Fiberglass boats are built from a composite material known broadly as fiber-reinforced plastic, or FRP. This consists of strands of fibers embedded in a matrix of a hardened plastic resin to form a rigid material that is stronger and has better mechanical properties than either material by itself. Both the fiber strands and the resin can be of several different types.
Larger bundles of coarser strands can be grouped to form a roving, which looks like untwisted twine. These rovings can be used in chopper guns or woven into a coarse, heavy cloth called woven roving. The fiberglass strands can also be chopped into short lengths and glued into a mat, much like felt. There are other configurations as well, maybe some of which will be discussed in a later article.
Each configuration has different properties and is used in different parts of a boat and in different ways. Most fiberglass boat hulls these days are laid up using alternating layers of mat and woven roving, often beginning with two layers of mat under the exterior gelcoat to prevent the weave of the roving from “printing through” the gelcoat to be visible on the exterior.
In the early decades of composite boatbuilding, glass fiber reinforcements were the only choice. Through the miracles of modern chemistry, however, we now have several additional materials, each with different properties. Carbon fiber is used in high performance composite parts. Carbon fibers are three times stronger and four times lighter than steel. They add stiffness as well as strength to properly designed and built laminates. These characteristics can be achieved only with a careful orientation of fibers in the resin matrix. Prepregs are carbon fiber materials suspended in a partially cured resin matrix (usually epoxy). The prepreg material can be handled, cut, and precisely applied to a mold. The resulting laminate is then cured under heat and pressure to form a completed part.
Kevlar is a DuPont brand name for aramid polymers. Kevlar is widely used in personal body armor as well as reinforcements in boat hulls. Kevlar is five times as strong as steel for a given weight. It requires careful application, as it tends to float on top of wet resin, and it is difficult to sand and fair. Generally speaking, the higher tech the reinforcing material, the more precise its application needs to be to achieve its highest potential. Naval architects and boatbuilders spend a great deal of time optimizing the exact composition and configuration of hull laminations and reinforcements.
Heat affects the cure time of any resin system. The higher the temperature, the faster the cure. These resins are exothermic, meaning that they generate heat while they cure. This becomes a problem if you have a large amount of resin in a container. I have seen epoxy and polyester get dangerously hot and even boil over when too much was mixed and not applied soon enough. By the way, the amount of time the resin remains workable in a pot after mixing is referred to as pot life.
Polyester resin was one of the first resins to be widely used with fiberglass. A small amount of catalyst (typically 1 to 2 percent by volume) is added to trigger polymerization and cause the resin to harden. The resin-to-catalyst ratio is highly dependant on temperature and humidity and should be adjusted accordingly for best results.
The most common fiberglass reinforcing materials used in boatbuilding are (left to right) mat, woven roving, and cloth, all three of which come in various weights. Because woven roving is coarser and heavier than cloth, boatbuilders use it to build thickness in a hull or deck laminate. But layers of roving must be sandwiched between layers of mat to ensure uniform adhesion and prevent delamination.
The use of polyester resin began with attempts to develop alternatives for aluminum during World War II. After the resin was perfected in the late 1950s, fiberglass boatbuilding exploded. Early fiberglass boats were touted as being impervious to water and maintenance-free. We know better now, but perfecting this resin fueled the conversion of recreational boatbuilding from wood to plastic.
As time has proven, boats built from polyester do sometimes absorb water and develop osmotic blisters. Early resins were less susceptible to the problem, but resin reformulations in the late 1970s and 1980s produced hulls that were more likely to blister. Blistering problems were worsened by less-than-adequate quality control in some of the lamination processes. Polyester resins usually come with a styrene monomer added to make the resin less viscous and easier to handle, and to aid in hardening. This styrene was the cause of the familiar “polyester” smell in most mold shops in the past. The fumes
are now regulated as volatile organic compounds (VOCs), which has forced boatbuilders to add safety and antipollution equipment and develop new molding methods that greatly reduce the pungent odor. Polyester resin, by itself, is air-inhibited: it won’t cure in the presence of air, and the surface will stay tacky. That is a useful characteristic when you are laying up a hull with successive applications of resin and fiberglass reinforcements, because each layer will chemically bond to the next. Such resins are identified as laminating resins.
A wax compound can be added to the resin to create a finishing resin. While curing, the wax migrates to the surface of the resin and seals it from contact with air, allowing the surface to cure hard. (This can also be accomplished by spraying on a mold release compound such as polyvinyl alcohol, or PVA.) Finishing resin is used for the final laminations. Whether building a new boat or making repairs, it’s important to select the right resin. When building up successive laminations, you don’t want to use a finishing resin, because the wax will prevent additional layers from adhering and must be sanded off before the next layer can be applied. On the other hand, for a final layer, you do want to use a finishing resin to ensure a complete cure.
The vinylester molecule is chemically similar to polyester but has fewer ester groups. Since ester groups are the resin component most subject to water degradation and osmotic blistering, vinylester resins are more resistant to blistering. Many boatbuilders now use vinylesters, at least in the outer layers of hull laminations. Boats built with these resins often offer extended hull warranties against blistering. The downside is that vinylester resins cost more than polyester.
