In Cored Hull Construction, Does High-Tech Mean High-Quality?

Despite the numerous advances in modern boatbuilding technology, sailboat construction is far from being a perfect science.


There is nothing inherently wrong with core construction when it is done right. Over the years, however, Practical Sailor has seen a variety of core failures resulting in costly repair. There are five common types of failure: poor workmanship, excess flex (poor engineering), point loading, bad chemistry, and water intrusion. Even with a generous safety factor, things can still go wrong at the factory. High-tech materials and construction methods do not necessarily solve quality control problems.


Think you have boat problems? Consider this one: A high-end ocean racer, barely a year old, shows up in a boat yard with sauce-pan-sized blisters bulging up from the slick, blue skin. A prominent surveyor is hired to define the problem. After some testing, he announces that the solution is fairly straightforward.

All that this new, nearly $1 million boat needs is a complete face lift. The entire outer laminate will have to be removed. Any core that is damaged when the outer skin is peeled away will have to be faired again. Next, a new Kevlar and S-glass skin will have to be vacuum-bagged in place, and the hull will need to be post-cured in an oven. Finally, the finished surface will have to be faired (requiring hundreds of man hours), before the hull can be primed with epoxy and the topsides can be painted with linear polyurethane.

One can almost imagine dollar signs registering in the boatyard managers eyes when this news is announced.

In this actual case, the resin manufacturer blamed the maker of the core material. The core producer blamed the resin chemistry. No one was happy with the boatbuilder. It is just this type of astronomical repair job that has led to many new insurance policies limiting the amount of coverage on this sort of hull “deterioration.”

The engineering philosophy behind any composite construction is that the strength of the finished product is greater than the sum of its individually tested components (see “Core Concept,” page 34). Initially, the garboard of a fiberglass boat was laid up as thick as the plank it replaced. Performance-oriented boaters soon saw that lighter was faster, and some&emdash;but, unfortunately, not all&emdash;boatbuilders recognized that better engineering had to go hand-in-hand with any plans to strip out structure. The shift toward cored hull and deck construction brought great potential for weight savings, but also introduced new challenges. In core construction, a low-density material is sandwiched between high-modulus, fiber-reinforced-plastic (FRP) skins, resulting in a strong, but light panel. So long as the bond between the outer skins and core remains intact, this light component will exhibit extraordinary structural properties. Cored panels are now the norm for most sailboat decks. Hulls, on the other hand, can be completely cored, partially cored, or solid FRP laminate.

There is nothing inherently wrong with core construction when it is done right. Over the years, however, Practical Sailor has seen a variety of core failures resulting in costly repair. There are five common types of failure: poor workmanship, excess flex (poor engineering), point loading, bad chemistry, and water intrusion. Each of these results in the loss of connectivity between the inner and outer skins, much like losing the span connecting the top and bottom of an I-beam. The result is a massive drop in stiffness and increased flex, and if left unattended, this can result in the critical failure of the component. If the region in question happens to be the garboard area of a cored hull, the failure may include the ballast tearing free from the hull. Such failures are rare among cruising boats, but not unheard of among racing vessels. The reasons for this drive home a good lesson about stress and scan’tlings&emdash;the specified measurements for structural components.


As explained in our recent special report on fiberglass hull failure (“Lessons from the Boneyard,” August 2007), a stress riser is a focal point of energy. On sailboats, the areas near keelbolts, chainplates, rudder bearings, and the mast step are good examples of such high-energy hot spots. The best builders use well-reinforced solid laminate in these areas, creating a tapered junction between solid and cored laminate that better distributes the forces focused on such hot spots.

By using computer software that carry out finite element analyses, naval architects can get a clear picture of where hot spots might lie in a new design. These programs yield graphic images of the hull that is “painted” with color-coded hot zones associated with high loads. Using this data, designers and engineers can change hull and deck laminate schedules to better respond to changing load dynamics. Although these computer models can be confirmed by actual data collected by onboard sensors (as well as post-failure forensics), computer modeling of sailing loads is not an exact science. The frequency and degree of failures in the most recent Volvo Ocean Race (“Life at the Extreme,” August 2006) bears this out.

