Ins and Outs of Marine Fiberglass

Ever wonder how your boat was built? Probably not because we depend on the manufacturer to build a safe, quality boat that can manage whatever it’s supposed to manage. Some boats are manufactured with steel and aluminum, and wood boats are still manufactured, but today the majority of boats larger than a small fishing boat have a lot of fiberglass from top to bottom. Some are all fiberglass while others are made with a mixture of wood and fiberglass laminate.

Fiberglass is a man-made fiber made of reinforced plastic materials consisting of glass fibers embedded in a woven material. It may be referred to as glass reinforced plastic (GRP) or fiberglass reinforced plastic (FRP). The terms are a bit confusing because glass fiber in and of itself is sometimes called fiberglass, so the fiberglass composite is usually called fiberglass reinforced plastic (FRP).

In the marine industry, fiberglass is used for all types of boats and yachts, surfboards, fishing dinghies, dock boxes and various other marine components. Glass fibers have many desirable properties, like high electrical insulation, low susceptibility to moisture and a variety of high mechanical properties.

Though there may be some variations, generally FRP is made by setting fiber material in a resin which serves as the binding substance. This is why it’s called fiberglass composite. Fiberglass composite refers to the fact it combines fiber (the reinforcement material (which in this case is glass) and resin (a matrix). There is also fiberglass laminate which is thin sheets of fiberglass used to sandwich a core material (more on that later).

Fiberglass cloth is a woven material that is exactly what it sounds like. Think of woven material used in clothes, and like clothing material, there are a variety of weave patterns and fiber weights which influence stiffness. The cloth is made with continuous thin strands and weighs anywhere from 4-15 ounces per square yard. Heavier weaves, called roving, are made with untwisted fiber yarns, and can weigh as much as 48 ounces or more per square yard.

There are different types of fibers and fiber arrangements used in boat construction and for marine fiberglass repairs. The glass fiber can be woven into a fabric, flattened into a sheet, or randomly arranged. The three most common fiber cloths are:

• E-glass – E-glass is made of melted plastic that was spun into fine fibers that are woven into fiberglass cloth. It is one of the most common types of fiberglass cloth used in boat building and is woven into lightweight and heavier weight cloth. The lightweight fiberglass cloth provides a smooth finish and, when combined with a resin, will produce excellent waterproofing. The heavier (roving) e-glass fiberglass cloth offers more rigidity and strength.

• S-glass – S-glass is considered high performance fiberglass. The fiberglass cloth will have an approximately 25-30% (some even reach 40%) increase in tensile strength compared to E-glass. Tensile strength is the amount of stress the plastic material can handle before breaking. It also has up to 40% higher flexural strength which is the bend strength. The cloth consists of the S-glass fibers and a resin. S-glass is used when strength is needed but boat weight must be kept to a minimum.

• Mat - (or chopped-strand mat) is made of E-glass, with very short (2-3 inch) fibers placed randomly. The fibers are held together with a resin soluble binder, but this type of cloth requires a lot of resin. It’s relatively waterproof and can be sanded smooth. Mat is used mostly for getting thickness in fiberglass layups.

Of course, there are other options. They include unidirectional fibers (fibers run in one direction and are held together by a low number of single fibers); bi-axial fibers (layers of unidirectional cloth); tri-axial fibers (fibers are oriented in three directions); and veil mats (continuous strand fibers looped randomly).

It should be noted that Kevlar is another fiberglass used in boat construction. There are two types of Kevlar, and it’s Kevlar 49 that is used as reinforcement in plastic composites. It boasts the highest tensile strength among commercial fibers and is twice as strong as E-glass and much lighter in weight. Kevlar is found in a variety of boats, and especially in race and sail boats. However, it’s being used less and less in boat building as carbon fiber (discussed in another section) becomes more affordable. Carbon fiber offers many more advantages.

Types of Resins Used in Fiberglass

The resin is the second component of the fiberglass composite and serves as the matrix. There are three main types of fiberglass resin used to hold the fiberglass strands together. Most service providers know how to match the strengths of the reinforcing material (i.e. E-glass or S-glass) to the resin.

• Polyester resin – Commonly used in the boat manufacturing industry, it is affordable and has good adhesive quality. It’s basically a solution of a polyester (polyol with dibasic acid and a hardener) with the addition of styrene to reduce viscosity and make it easier to handle. There are two types.

