Before considering the different fibreglass boat building techniques, you will need a basic knowledge of the materials you will use to build your boat. On this subject, we are not going to get too technical other than to explain the rudiments of the materials and techniques. If you want to know more, there are many good books available and evening classes or other courses where you can enrol for hands-on experience or sound out your local fibreglass materials supplier who should be up to date with the latest materials and techniques.

Boat designers with experience in steel and aluminum will immediately notice that most fibreglass materials have lower strength and stiffness values than the metal alloys they are used to working with. Because glass fibre materials are much lighter than metals, thicker laminates can be designed so that the stiffness can match or exceed that of metal hulls.

   There are a number of types of fibre used in reinforced plastics but glass fibres are the most common because they are inexpensive to produce and have relatively good strength to weight characteristics.

   With the exception of chopped strand mat (CSM), reinforcements used in a marine glass fibre application usually utilize bundles of fibres oriented in distinct directions such as glass cloth and woven roving. Some are aligned in a single direction others multidirectional and the strength of the laminate will vary accordingly.

   There is a considerable variety of glass reinforcements but we are mainly interested in what is known as E-Glass or electrical grade glass that was originally developed for insulators, for electrical wiring, and is now used almost exclusively as the reinforcing material commonly known as fiberglass. E-glass is the most common reinforcement used in marine laminates because it is relatively inexpensive, has good strength properties and resistance to water degradation.

   Another glass fibre known as S-Glass is a structural glass typically used in higher strength applications. It has a greater tensile strength and stiffness than E-Glass and in general, demonstrates better fatigue resistance but at a considerably higher cost which means that it is limited to selected applications.

    There are other types of fibre such as carbon fibre and graphite fibre, used as reinforcement and known as Multi-axial Engineering Fabrics or just plain Engineered Fabrics which, when knitted stitched or woven into materials, include names such as Double Bias, Biaxial and Tri-axial Fabrics and Woven Fabrics and so on but these are specialised materials which probably won’t concern you. There is even an aluminised fibre used primarily for its cosmetic appearance which has a thin coating of aluminium to create a highly reflective surface but, so far, this is not used in boatbuilding.

   When you decide to build a fiberglass boat you should, primarily, be guided by the boat’s designer and the technical knowledge of your material suppliers. Don’t be confused by the vast array of materials on the market, most will never concern you. The majority of readers of this book will be concerned with building a strong, practical boat, so unless you are considering a specialised race boat, lightweight flyer or multihull, you can concentrate on E-glass and use the more traditional fibreglass boat building materials and methods.

Unlike continuous fibres, Chopped Strand Mat is literally short chopped strands sometimes described as random discontinuous fibres (about 1½” – 37mm long) and held together with a soluble resinous binder.  CSM is available in varying types and weights from ¾ ounce per square foot [225 grams per square metre] upwards however, 1½ oz. [450 g/m 2] and 2 oz. [600 g/m 2] are the weights you will most often see expressed by designers and manufacturers of boats.   In our own material lists we simply say 1½ oz. Mat [450 g/m 2] and so forth.

    CSM can be used as a “bulk builder” in a laminate where build-up is required but without great strength.  In a laminate, layers of CSM should be used between layers of woven roving as a cushion to promote a good bond and where the strength (or lack of) in the CSM, is complimented by the strength of the roving.  

The amount of resin required to impregnate CSM is approximately 2 ½ times the weight of the mat.

Supplied coiled in square boxes which are referred to as “cheeses”, this material resembles a coil of light rope and is used with a fibreglass depositor machine (chopper gun) thus the alternate name “gun roving”.  Over the years, the lower price and availability of these guns has made it worth considering their use even if you are building a single boat. You will always get your money back when the boat is finished and, for a modest outlay, save a lot of time and expense in the process.

   Using a depositor gun, the continuous roving is cut into short lengths (like CSM) and deposited by the gun, which also mixes the resin and catalyst. They all come together as they leave the gun head, and are sprayed simultaneously on to the job.   The result is a quickly applied chopped strand mat lay-up. This same gun can also used as a resin depositor only, to wet out the alternate layers of roving or fabric.  This is the procedure used for production moulding but is equally suitable for one-off male moulded boats.   If you are considering using a female mould or laying up your hull using the “Panel Construction” methods, then the chopper gun and continuous roving may be a great investment.  

   Gun laying requires an experienced operator to get a perfectly even layer of mat and resin to the job.   When building on a male mould, evenness of the application is most important, so some experience with the gun is an advantage although not difficult to learn.  

Woven roving is much like woven cloth except that it is much heavier and woven differently.   It looks much like basket weaving with heavy bundles of non twisted strands of glass fibres woven loosely at right angles so that there is relatively, a lot of space between individual bundles of strands.   These spaces allow resin to flow through and more easily wet out of the roving.   The amount of resin required to impregnate woven roving is approximately equal to its own weight.