Over the years, Practical Sailor’s contributing editors have seen boats – both with solid laminate hulls and cored hulls – that, either by defect or design, are clearly under-built for their job. In one extreme case, a current editor and a friend were able to place their feet on the keel of an early model Pearson 365 that was hanging in the Travel Lift slings. With shockingly little pressure, the two were able to make the keel swing side-to-side like a pendulum. The hull skin in the garboard area dimpled in and out with each oscillation. Each time the vessel tacked, the flexing garboard destroyed more and more of the FRP-resin bond, a process that would have eventually caused a catastrophic failure.

The good news is that most builders have gotten better at composite boatbuilding and their efforts to beef up high-load areas have improved. In the case of keel attachment points on cruising boats, theres usually a long, wide junction point where lead meets fiber-reinforced plastic, and the weight of additional reinforcement from extra laminate is of little concern.

However, the modern bulb keel on a racing boat or racer-cruiser is usually heavier and deeper, yielding a greater righting moment and more pressure on the keel-to-hull attachment point. The upper portion of the keel is usually narrow and thin, meaning that heeling forces are focused on a small area of the hull. An interior structural grid and well-engineered keel junctions are essential, but even the best built hulls of this design can be vulnerable in a severe grounding. A smack on a hard bottom can result in hidden damage well beyond any obvious failures in the garboard area.


Aside from the reality that cruising sailors run aground, strike submerged objects, and sail into storms, another strong argument for a generous safety factor when designing a voyaging boat is the relative lack of quality control in the boatbuilding industry. In some ways, the aerospace industry has gotten boatbuilders into trouble by being too good at producing composite structures. Skilled workers in near-laboratory conditions are able to consistently create fiber-reinforced structures that meet the highest standards. Product inspection includes the careful analysis of test samples and X-ray inspection.

This meticulous approach yields composite panels that reliably resist the loads that they are designed to handle. These include ordinary tensile and compressive stress, which run perpendicular to the panel, as well as the more complex shear and “twisting” loads that run tangentially to the material face (see “Core Concept”).

Unfortunately, the data collected from the aviation industry often gets directly transferred to boat designers and builders, where the manufacturing process is far less controlled. In many instances, laminators are the lowest paid workers with the highest turnover rate of any employee in the company. Ultimately, manufacturing lapses result in a hull that falls far short of the engineers ideal.

Solid hulls are not immune to problems of quality control. Prior to the mid-1980s there was little climate control, so temperature and humidity at the plant often followed Mother Natures seasonal profile. The result was, and continues to be, less stringent control of manufacturing variables and more chance of panel inconsistency. In those early days, overbuilding tended to cancel the downside of inexact methodology. Early fiberglass boatbuilders were skeptical of the new material and tended to add extra layers to solid FRP hulls. The simple, wet, hand lay-up process may have been resin rich, but if the crew used their serrated rollers diligently and made sure that edges of mat and roving overlapped, all turned out pretty well&emdash;if the chemistry was right.

Polyester and vinylester resin cures or thermo-fixes when a catalyst and a promoter react, producing oxidation and subsequently generating the heat that causes the resin to harden. If theres too much of one ingredient and too little of the other, an impartial cure results, and un-reacted chemicals are left in the material. The resin never reaches the prescribed hardness.

Some manufacturers purchase resin that has already had promoter added while others prefer to mix in their own promoter according to ambient temperature and humidity. In quite a few unfortunate situations, applicators either added extra promoter or, worse, left it out completely, resulting in an uncured or under-cured laminate. All too often, the solution was to apply the following layer with a “hot” batch of resin that would, hopefully, completely cure the layer below. Often it didnt and the boat left the factory with uncured or under-cured laminate trapped between hardened layers&emdash;in essence, a polyester time bomb that would eventually erupt into mega-blisters, causing a colossal headache for some future owner.

Core construction introduces a whole new set of challenges. This I-beam-like structure greatly reduces the amount of FRP material required to achieve the desired hull rigidity. To reduce flex, early, solid FRP hulls required two to three times more resin and fiber than a modern cored hull needs today. Unfortunately, the core materials and techniques used by boatbuilders generally make these boats more vulnerable to water intrusion, a major drawback.