  1. Orthophthalic polyester resin - Right now this is the polyester sold most often. It is a tolerant resin during lay-up (see boat construction) and can handle temperature and catalyst variation. It’s easy to use for fiberglass repairs too.

  2. Isophthalic polyester resin- This type of resin is quickly catching up in popularity because…well…it’s just better in all ways. Isophthalic resin offers more water resistance, more abrasion resistance, higher strength, better UV resistance, higher impact resistance and higher flex performance. It’s good for use in molds and, due to its higher water resistance, isophthalic polyester resin is good for underwater repairs.

• Vinyl Ester (or vinylester) resin – This resin is basically a polyester resin that is strengthened with epoxy molecules and contains a hardener. It’s also thinned with styrene for ease of use. You can think of Vinyl Ester as a resin that’s between polyester and epoxy resins. Vinyl ester resin is more durable and stronger than polyester resin, can bond to epoxy resin and has higher resistance to UV and water compared to polyester resin. Vinylester resin offers better resistance to vibrations too. A stronger resin has better flexural and tensile strength. One of its great features is that the high resistance to water absorption means less chance of osmotic blistering.

• Epoxy resin – This is a two-part product that consists of an epoxy resin (made through sophisticated chemistry) and a hardener. It cures to a hard, solid plastic with the hardness determined by the type of hardener you decide to mix in. You can choose a slow hardener for application during hot weather or a fast hardener during colder weather. In other words, epoxy resin needs to match the weather, so it doesn’t harden too slow or too fast.

  Epoxy is the most expensive type of resin but offers a number of advantages. It’s stronger and more flexible than polyester and vinyl ester; can withstand higher impact without cracking; has higher resistance to degradation from temperature or environmental conditions compared to polyester resin; is resistant to moisture absorption; and has lower shrinkage. It’s also great as an adhesive for covering wood with fiberglass and adheres to steel, aluminum and wood.

Where does marine gelcoat fit into all of this? A gel coat is a polyester resin that is formulated to have increased viscosity and non-sagging properties. Gelcoat is the final layer on a boat. Most of today’s gel coats use isophthalic polyester resin because, as discussed, the resin is more durable and stronger. The special formulation with thixotropic ingredients sets it apart from the standard polyester resin. Gel coat can stand on its own, whereas polyester resin must be used with fiberglass to gain structural integrity. Gelcoat comes in a variety of colors.

Gelcoat is used during the boat building process and for boat repairs. It is a durable and repairable sealant that saturates fiberglass and is also applied as a coating to give the boat a shiny finish.

Building a Boat from the Jig or Mold Up

Fiberglass boats are the most popular boat today. There’s a process called hand lay-up in which fiberglass and resin are applied to a reusable mold through the use of a hardener. Fiberglass mat is layered until the desired depth is achieved and allowed to solidify. By hand, it’s a long, tedious process that is mostly undertaken by hobbyists or specialty commercial boat builders.

Obviously, most boats are built with high-tech assistance in a manufacturing plant. Boats are either all fiberglass or fiberglass is laid over some other material like wood. Following are the most common ways fiberglass is used in boat building:

Cold-Molded Boat Building

The cold-molded process is basically a wood boat encapsulated in fiberglass. A jig is built which is a frame designed to support building a hull in a particular shape. It holds the boat in progress in alignment. Using epoxy resin, two or more layers of wood veneers are glued together to build the hull. The veneer hull is then reinforced with layers of epoxy resin and fiberglass inside and outside the hull. The hull is turned over, the jig removed and the interior hull is finished with fiberglass, stringers, under deck components, etc. Once the hull is completed, the rest of the boat is assembled and components installed – deck, cabin, and components like the steering wheel, seats, etc. This process is used mostly for building custom boats.

Fiberglass Construction

Fiberglass boat construction is used for all sizes of boats, from the small fishing boat to the gorgeous yacht. The process in very general terms is as follows:

• A female mold is the starting point
• The mold is coated with mold release wax so the hull can be pulled out of the mold without sticking (kind of like spraying non-stick coating in a cake pan)
• The mold is sprayed with a thick layer of gelcoat
• Before the gelcoat cures, a layer of catalyzed resin (polyester resin or epoxy resin) is applied
• Alternating layers of fiberglass cloth or mat, along with resin, are placed in the hull to create a strong structure
• Stringers and bulkheads are included or added
• Components that sit below the deck level are installed, i.e. fuel tank, water tank, inboard engines, plumbing, wiring, etc.
• Deck or liner is installed over the hull and secured in some manner
• Remaining components are installed, i.e. seats, steering wheel, cabinets, appliances, electronic equipment, etc.

