August 2007 Issue
Storm Damaged Boats Reveal the Limits of Fiberglass Hull Construction
Storm-damaged boats offer a road map to key areas of concern in fiberglass boats. From downed rigging to sheared-off keels to gaping holes in the hull, these victims prove that fiberglass is far from indestructible and that no fiberglass boat is designed to withstand the point loading resulting from collisions, groundings, and poundings.
An inspection of storm-damaged boats reminded Practical Sailor editors that sailboats are designed, engineered, and built to handle sailing loads, and the point-loading that occurs during collisions or fetching up make all promises of ruggedness and survivability a tenuous crapshoot at best. Fiberglass hulls have many redeeming qualities, and itís these positive traits as well as their limitations that boat owners need to understand. One thingís for sure: Itís easier to become a proficient navigator, install a secure storm mooring, or transit to safe shelter before a storm, than it is to build or buy a catastrophe-proof vessel.
Contrary to the testing-oriented automotive industry, sailboat builders seldom willingly destroy boats to validate construction quality. High per-unit cost and limited R&D budgets cause destructive evaluations of new sailboats to be relegated to laminate samples, test coupons, and small subsections of new models, rather than experimenting with the sacrifice of a complete vessel. Just the thought of taking a brand-new, marketable sailboat and subjecting it to an intentional grounding, dismasting, or capsize is enough to give most yacht brokers chest pains.
Instead, baseline scantlings, laminate schedules, and building practices, such as those promulgated by the American Bureau of Ships (ABS), help to guide pre-construction engineering. Some companies voluntarily comply with the International Standards Organization (ISO) system of certification, while others use finite element analysis to digitally analyze the structural features of a new vessel. Past production results and owner feedback also help a builder define just how "strong" a new boat should be. The goal is to create a vessel for a specific type of use that is capable of withstanding the associated natural forces, and some degree of operator error. The wear and tear associated with time is also factored into the equation, as are the designerís and builderís usually unstated views of the lifespan of the boat.
Because real-world destructive testing goes beyond the scope of most builderís R&D efforts, the results of storm damage and operator error have become the next best alternative to controlled destructive testing. Insurance companies, standards bodies, and others involved in the big question of just how strong a recreational vessel should be made, look carefully at broken boats. Such after-the-fact analysis provides valid engineering feedback. A close inspection of critical failures and the factors that lead to their occurrence can explain how a boat should be built and what should be changed or maintained.
Marine surveyors and other industry experts employ a forensic approach to post-storm damage evaluations, noting easily observable as well as hidden problems that lead to or become apparent after an incident.
recently toured a yard that contained numerous storm-damaged vessels with the intention of finding areas of potential concern in FRP construction. It wasnít pretty. We saw shorn-off keels, ripped winch-base pedestals, AWOL chainplates, and bent rudder stocks. The boats came from a broad spectrum of production builders, yet itís difficult to draw definitive conclusions about how one maker deals with unusual stresses over another absent an understanding of what these boats actually endured.
Accordingly, itís impossible to state that any of the boats pictured here are "defective," because the treatment each received was catastrophic, highly variable, and well outside the conditions they were designed for.
Nonetheless, understanding failure points can give us a better picture of common vulnerabilities and how builders might address them.
The first of many lessons learned is that sailboats are designed, engineered, and built to handle sailing loads, and the point-loading associated with collisions or fetching up on a coral reef, granite ledge, or surf-swept sandy shoal make all promises of ruggedness and survivability a tenuous crapshoot at best. Just one look at what happens to FRP when its elastic deformation ability is exceeded, and critical fracturing of the hull skin results, conveys much about recreational boatingís most favored material. Its inability to survive heavy point loads is one reason why there arenít many FRP tugs, workboats, or warships. Fiberglass has many redeeming qualities, and itís these positive traits as well as their limitations that boat owners need to understand.
Fiberglass molding revolutionized recreational boatbuilding not simply because the new material was rot resistant and offered a good strength-to-weight ratio, but because it provided a means of generating smooth curves and complex shapes that could be reproduced cheaply and efficiently.
Unfortunately, the material was anything but ideal, and itís the downside to FRP construction that tends to get little press. Far from a perfect composite, glass fiber and resin have little affinity for each other. To make the bond work, a chemical coating must be added to the glass filaments and other synthetic fibers used as reinforcement in the resin matrix, whether itís polyester, vinylester, or epoxy resin. This water-soluble chemical finish allows the resin to form a covalent bond with the fiber, and when solidified, the composite forms a hard but flexible skin just a little denser than hardwood. The finish used to link glass filaments to resin is the weak link in the adhesion process. This is why when destructive loads cause an FRP structure to tear, the ragged, resin- free filaments look as if they were never properly "wet out," making it appear as if the failure was due to poor lamination. In reality, the shearing likely occurred at the critical interface between the glass filaments and their chemical finish.