The water doesn’t have to come from rain or seawater. One common boatbuilder shortcut that can lead to eventual problems is encapsulated tanks (those that use the inner skin of the hull as one side of the tank). In one particular case, a marine surveyor found holding tank residue leaking from the forward portion of the hull after a grounding had damaged the laminate. The point of impact was 20 feet away from the encapsulated holding tank, and an internal leak had filled all the void slots (described below) in the hull skin with effluent. That pungent surprise is actually not as big a disaster as when the same problem occurs with a diesel fuel tank and spreads oily residue throughout the core.


One persistent challenge when constructing a cored hull or deck is how to minimize voids in the kerfs, the channels that are scored into a cored panel so that it can be shaped into complex curves (see photos, page 32). To better ensure the integrity of the composite structure, these channels should be filled. Typically this is done using either resin-rich chop-strand mat or a core bonding adhesive, which also bonds the core to the outer skin. When laying up a hull inside a female mold, these kerfs face down (toward the outer skin), resulting in a “blind bond” that cannot be visually inspected during construction unless a clear gelcoat is used and the outer skin LPU painted.

Not only do unfilled kerfs and other voids compromise the integrity of the panel, they offer an effective channel system for water to travel and weaken larger areas of core. One marine industry expert has been recently making the pitch that wet core is nothing to worry about. To the contrary, wet core is a precursor to balsa rot and structural failure.

In cold climates, the expansion that occurs when water freezes splits the bonds in FRP. Even in its liquid state, water does interesting things to residual promoter and/or catalyst trapped in the laminate. It also can attack the water-soluble compound that coats each glass filament and allow it to bond with resin. Finally, contact with wet core material can cause capillary action to wick water deep into the laminate, weakening the mechanical properties that help hold it together.

Today, vacuum-assisted lay-up techniques like vacuum-bagging (in which vacuum pressure is used to squeeze resin-impregnated laminates together during the cure phase) and resin-infusion (in which vacuum pressure vacuum is used to draw resin through dry laminate as well as squeeze the laminates together) can help fill kerfs. These processes can markedly reduce voids, creating a better quality composite. However, these new building techniques require more skilled technicians. And if things go awry, the consequences can be severe.

In one case at Pearson Yachts, the resin flow in a resin-infused hull had ceased before it had reached all of the shear line. A 30-foot length of deck flange and hull remained as dry as when the laminate came off the roll. The repair required a secondary bond&emdash;an inferior bond between a new layer of FRP and laminate that is already cured&emdash;eliminating a key advantage of resin infusion.

In another instance, Ted Hood complained that when fiber/resin ratios get too high and fibers are not being sufficiently saturated, it can actually increase the likelihood of delamination. Clearly, when the resin-fiber ratio is intentionally held to a minimum, careful quality control is essential.

Point loads such as the tension of a sheet block on under-structured portions of the deck are another notorious cause of core problems. The repeated on-and-off bending caused by point loads can crush core material and result in delamination. Water intrusion exacerbates the problem and telltale cracks usually appear (see, “Curing Core Problems,” at right). If left unattended, catastrophic failures can occur, such as a genoa track actually being torn from the deck.

Repairs can be complicated because the failure was actually more a symptom than the cause of the problem, but its not always seen that way. The real villain is often a poorly engineered structure and returning the damaged area to “good as new” condition isn’t good enough in these cases. The correct fix must include additional reinforcement to the area in order to prevent the same problem from reoccurring.

With metal boats, extra strength is generated by using thicker plates or welding in more longitudinal and transverse supports. The same principal applies to FRP construction, and naval architects have reference tables that relate the size of a span or unsupported panel and what its core and skins must be composed of in order to handle the anticipated loads. Equally important is the fatigue cycle or rate of flex that the structure will endure and the time frame in which the object is to remain viable. These same designers also build in a safety margin that increases the strength of the structure by a factor of two, three, or more depending upon how rugged the vessels use will be or how dangerous its working environment might become. Naturally, the average production boat made for the inshore weekend sailor or powerboater will be less ruggedly built than a Southern Ocean expeditionary vessel.

Automakers have reverse-engineered product viability and attempted to come up with a pattern of uniform systems decay. Ideally, the life span of all ancillary components, from exhaust system to upholstery, match the longevity of the engine and drive train.

Fortunately, most boat designers and boat builders havent fully embraced the concept of intentional product obsolescence. However, cost-cutting and the age-old art of putting lipstick on a pig are a very real part of the industry. Hence, savvy boat buyers know that they need to look through the bilge and crawl into tight spots with a bright flashlight to see the hidden parts of the hull and deck laminates, and hunt for good and bad indications of the builders best kept secrets (See “Core Check” ).

