Rethinking Sailboat Structure

New designs, construction techniques are reshaping our hulls.


When it comes to describing a sailboats most valuable attribute, its surprising how varied opinions can be. Staying afloat should be our first priority, and although you seldom read or hear much about it at boat shows, the structural elements that hold a sailboat together are an all-important consideration.

Ironically, the evolution of boatbuilding techniques engendered by the fiberglass reinforced plastic (FRP) revolution has brought us closer to the primitive dugout canoe. Like the dugout canoe, the contemporary fiberglass sailboat is a skin-stressed structure: Most of the structural loads are spread and dissipated in the same material that works to keep water out and provide buoyancy. Building boats with a heavy timber framework and adding planks to create a watertight skin have gone the way of cotton sails.

keel loads

Photo by Jonathan Neeves

However, wood is still a viable material, especially when used in veneers or sheets and bonded with epoxy glue. This approach allows a builder to easily bend batten-like veneers over a male jig to cold-mold a seamless, monocoque hull. Multi-chine metal boats are also built over a male jig that often becomes part of the framework, and plating is attached with an arc welder rather than epoxy resin. Round-bilge development made of aluminum or steel is a more challenging process, as shaping compound curves in flat metal requires specialized jigs and shape formers, plus highly skilled metal workers.

The bottom line is that well over 90 percent of recreational sailboats built today are fiberglass, built in female molds; these boats will be the main focus of this discussion of sailboat structure.

To better recognize how sailboat structures vary, we need to understand what is meant by scan’tlings. This traditional boatbuilding term originally referred to the thickness of planks and the spacing and cross-section of ribs, frames, and other key timbers used to reinforce the hull. Higher scan’tlings correlated with stronger and heavier hulls and decks.

From the earliest days of boatbuilding, there were appropriate scan’tlings for inshore light-duty craft and higher scan’tlings for ocean-going vessels enduring more arduous conditions. This habit of designing and building to the demand of a vessels mission continues today, and its no surprise that an around-the-world raceboat, which must endure bone-jarring slamming loads, incorporates structural details that are alien to run-of-the-mill sailboats at local boat shows. A crew preparing to wander down the Intracoastal Waterway has no need for a hull and deck thats fortified to endure the torment of the Roaring Forties, but that’s no excuse for shortcuts in critical load-bearing areas.

The Design Process

When approached by a builder with an idea for a new recreational sailboat, naval architects prefer to have a clear picture of the boats mission spelled out in the specifications (specs). Vague or wide-ranging specs understandably make a naval architect nervous. The problems of an ill-defined or vaguely defined mission are compounded when a boat that was intended as a superior coastal cruiser is touted by over-enthusiastic brokers as a go-anywhere passagemaker. This sort of mission creep can lead to serious trouble for the crew.

Working under the auspices of the International Organization of Standards (ISO), European boatbuilders have developed a Small Craft Directive that defines four categories of usage based upon specific structure and stability attributes: ocean, offshore, inshore, and sheltered waters. The consensus among most naval architects is that this is a good approach, but there are some questions over the efficacy of vessels that just squeak into the bottom-end of the ocean category-Category A.

Ocean-approved Category A boats are defined as designed for extended voyages where conditions may exceed wind force 8 (Beaufort scale; fresh gale, 34- to 40-knot winds) and significant wave heights of 4 meters (approximately 13 feet) and above, but excluding abnormal conditions.In our view, this abnormal label is too vague, rife with waffle words that leave the consumer wondering why force winds and 4-meter seas were spelled out rather than putting an upper limit on the operational range as exists with the three other categories.

Interestingly, in the original draft of the guidelines, there was a reference to force 10 and under conditions, but this was watered down in later drafts. Now, terms like avoiding abnormal conditions leaves a consumer wondering if a severe thunderstorm, gale, extra-tropical storm, or tropical storm, fall under the abnormal label, or whether its hurricanes and the worst of extra-tropical storms that deserve such billing. Naturally, a hurricane would be regarded as abnormal, but what about a severe thunderstorm?

The EU did a much better job specifying missions with empirical references in Categories B, C, and D. So why not Category A, arguably the category in which hull structure is most critical?

