. And, of course, you must find and seal the source of the water; this is often as simple as re-bedding deck fittings. The money you save by not having to cut out and replace even a small area of bad core will pay the surveyor’s fee many times over.

Use Desert-Dry Air

If you have wet core, how do you dry it? The best way is to call in an expert like Jon Bartnick, co-owner of Dryboat, a mobile service for, as the name suggests, drying out boats. The patented Dryboat method injects “desert-dry” air into wet sections of coring to drive out the moisture. It’s done using many small air tubes inserted into 1/4-inch holes, ideally drilled in areas that are unobtrusive and easy to repair, maybe on the underside of a chine. Once the core is completely dry (the process takes five weeks on average), the Dryboat crew injects filler into the core, followed by epoxy resin, and then seals up the holes. “We don’t do cosmetics,” said Bartnick. “We leave that for the boatyard.” But, he added, the crew drills the air holes in the least invasive areas possible. Even if holes must be drilled in the topsides, and the Dryboat process often requires many holes, they are small and easy to repair.

Bartnick started Dryboat when he was trading in his own boat and the surveyor discovered the wood-cored engine stringers were soaking wet. “The repair would cost more than I was getting for the boat,” said Bartnick. But he had a friend who could fix the stringers if Bartnick could dry them. “I’ve been drying things out since I was in high school,” he said. “Not boats, but houses and things.” He used his house-drying techniques on the stringers, his friend injected them with resin, the boat passed survey and the deal went through. “The boat dealer said, ‘You should do this as a business,’ so I did,” said Bartnick. Today, he has Dryboat trailers in the Pacific Northwest, New England, Florida and Maryland; they can be towed to wherever the job is, sometimes even to the boat owner’s backyard.

Bartnick said that Dryboat won’t work for every job. If there’s really serious delamination or rot, he doesn’t recommend the process, and while he can dry out an outboard or stern-drive boat’s plywood transom that’s just wet, “If it’s rotten, tear it out,” he suggests. There’s more leeway to repair transoms on inboard boats, since they don’t have the stress from the motors. His advice is to check cored areas for moisture frequently using a meter, and recommends the Protimeter Aquant, an investment of about $400 that can save you a lot more. Bartnick drills test holes for almost every job, and he’s found the Aquant is very accurate, but not so sensitive that it reads false positives, as some meters do, tagging dry areas as wet. “Boatyards love those meters,” he said.

Time to Start Cutting

If drying the core doesn’t fix the problem, you’ll need somebody with the chops, and the chop saws, to cut up your boat and put it back together good as new—maybe somebody like JB Turner, owner of Front Street Shipyard in Belfast, Maine. His crew simply replaces the core, Turner said; they’ve done it hundreds of times, and have the process down pat. “It’s a straightforward repair: Pull off one skin, strip out the core and replace it.” Turner makes it sound easy, and I guess for him it is, but I’d rather perform an appendectomy on myself than take on the job. (The fact that just this one yard has repaired hundreds of cores suggests there’s a lot of failed laminates out there; see “Bonding the Core.”)

Turner always tries drying out the core first, and about 25 percent of the time it does the trick. If it works, the cost of repair is much less, so it’s worth taking the shot, he said. But if a balsa core has been wet for more than a year, usually it starts to rot and needs replacement. “You can pull it out with a spoon,” he said. When repairing a hull core, Turner works from the outside, cutting away the skin over the wet area and then replacing it when the job is done. On deck, though, it’s not always so easy. Some owners want to preserve the boat’s original nonskid, he said, which means working from the inside. I don’t know about you, but I find the prospect of working overhead with dripping epoxy and sagging fiberglass fabric particularly unappealing; boatyard guys really earn their money on jobs like this.

A moisture meter and percussion sounding are the typical means of discovering core problems, which can hide behind a healthy-looking outer skin. But it’s easy to see when an outboard boat’s cored transom has started to fail: There will be stress cracks all over the place. Water generally gets into the core via failed bedding, but the design of the hull-to-deck joints on some boats will let water in, too. (Turner’s crew reworks the joint to prevent this.) The only cure is to cut out the bad core and replace it with a composite material like Coosa, a high-density polyurethane foam that’s reinforced with fiberglass. It’s lighter and stronger than plywood, won’t rot and comes in sheets, so it can be cut and fitted easily. Today, most boatbuilders are using this kind of material where they once used plywood, but until 2010 or even 2015, some were still coring with wood.

The job’s only half done when the core’s replaced. A new skin has to be relaminated, faired and refinished, a skilled, time-consuming job in itself. All this means replacing a bad core is expensive. Turner couldn’t give me an estimate of the cost, since every project is different. I asked him if we’re talking four figures or five. “More often five,” he said. “It’s not free.”

So, a word to the wise: Maybe it’s time to invest in an ounce of prevention and buy a moisture meter and learn how to use it before a wet core sinks your boating budget. Either that, or get real friendly with a skilled local surveyor. Both options are more expensive than a “For Sale” sign, of course, but a lot cheaper than paying for the repair.

2018 Boatyard rule

To Dry or Not to Dry?

If you’ve got wet stringers or frames, sometimes you get lucky, and sometimes you don’t, depending on how they were built in the first place. A stringer with a heavy lamination of fiberglass over its core gets most of its strength from its shape and from the glass, not from the core. In cross-section, the stringer’s profile resembles that of a stovepipe hat (that’s why they’re called “hat-section stringers”), taller than it is wide, and it’s very stiff in its tall dimension, like how a toilet paper tube on end is hard to crush relative to the strength of cardboard. The stringer will do its job whether the core is wet or dry, or maybe even missing entirely. But all stringers are not created equal—some builders skimp on the fiberglass and depend on the core to take much of the load. When these cores start to deteriorate, drying them out and injecting them with resin might not be enough. It’s important to call in a pro on any core-repair job—don’t just dump epoxy into the core and then head out into Victory at Sea conditions. Ask an expert.

Gray rule

Bonding the Core

Fiberglass allows easier building of components with curves, one of its major benefits in boatbuilding—most boats, especially sailboats, have compound curves in their hulls, tricky to build using wood or metal. Flexible fiberglass fabric will drop right into place in the mold, though, and be locked into shape by the resin. But a sheet of coring is stiffer than ‘glass fabric. It’ll bend, but isn’t always happy doing it, and it’s often difficult to force a sheet of coring into the curved sections of a hull or deck and keep it there while the resin cures. Boatbuilders get around this by using flexible coring that’s made up of very small blocks of material glued to a polyester scrim. Coring manufacturers call this material “drapeable,” and Baltek ContourKore is one example.

Drapeable core will conform to curves, but when it’s bent, the cuts between the blocks—or the kerfs—open. If they’re not filled during construction with resin or bonding putty or both, they provide easily navigated channels for any water that enters the core to migrate through the laminate. When production boatbuilders first started to core their hulls in the 1980s and early ‘90s, many used rudimentary methods to bond the core, sometimes as basic as piling on scrap metal to press the core into place. Empty kerfs and incomplete bonding were common in those early cored boats, and this resulted in built-in delamination and frequent water intrusion problems. Usually, the core material took the blame, but it was often as much a result of poor construction. If you decide to restore a cored hull from those long-ago days, don’t be surprised if the survey reveals areas of delamination, even without water intrusion. They might have been there since the boat rolled out of the factory.

This article originally appeared in the Summer 2021 issue of Power & Motoryacht magazine.