So whats the bottom line? Even with a generous safety factor, things can still go wrong at the factory. High-tech materials and construction methods do not necessarily solve quality control problems. Ultimately, hiring a marine surveyor who is skilled in FRP construction can be as wise a choice for new boat buyers as it is for those hunting for a 20-year-old bargain boat.

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at


  1. A common failure in foam sandwich laminates is due to air entrapment. Balsa sandwich does not have this problem as it uses individual blocks which allows the air to escape (but balsa suffers from rot if it gets wet). I used to build Tornadoes and the air entrapment problem was easily solved by drilling the foam sheets 3/16″ diameter on 4″ centres. A pair of Tornado hulls were built with clear gelcoat, one hull had drilled foam the other had undrilled. After vacuuming large areas of air entrapment were visible on the undrilled foam hull and not a single bubble in the drilled foam hull. This test was done in 1971 and still is not being employed by most builders. Also, at all points of high stress the foam was tapered to zero and extra glass layers added. The Tornado is one of the most demanding hull structures having to take high stresses but keeping to class minimum weight. The fact that this was achieved shows that with good detail design and techniques a good structure can be achieved.

  2. Balsa Core is a useful lowest cost core material BUT SHOULD NEVER NEVER
    EVER be used if the structure could be accidentally punctured or cracked to allow FRESH (not salt) water to reach the core in room temperature conditions. WHY? you say Because Balsa (The Tree) was created by whoever to be self disposing. It grows self including spores of the fungus that we know as “Dry Rot” which grows fastest in the presence of luke warm (body temperature) fresh water and balsa. It then becomes a mush much like over watered Breakfast Porridge. —— An outer skin or inner skin can and does then flex and bend sufficiently to cause total failure and death of crews. I have seen so many cases and worried that other friends lost at sea were the other cases. BIG MONEY SUPPORTS THE USE OF BALSA and it is the cheapest. I am not popular but I am retired and can say USE OF BALSA CORE IN YACHTS/ PLEASURE VESSELS Should be made CRIMINAL And Suppliers who sell into that industry should be Persecuted
    Michael Anthony Pope aged 87 a shipwright and sailor, not so smart, but honest!

    • Mr. Pope, with all due respect to your age, you should be aware that there’s no such thing as dry rot. Fungal organisms which degrade cellulose fiber, such as wood, depend on moisture and not just a little bit of it, but above 20%. Dry rot is a myth that has been perpetuated in the housing industry because many times there is wood that rotted because of staying wet, which then subsequently dried because of removal of a moisture source. A lay person looks at this rotted wood which is dry and says, oh this must be dry rot. On boats, if the balsa core can be kept dry, there is no better core material considering all aspects of construction, performance and ownership.

  3. how fortunate my CSY 44 was built of solid glass. Some blisters when I purchased when I purchased it, where the price of blister repair was negotiated, a great repair yard; Yacht Maintenance in Cambridge MD and I am blister free all these years.
    Ernest M Kraus
    sv Magic Kingdom
    CSY WO #123

  4. My 1978 Mainship MK1 is a solid glass hull with a balsa cored deck. The only issues I have are in the cabin top do to the screws that held the flybridge down. I have some cutting and removing to do. I recently installed a windlass, as the boat never had one. I had to drill a new hole for the windlass. The 1 inch of Balsa was as good as the day it was installed, 52 years ago.

  5. Mr. Pope was right and Wayne Richard used an attack on a term to substitute for logic, experience, and science. “Dry rot” is simply rot. Don’t argue about terms, argue about facts. In point of scientific fact, balsa rots very easily and quickly becomes structurally unsound post water penetration. Building a balsa-cored boat is cheaper but the repairs to a wet core, almost inevitable over time, are an expensive nightmare. No boat should ever be built with a core that rots. Cores in general are a bad idea for any boat exposed to the open ocean or a large lake environment. The inferior performance in high flex areas, to sharp blows, in propensity to allow water penetration post damage, etc. keep me from even considering ownership of such a vessel. Weight in a displacement vessel is not a huge consideration and racing with sails is sort of an oxymoron.