Some say theres nothing wrong with waffling on the structural mission for Category A, especially since most coastal cruisers arent launching off wave faces at 20 knots and dropping into troughs. However, when youre caught offshore in a nasty gale or storm, and can hear and feel the stress and strain wracking the hull, knowing that your boat barely makes it over a vaguely defined threshold for an ocean-going boat offers little reassurance. When the chips are down, staying afloat is the number one priority, and the boats structure becomes paramount.

An Absence of Data

One of the reasons we assume that our boats-even those built to deliberately vague standards-are quite bulletproof is the encouraging statistics revealed by loosely compiled data. New boats sailed in inshore or coastal waters have a very good track record when it comes to tallying up an equal number of departures and arrivals. The cheerful brokers claim that coastal cruiser X has seen hundreds of thousands of miles at sea is true-to a point. Look closer, and youll find that those so-called sea miles include hour upon hour of summer bay sails.

For better or worse, accidents at sea tend to grab headlines-at least those in which lives are in peril. We can all recall news stories about a one-design keelboat swamping, a crew colliding with a reef, or a vessel run down at sea. More often than not, though, these offer an example of operator error rather than a structural shortfall. And if there is a structural issue, it rarely gets publicized.

There are other instances, however, in which a structural failure seems obvious (at least to the knowledgeable sailor), and apparently induced by nothing but the sea and wind-the keel of a racing sailboat snaps off, the oversized window of a cruising boat gives way to a boarding sea.

When these types of incidents occur, justifiable outrage ricochets through the blogs, and for a short time, theres some serious thought given to a wide range of structural changes. So far, however, regulatory bodies in the United States, most noticeably the U.S. Coast Guard, American Bureau of Shipping, and voluntary agencies such as the American Boat and Yacht Council (ABYC) have side-stepped the structural issue. As a result, recreational boat builders in the U.S. are left to adopt their own version of appropriate scan’tlings. When failures like a spate of broken rudders occur, only then are changes made.

In the end, U.S. sailors must often depend on the courts to determine whether a builder or design is at fault. Sadly, many court cases involving major structural failures are resolved through out-of-court settlements that conclude with a gag order. Gag orders keep the problem and potential solutions from becoming part of the public record for a specified period of time. In the realm of production boats, this approach ignores owners of sisterships that might suffer the same defect. Recently, the U.S. Coast Guard has begun taking a closer look how the marine version of an automotive recall should be handled.

One of the greatest impediments to more rigorous structural standards is the lack of data. Despite the headline-grabbing nature of maritime accidents, many structural failures involve no loss of life, and are resolved in the boatyard rather than the courts. When it comes to older boats and more off-the-beaten-path voyaging, the data gets even murkier. Even the Coast Guard is not tracking incidents involving recreational vessels voyaging beyond territorial waters.

Then, there are the tragic incidents in which the details are lost at sea. A boat and crew that set sail on a lengthy passage never makes landfall, leaving more questions than answers in their wake. It is up to others to speculate whether the vessel was run down, consumed by fire, torn apart on a surf-swept reef, or succumbed to a structural flaw that left it open to the sea. With no advocates surviving the incident, theres no feedback as to what actually went wrong.

Standard Industry Practice

Although less regulated than European boatbuilding, U.S. boatbuilding still has a pretty good track record when it comes to ensuring structural adequacy. However, the paradigm for building fiberglass boats has shifted over time, presenting new challenges.

The modern era in boatbuilding abides by a lighter-is-faster theme, presenting a two-fold challenge. First, there is cost: Legitimate lightweight boats are simply much more expensive to build. Second, there are technical challenges: Creating a structure thats just as stiff, strong, and seaworthy as a structure that is 30-percent heavier requires specialized equipment and individuals skilled in fabrication with high-modulus materials.

These efforts make the most sense when it comes to racing boats or high-end performance cruisers. However, when a plant that has historically focused on high-volume production for the mainstream market tries to adopt the light-boat approach, the result is too often a lighter boat that misses out on the stronger element of high-tech construction. By cutting down on the materials and technical skills that go into building a lightweight structure, the safety margin shrinks, and the end result is a fragile product that isn’t cut out for the sea-an approach most builders and boat buyers would rather avoid.