Epoxy differs from ester-based resins in that it requires a hardener as opposed to a catalyst. The hardener is added to the epoxy resin in a critical, defined ratio, typically ranging from 5:1 (resin to hardener) to 1:1, depending upon the manufacturer. (In contrast, just a few drops of catalyst suffice to make a pot of polyester resin “kick.” Although adding extra catalyst will accelerate the cure of polyester, adding extra hardener to epoxy will not accelerate the cure but will simply weaken the resin structure.)
When mixed and applied correctly, epoxy is much stronger and more water resistant than either of the ester type of resins. Epoxy’s water resistance is so good, in fact, that epoxy can be used as a barrier coat over polyester, and this is often done as a repair in cases of extreme blistering. Epoxy is far more costly than the ester resins, however, so it is not often used as the primary resin in composite boats, with the exception of very high-end racing sailboats where light weight is critical and cost is less of an issue.
Since both components in epoxy—resin and hardener—completely react, there are very few VOCs to worry about. Unfortunately, some people become sensitized to epoxy resin and break out in hives or experience other difficulties in the presence of uncured resin. For that reason, gloves, masks, and other protective gear are recommended when using epoxies. Resin bonds between two composite parts are referred to as primary bonds when there is a chemical reaction between the parts and the resin, so that the two parts become essentially one. A secondary bond exists if the resin simply serves as an adhesive that mechanically connects the two parts. During construction, primary bonds are generally preferable, since they are stronger; but to accomplish this, the construction schedule must be carefully planned so that subsequent parts or laminations go together before the previous ones are fully cured. Bonds between parts in repairs are necessarily secondary bonds, and epoxy should be used whether the substrate is epoxy or polyester, because epoxy forms a much stronger secondary bond with either material.
Epoxy has poor resistance to ultraviolet (UV) rays, and sunlight causes it to break down fairly quickly. All epoxy-coated surfaces that are exposed to sunlight must be protected with paint or with a varnish that contains UV-blocking compounds.
FIBERS
“Glass-reinforced plastic,” or GRP, is a slightly more specific term than FRP, and it can be accurately applied to the majority of boats, because fiberglass is by far the most common reinforcing fiber in FRP. Fiberglass is real glass that has been melted and forced under pressure through fine holes in a die. After exiting the die, the glass fibers can be stretched or blasted with steam or air to make them even finer, and the resultant strands are then coated with a sizing and wound onto bobbins. Fiberglass for boatbuilding comes in a variety of constructions. Tight bundles of fine strands can be woven into fiberglass cloth, which formerly (but not so much any longer) found frequent use as the finished interior surface of a molded hull or component. It is still used in small boats, where its high tensile and flexural strength works well in conjunction with lightweight mat.Larger bundles of coarser strands can be grouped to form a roving, which looks like untwisted twine. These rovings can be used in chopper guns or woven into a coarse, heavy cloth called woven roving. The fiberglass strands can also be chopped into short lengths and glued into a mat, much like felt. There are other configurations as well, maybe some of which will be discussed in a later article.
Each configuration has different properties and is used in different parts of a boat and in different ways. Most fiberglass boat hulls these days are laid up using alternating layers of mat and woven roving, often beginning with two layers of mat under the exterior gelcoat to prevent the weave of the roving from “printing through” the gelcoat to be visible on the exterior.
In the early decades of composite boatbuilding, glass fiber reinforcements were the only choice. Through the miracles of modern chemistry, however, we now have several additional materials, each with different properties. Carbon fiber is used in high performance composite parts. Carbon fibers are three times stronger and four times lighter than steel. They add stiffness as well as strength to properly designed and built laminates. These characteristics can be achieved only with a careful orientation of fibers in the resin matrix. Prepregs are carbon fiber materials suspended in a partially cured resin matrix (usually epoxy). The prepreg material can be handled, cut, and precisely applied to a mold. The resulting laminate is then cured under heat and pressure to form a completed part.
Kevlar is a DuPont brand name for aramid polymers. Kevlar is widely used in personal body armor as well as reinforcements in boat hulls. Kevlar is five times as strong as steel for a given weight. It requires careful application, as it tends to float on top of wet resin, and it is difficult to sand and fair. Generally speaking, the higher tech the reinforcing material, the more precise its application needs to be to achieve its highest potential. Naval architects and boatbuilders spend a great deal of time optimizing the exact composition and configuration of hull laminations and reinforcements.
RESINS
Reinforcements are only half the picture. The other half is the resin, which, in boatbuilding, is either polyester, vinylester, or epoxy. These are applied as a thick liquid that has been catalyzed or mixed with a hardener, either of which turns the resin hard so that it encapsulates the reinforcing materials in a rigid matrix. Generally speaking, the reinforcements contribute tensile strength while the resins contribute stiffness. Together these components are more than the sum of their parts, and they are the basis of modern fiberglass boatbuilding.Heat affects the cure time of any resin system. The higher the temperature, the faster the cure. These resins are exothermic, meaning that they generate heat while they cure. This becomes a problem if you have a large amount of resin in a container. I have seen epoxy and polyester get dangerously hot and even boil over when too much was mixed and not applied soon enough. By the way, the amount of time the resin remains workable in a pot after mixing is referred to as pot life.