Axes of Attack
In order to achieve a lighter but stiffer structure, low-density foam or other core material may be sandwiched between inner and outer FRP skins. In essence, these laminates are stiff and can have a great strength-to-weight ratio in two of the three planes often referred to with X, Y, and Z coordinates. The X and Y planes are aligned with the warp and weft of woven or stitched reinforcing fabrics, and they cope nicely with loads aligned in the same plane. The nemesis of FRP construction is the Z axis, a plane that is solely reinforced by the resinís adhesive quality rather than by reinforcing fibers aligned with the axis. Metals, on the other hand, are isotropic in nature showing equal strength in all directions. Bond failure due to stress along the Z axis is one of the most common causes of FRP deterioration, and the term "de-lamination" is part of the boatyard lexicon all over the world. Boats driven hard at sea, or inadvertently grounded, often display critical failures that are partly due to the physical nature of the material from which they are made and partly due to the actual building process.
Itís important to note that high loads migrate through a structure and focus where the stiffness is the greatest. For example, global sailing loads create energy hot spots around the keel stub, mast step, chainplates and rudder bearing points.
Most skilled marine surveyors carefully inspect these locations, especially when storm damage or grounding-related problems are an issue. Point loads associated with collisions and grounding can subject FRP structures to loads that they may be incapable of weathering. When it comes to toughness and abrasion resistance, a resin-fiber composite will prove to be less than an ideal material. Ironically, itís often an external lead ballast keel that saves a fiberglass sailboat in a serious grounding. As long as the hull pounding is mitigated by malleable lead making contact with a hard, tough planet, the chance of survival is good. Once the FRP hull skin starts taking the punishment, the odds for survival drop significantly.
Aluminum, though tougher than FRP and wood, also has a proclivity to tear eventually. Compared with wood and fiberglass, its tolerance for pounding against a hard surface is superior, but its durability is also directly linked to design scantlings. Sailboats with thicker plate and closer spaced frames do better than lighter alternatives. But, as the military and commercial sides of the marine industry have long known, steel is the only material that can give those prone to hitting things a fighting chance.
Ductility is one reason that steel resists being holed. It is known as a high-yield material, which means that thereís a wide range between the point where its elastic quality ends and where its plastic deformation ability finally ceases and a crack occurs. This property of steel is much greater than whatís found in wood, FRP, and even aluminum.
The engineering term "yield" is a measure of a materialís ability to cope with loads that exceed its "spring back" point, and defines how much permanent distortion it will go through prior to failure. This range of deformation leads to dents and caved in sections of the hull, but often keeps a boat from being holed. Ironically, in engineering terminology, steel has a greater range of plastic deformation than fiber-reinforced plastic happens to have. Unfortunately, steel is heavy, rust-prone, and expensive to keep in Bristol fashion, not to mention its unsuitability for fast production-boat building. Consequently, it remains on the fringe of recreational boating&emdash;more of the material of choice for high-latitude expeditionary cruisers than for casual inshore sailors.
Designers use computer-aided design and finite element analysis to engineer increases in baseline laminates for hull and deck areas where highly focused loads occur. For example, carbon-fiber deck beams and chainplates, molded grid structures bonded to the keel stub, and rebated cores with solid laminates can be added to stiffen and strengthen specific areas. One of the most important tricks of the trade is the art of building it in accordance to the design specs, and over the years, there have been many examples of errors between the "as designed" and "as built" aspects of boatbuilding.
Unfortunately, the material that offers immense stiffness, light weight, and high yield plus construction simplicity is jokingly referred to as "unobtainium." Chemists continue their quest for the ideal boatbuilding materials, but in the meantime, racing sailors want light, stiff sailboats, cruisers want rugged, easy-to-maintain hulls, and when either fetch up on a rocky ledge, they wish they had a stout steel boat. The right compromise continues to be debated, and each builder promotes his own solution to the challenge of structural adequacy. Building a boat to take surf-swept groundings in stride would lead to a sailboat that behaves like a boulder, and at the other end of the spectrum are race boats that simply lose their keels under the load of sailing.