The 1960s ushered in a Wild West revolution in materials combined with a cautious East Coast approach to construction. On one side was a full-speed transition to molded FRP boatbuilding steered by some gifted, seat-of-the pants engineering. On the other, there was an allegiance to traditional boatbuilding techniques. Many of the boats from this era have stood the test of time because of their solid FRP hulls, built with hand layups of alternating layers of 24-ounce woven roving and 1.5-ounce mat.

Built before the widespread use of the chopper-gun (an apparatus designed to apply a stream of resin and fiberglass filaments), the boats of this period involved a lot of hands-on elbow grease. Serrated rollers were used to force air bubbles out from between the layers of laminate to reduce the number of voids. A variation of this hand-layup process is still used at many builders today. In the 60s, with oil at $15 per barrel and polyester resin costing only a couple of dollars per gallon, thicker hull skins became the answer to most engineering challenges.

Increasing skin thickness was used to deliver both strength and stiffness throughout the boat. During this period, sailboat buyers had yet to latch onto lighter, faster boats, and builders had yet to equate less material with more profit.

In the end, what these overbuilt arks delivered was longevity. Its no surprise to see a nicely refit 40- to 50-year-old, solid FRP sailboat still going strong.

As for speed, this summer, the Newport to Bermuda Races prestigious St. Davids Lighthouse Trophy went to the crew of Actaea-a well-maintained, 42-year-old Hinckley Bermuda 40. Some allude to her carbon spar and other go-fast modifications, but the bottom line is that the hull and deck are original, and after 40 plus years of tacks and jibes, shes still able to handle the increased rig loads imposed by modern, low-stretch sails and cordage.

Varying Density

In past decades, better engineering has led to the use of different-density material in different regions of a sailboats hull and deck. In high-stressed areas, such as where the keel joins the hull and where chainplates secure standing rigging, all lower-density core material is often removed, and a thicker solid laminate prevails. This is also true for hull-to-deck joints and where the rudderstock and prop shaft exit the hull. Elsewhere, the core material, and number and type of units (layers) of reinforcement depend upon the loads carried by that part of the hull skin.

Computer-aided finite element analysis helps engineers determine what locations in a specific panel or area of the hull will be subject to higher loads, and how these loads will spike when the boat is sailing or the panel is impacted by a point-load such as a rock or a reef. Factors such as righting and heeling moments significantly influence this data. And as time goes on, the cycle loads that continually pass through the structure, slowly but surely break down the resin bonds. The less bending a hull and deck endure, the slower the deterioration of the laminate. Keeping water, the universal solvent, out of the structure is vitally important. Freezing winter temperatures can further exacerbate inter-laminate shear issues: Balsa core will rot when wet, and heat and flex can damage foam. When a sandwich structure fails over a large area, associated repairs can be very costly and time consuming.

The hull and deck are the meat and potatoes of a sailboat, and when something is wrong with the engineering or build process of these structures, its a big problem that only gets worse. In many cases, the worst problems are localized to certain areas of the hull or deck, areas that might easily escape the notice of an untrained eye.

All it takes is a basic understanding of where forces are focused on the hull and deck of a sailboat to get an idea of where problems often arise and what to inspect. For example, imagine what goes on as a keel silently hangs day after day from a buoyant hull, putting the nearby structure in tension for years or even decades with what might be equivalent to the weight of a submerged pickup truck. This tension alone starts to flex and torque as the vessel heels and begins pounding to windward.

Next, think about the rig loads that induce this heeling moment, and youll quickly come to the mast step, chain plates, and points around the deck that support winches, rope clutches, and other highly stressed hardware. With a little head-scratching, and follow-the-load-path logic, your own mental image of potential trouble spots will begin to mirror the graphic image generated by finite element analysis software. And what youre after when you look closely at the hot spots are signs of cracking and crazing on the FRP skin around these high-stress areas.

Deflection of the coachroof under a deck-stepped mast or torn tabbing on a chain plate supporting a bulkhead need attention-as does a rudder blade showing signs of horizontal cracks in the skin or rust weeping. In short, regular close inspections of highly loaded points on the hull and deck can alert an owner to problems that will only grow more serious.