Polyester resin was one of the first resins to be widely used with fiberglass. A small amount of catalyst (typically 1 to 2 percent by volume) is added to trigger polymerization and cause the resin to harden. The resin-to-catalyst ratio is highly dependant on temperature and humidity and should be adjusted accordingly for best results.
The most common fiberglass reinforcing materials used in boatbuilding are (left to right) mat, woven roving, and cloth, all three of which come in various weights. Because woven roving is coarser and heavier than cloth, boatbuilders use it to build thickness in a hull or deck laminate. But layers of roving must be sandwiched between layers of mat to ensure uniform adhesion and prevent delamination.
The use of polyester resin began with attempts to develop alternatives for aluminum during World War II. After the resin was perfected in the late 1950s, fiberglass boatbuilding exploded. Early fiberglass boats were touted as being impervious to water and maintenance-free. We know better now, but perfecting this resin fueled the conversion of recreational boatbuilding from wood to plastic.
As time has proven, boats built from polyester do sometimes absorb water and develop osmotic blisters. Early resins were less susceptible to the problem, but resin reformulations in the late 1970s and 1980s produced hulls that were more likely to blister. Blistering problems were worsened by less-than-adequate quality control in some of the lamination processes. Polyester resins usually come with a styrene monomer added to make the resin less viscous and easier to handle, and to aid in hardening. This styrene was the cause of the familiar “polyester” smell in most mold shops in the past. The fumes
are now regulated as volatile organic compounds (VOCs), which has forced boatbuilders to add safety and antipollution equipment and develop new molding methods that greatly reduce the pungent odor. Polyester resin, by itself, is air-inhibited: it won’t cure in the presence of air, and the surface will stay tacky. That is a useful characteristic when you are laying up a hull with successive applications of resin and fiberglass reinforcements, because each layer will chemically bond to the next. Such resins are identified as laminating resins.
A wax compound can be added to the resin to create a finishing resin. While curing, the wax migrates to the surface of the resin and seals it from contact with air, allowing the surface to cure hard. (This can also be accomplished by spraying on a mold release compound such as polyvinyl alcohol, or PVA.) Finishing resin is used for the final laminations. Whether building a new boat or making repairs, it’s important to select the right resin. When building up successive laminations, you don’t want to use a finishing resin, because the wax will prevent additional layers from adhering and must be sanded off before the next layer can be applied. On the other hand, for a final layer, you do want to use a finishing resin to ensure a complete cure.
The vinylester molecule is chemically similar to polyester but has fewer ester groups. Since ester groups are the resin component most subject to water degradation and osmotic blistering, vinylester resins are more resistant to blistering. Many boatbuilders now use vinylesters, at least in the outer layers of hull laminations. Boats built with these resins often offer extended hull warranties against blistering. The downside is that vinylester resins cost more than polyester.
Epoxy differs from ester-based resins in that it requires a hardener as opposed to a catalyst. The hardener is added to the epoxy resin in a critical, defined ratio, typically ranging from 5:1 (resin to hardener) to 1:1, depending upon the manufacturer. (In contrast, just a few drops of catalyst suffice to make a pot of polyester resin “kick.” Although adding extra catalyst will accelerate the cure of polyester, adding extra hardener to epoxy will not accelerate the cure but will simply weaken the resin structure.)
When mixed and applied correctly, epoxy is much stronger and more water resistant than either of the ester type of resins. Epoxy’s water resistance is so good, in fact, that epoxy can be used as a barrier coat over polyester, and this is often done as a repair in cases of extreme blistering. Epoxy is far more costly than the ester resins, however, so it is not often used as the primary resin in composite boats, with the exception of very high-end racing sailboats where light weight is critical and cost is less of an issue.
Since both components in epoxy—resin and hardener—completely react, there are very few VOCs to worry about. Unfortunately, some people become sensitized to epoxy resin and break out in hives or experience other difficulties in the presence of uncured resin. For that reason, gloves, masks, and other protective gear are recommended when using epoxies. Resin bonds between two composite parts are referred to as primary bonds when there is a chemical reaction between the parts and the resin, so that the two parts become essentially one. A secondary bond exists if the resin simply serves as an adhesive that mechanically connects the two parts. During construction, primary bonds are generally preferable, since they are stronger; but to accomplish this, the construction schedule must be carefully planned so that subsequent parts or laminations go together before the previous ones are fully cured. Bonds between parts in repairs are necessarily secondary bonds, and epoxy should be used whether the substrate is epoxy or polyester, because epoxy forms a much stronger secondary bond with either material.
Epoxy has poor resistance to ultraviolet (UV) rays, and sunlight causes it to break down fairly quickly. All epoxy-coated surfaces that are exposed to sunlight must be protected with paint or with a varnish that contains UV-blocking compounds.
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