For the average cruising sailor, thereís value in a reasonable length of garboard joint that allows keel loads to be dissipated through a larger portion of the hull. This can be aided with floor frames, grid structures, and a well-tapered laminate build-up near the keel stub. Cored hulls that continue all the way to a keel stub create a stress riser that does not dissipate loads as effectively as a heavier, more gradually tapered laminate.
For too long, thereís been an enduring misconception that rudders attached to the trailing edge of a keel are more protected in grounding situations and consequently suffer less damage. The truth is that these rudders reach down almost as deep as the keel itself, causing both the heel gudgeon and rudder to be quite vulnerable in a grounding situation. Shoal-draft vessels also have rudders extending nearly as deep as the keel, and they too will collide with the bottom, even in minor grounding scenarios. Ironically, deeper-draft vessels tend to have a larger offset between their max draft and the draft of a separate rudder. This offset often prevents bottom contact by the rudder during serious groundings.
Some older cut-away forefoot designs had their fore-and-aft center of gravity far enough forward to cause the vessel to rotate bow down as the tide ebbed, and the vessel would heel and rotate forward, holding the rudder out of harmís way. Centerboarders with a separate rudder face the same grounding problems affecting shoal-draft vessels and can suffer serious rudder damage in a grounding.
Keel attachment has proven to be a critical factor in surviving a heavy-weather grounding. Wave action causes the full weight of the hull to act like a jackhammer, creating massive energy transfers between the keel and hull. Lead and iron keels are well-suited to such pounding, but hull skins do not have the same tolerance for intense impact loads. In addition to the forces generated, the hulls of modern sailboats often have a near-90-degree change in shape exactly where the keel/hull interface occurs. When these modern hull shapes pound on hard bottom, the critical junction between the hull and keel becomes a focal point for stress loads. The shearing that occurred at the keel-hull interface on the Beneteau on page 24 shows the location of stress risers on a flat-bottom, fin-keel hull when subjected to pounding that is far from ordinary. Wayne Burdick, of Beneteau USA, commented that, based on his observations of other storm-damaged boats, the photographed boat was clearly the victim of a "remarkable force."
"This is, for the most part, the standard keel attachment for the majority of the industry," Burdick said.
High-stress areas such as winch base pedestals and chainplates must be well-engineered and able to take significant loads. A well-made yacht is capable of being lifted by its chainplates&emdash;the loads imposed are equal to the displacement of the vessel.
Practical Sailor believes the chainplate structures should be the strong point in the rig, able to support any load that the rig might be capable of transferring to the hull before some other element of the rig itself fails. FRP-bonded chainplates, like those that are missing from the Island Packet 44 pictured on page 25, can present a special engineering challenge in this regard. Island Packetís Bill Bolin pointed out the extreme conditions that led to the rigís demise, and that not too many hulls would have survived being crushed beneath the steel beams of a bridge and pounded stern first against rocks, the event that befell the IP44.
Similarly, winch pedestals, bow cleats, and mast partners must be heavily reinforced because these structures may be called on to keep a vessel off a surf-breaking reef or to tow it to safety.
No boat can be built to withstand the worst of a stormy pounding on an unforgiving reef, jetty, or breakwater, but every designer and builder should give serious thought as to how the garboard region of a vessel should be structured in order to endure more than just average sailing loads. Thickening panels or plate, removing core, and adding more units of FRP in this region, and bonding in or welding up a reinforcing grid are options that can make the hull-to-ballast keel connection the most rugged part of the vessel. External lead ballast can act as functional shock absorber, accepting the cutting and grinding effect on the keel and spreading the load over the entire garboard joint. A thick FRP hull skin and sealed, encapsulated ballast effectively transfer the rig and righting moment forces throughout the hull, but FRPís lack of toughness in comparison to external lead or iron make it a poor material to put in contact with a reef or rocks.
Itís easier to become a proficient navigator, install a secure storm mooring, or transit to safe shelter before a storm, than it is to build or buy a catastrophe-proof vessel. And with the busiest part of the hurricane season dead ahead, followed up by the promise of violent fall noríeasters, itís time to make sure that mooring pendants are in tiptop shape and anti-chafe gear is used wherever line contacts the rail or other hard surface.
Whenever thereís a warning of severe weather, all efforts should be made to double up lines, beef up chafing gear, and reduce your boatís windage. More and more sailboat damage is associated with unfurling headsails and the windage that they create. Stripping headsails from furling foils during the proverbial calm before the storm ups the odds of survival and helps keep your boat where it belongs. And with some luck, in one piece.