Bottom Line

New boatbuilding techniques such as resin infusion, vacuum bagging, and other closed mold processes lead to better laminates. They reduce void content and can increase the ratio of fiber to resin, resulting in a higher strength-to-weight ratio. Make sure that this improvement in laminating a hull is not offset by a heavy compliment of resin-rich, low-fiber-content hull liners, pans, and other non-structural components.

It is encouraging to see custom cruisers and race boats that show off their inside hull skin by carefully finishing the molded surface, instead of trying to hide everything with heavy, chop-strand-sprayed liners and pans that contribute little to keeping the water on the outside of the hull, but add a lot of weight.

If youre buying a new boat, think twice before buying if its the first unit and the largest model the builder has ever made. More often than not, this is less familiar territory and reflects a scaling up of what was been done on smaller models. Every builder has a mid-sized model and a boat size that they have a long track record of building. Buying a boat that has been in production long enough to have all the bugs worked out, delivers value that a customer will come to appreciate.

Builders who have been around for awhile have a good grasp on why new boat problems are a lose-lose situation, and they do all they can to avoid them. With the advent of better engineering approaches and manufacturing techniques, they provide products with adequate, if not exceptional, structure. The goal is to deliver a structure that is free of defects for at least 10 years.

A new builder may have a gifted approach to boatbuilding-or not-but the consumer doesn’t have hundreds of 10-year-old-plus boats in the used market to evaluate a builders expertise. When buying a new boat, especially one from a new builder, it makes sense to have a new boat survey done prior to final acceptance, and perhaps even enlist a consultant or owners representative to help evaluate procedures during key phases in the building process. Past Practical Sailor contributor Steve DAntonio is one of a few experts who offer these services.

Used-boat buyers need a marine surveyor who is very familiar with construction and repair issues. Price is an important variable, but in most used boat sales, it is not as important as the condition of the sailboat. Vessel structure often trumps the price tag.

Major structural flaws fall into several categories. One of the most critical is when the hull or deck laminate is so compromised that it requires the entire structure to be rebuilt. Core delamination that spans much of the sandwich structure can lead to repair costs that outstrip the value of the boat. Single-point structural shortfalls, like a keel stub or sump that is flexing too much and conveys a risk of complete failure is often able to be remedied in a cost-effective manner. The combination of available access and the localized context of such a keel problem often make it a justifiable repair.

Time and deterioration are directly correlated, but the better a boat was originally built, the more gracefully it shoulders the accumulation of both miles and years.

Rethinking Sailboat Structure
Pros & Cons: Core
Rethinking Sailboat Structure
Dealing with Spar Loads, Rudders, and Chainplates
Rethinking Sailboat Structure
Darrell Nicholson is Director of Belvoir Media Group's marine division and the editor of Practical Sailor. A lifelong thalassophile, he grew up sailing everything from El Toro dinghies to classic Morgans on Miami's Biscayne Bay. In the early 90s, he left a newspaper job to sail an old gaff-rigged ketch across the Pacific and has been writing about boats and the sea ever since. 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.


  1. Great article! I have been involved in designing high speed ship stabilization equipment including foundations like the one shown for 29 years, though mostly metals but also a lot of work with various composites. More recently I work at a local college and repair a lot small boats. Watch vacuum resin systems especially polyester, sometimes they use the resin to create a honeycomb that winds up plenty strong but brittle against impacts. I grind away the bad material until I find good, usually much further than I’d like on those repairs.

  2. For a serious cruising boat, I prefer a solid glass hulls and a properly done molded in Keel and ballast.
    For other types of boats, sandwich properly done is great. Like: serious performance boat, multi’s, coastal boats, fine. I know that the state of the art is improving on sandwich. But I probable can’t afford a state of the art vessel.

  3. Well written presentation of the challenges and issues surrounding the yacht design process. It would be helpful if boat buyers were more aware of the decisions made for designing and building boats so they find the best boat to fit heir use. There are many well built boats that are not used for their intended purpose that may decrease the owner’s enjoyment of sailing and bring unnecessary operating costs.


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