Stringers are lateral fore-and-aft beams installed on the bottom of the hull. The keel is a stringer that is placed in the center of the bottom and runs the length of the boat. They help reduce the weight of the boat by reducing the amount of fiberglass needed since they supply structural strength. Importantly, stringers also help the boat withstand wave impact.

The bulkhead is an upright wall in the boat’s hull that provides extra support to the overhead deck, the sides of the hull and the bilges. The walls also enable creating compartments and cabin space.

Stringers and bulkheads are either made at the same time the fiberglass is laid or can be molded separately and then installed.

Fiberglass Composite Construction

The fiberglass composite boat building process involves placing composite materials between layers of fiberglass. It creates a very strong hull that is lighter than an all-fiberglass boat. Using a jig, the sheets, panels and strips of composite foam are glued together to form the hull. Layers of fiberglass and epoxy are applied, the hull is turned over, the jig removed and fiberglass laminates with core material sandwiched in between, stringers and other forms are installed for strength. The under deck components are installed, the deck is built and installed, and the above deck components are put in place.

The purpose of core materials is to reduce the weight of the boat and provide other advantages like increasing the strength. In a cored hull or cored deck, a different material is used between two thin layers of fiberglass. The cored laminates are made with different types of materials that include:

• Balsa wood
• Plastic foam (usually made with PVC but there are other options)
• Aluminum (for race boats)

Stitch and Glue Construction

There is one more type of boat construction called “stitch and glue” which is often used in build kits. In this process, the jig is not removed and remains part of the boat as structural framing. Briefly, marine grade plywood is pre-cut into pieces for the frame, hull, deck and cockpit. The pieces are assembled and then held together with fiberglass and epoxy. Thick epoxy and then fiberglass cloth are used on the joints, while interior frames and panel interiors are coated with epoxy. Finally, a layer of epoxy resin and fiberglass are applied to the exterior.

Future of Boat Construction

The industry discussion is on whether boats will eventually be mostly manufactured with all-carbon fiber as opposed to traditional fiberglass. The reason is simple – carbon fiber weighs less, adding maneuverability and improved gas economy, plus it’s stronger than fiberglass. It’s five times stronger but lighter than steel.

Carbon fiber is a polymer made with strong, thin crystalline filaments of carbon. Carbon fiber replaces the glass strands in fiberglass. Right now, it takes large, high-energy use equipment hours to heat the carbon atoms to the point where they produce the carbon filaments that strengthen the material.

You may be familiar with carbon fiber in non-marine products like golf club shafts, tennis racket frames and bicycle frames. It’s used in fighter jets, body armor, wind turbine blades, medical equipment, prosthetics, household furniture and iPhone cases. Is carbon fiber in most boat construction next?

Holding back greater use of carbon fiber is the cost which is approximately five times that of fiberglass. Another drawback at this point is that carbon fiber is more rigid which could impact the ride, plus more rigid materials are more prone to severe damage when hitting something (hopefully not another boat!). Some manufacturers are currently using fiberglass for boat construction and using carbon fiber in components like radar arches and hardtops.

The future of carbon fiber in boat manufacturing remains to be seen, but it’s something to be aware of. When the boat dealer or broker proudly mentions state-of-the-art “carbon fiber”, you can nod your head intelligently and ask how much that added to the price.

Fiberglass Tree?

There is a lot to know about fiberglass, and this discussion only covered the main points. It’s a good start though to loving your fiberglass. L. Francis Herreshoff was an eccentric who designed long, narrow, fast and graceful sailing yachts. He built his boats with wood, eschewing fiberglass as a modern ugly material. He would not be happy knowing many of his sleek designs are only made in fiberglass today.

Herreshoff said, “If God had meant for us to have fiberglass boats, he would have planted fiberglass trees.” Now there’s a thought considering fiberglass is found in 95% of the boats in existence today.