   This material is the real meat of your fibreglass laminate and is sold in various weights per square yard or square metre.  Woven roving is available from 8 oz. per square yard [270 g/m 2] to 27 oz. per square yard [900 g/m 2], with a variety of intermediate weights.   It is supplied in a number of weave patterns such as, bi-directional, unidirectional, biaxial, triaxial, double bias and specially stitched fabrics 

Some glass fibre fabrics are available with a thin layer of mat already attached.   This makes it one “easy to install fabric” especially for hand laying as is can be applied quicker and more evenly than separate layers of the mat and roving.   You should check with your local fibreglass supplier to see which of these materials they recommend for your intended use.

Woven reinforcements generally fall into the category of cloth or woven roving. The cloths are lighter in weight and require more layers to achieve a set thickness. Their use in marine construction is usually limited to small parts and repairs or sheathing plywood, usually using epoxy resin. They are available in a variety of weights per square yard or in grams per metre.

Carbon fibre is an aramid which is an aromatic polyamide, better known by trade names such as Kevlar (DuPont) and is produced by spinning a solid fibre from solution. Applications include boat hulls, sails, bullet proof vests and aircraft parts to mention a few. The main difference between “Carbon” and “Graphite” fibres is that they have differing amounts of carbon in their make-up but, basically, they are not dissimilar so that they can be interchangeable. These fibres are not subject to stress rupture as with glass fibres and high temperature performance is exceptional. Carbon fiber offers the highest strength and stiffness of all commonly used reinforcement fibres but the major setback is their high cost. Not withstanding the cost, carbon fibre and engineered fabrics using carbon and graphite fibre, play an important role in many marine applications where certain design standards are demanded however, the price of this material would need to come a long way down the scale before we could recommend it for general purpose use in boat building.  

Balsa core is a closed cell structure that is available in sheet form for flat panel construction or in a scrim-backed block arrangement that conforms to complex curves.   This consists of small blocks of end grain balsa attached to a fine scrim netting. The flat panels can be used for bulkheads and furniture and the flexible scrim-backed core for shaped hull and deck construction.  End grain balsa has a high compressive strength, and is ideal as a core material for decks and power boat hulls. It exhibits good stiffness and bond strength however impact absorption is lower than for PVC foam and, in the case of damage, water absorption can be a problem. Best restricted to use in decks and superstructures where water penetration is not so likely to occur.

BALSA

DuraKore ™ is a product marketed by Baltek Corporation and provides the properties of an end grain balsa core material without the need of a mold.  It will form a compound shape over a set of temporary frames in the same manner as the cedar strip plank building method. It comes in planks that are made from sandwiching rigid sheets of end grain balsa between two layers of thin veneer. The sheets are then cut into planks or narrow strips, which have finger joints at each end to allow them to be scarf joined to make up the required length. The core is then covered on both sides with fibreglass to form an effective sandwich structure.  Due to water penetration, all forms of balsa when used as core materials have become discredited over the past few years so may best be avoided.

Bill Seeman of Seeman Fiberglass developed C-Flex in the nineteen seventies to help amateurs and fishermen build a cost effective one-off hull. Based in New Orleans USA his original idea was to be able to resurrect a worn out hull by inverting the boat, covering it with plastic sheeting and then attaching C-Flex and fiberglass to obtain an almost exact copy of the original hull. This then allowed all the existing engine and equipment to be reused.  Since that time we have offered C-Flex as an alternative method to build one off fiberglass boats.

   The C-Flex planking consists of rigid fiberglass rods and unsaturated strands of continuous fiberglass roving held together with a light fiberglass cloth. Like DuraKore and cedar strip planking, it will form a compound shape over a set of temporary frames.

Polyester resin is a thick viscous liquid like syrup to which a catalyst (and sometimes an accelerator) is added.   However, polyesters, like most plastics, lack the inherent strength of metals and are very brittle.   In order to improve their tensile strength and allow them some flexibility, they are often reinforced by the addition of fibres of carbon, glass, sisal, cotton or other suitable materials.  Once reinforced with glass fibre, their strength can far exceed that of steel.

   There are two basic polyester resins used in the marine industry, orthothalic and isothalic. The ortho resins were the original group of polyesters and are still in widespread use. The iso resins have better mechanical properties and show better chemical resistance. Their increased resistance to water permeation has prompted many builders to switch to this resin in marine laminates.

   Curing of polyester is accomplished by adding catalyst and accelerator (usually fixed amounts of catalyst and variable amounts of accelerator) – although most resins are now pre-accelerated. Gel times can be controlled through resin formulation to suit the climatic conditions. The gel time of a resin is the time taken after the addition of catalyst and accelerator for it to set to a jelly like state.   Most modern resins are pre-accelerated, and therefore, we only have to add catalyst except when using pigment or fillers which may require an additional quantity of accelerator.

   The pot life of resin is the time taken for the mixed resin to gel in the mixing bowl.

When polyester resins harden after going through the gel stage, they produce their own internal heat which is called “exotherm”.   This exotherm is much greater when there is a large bulk of resin such as in the mixing bowl.   In thin layer form such as when it is spread out onto a mould with glass reinforcement, the heat escapes easily from the large surface area before it can build up to a very high temperature.   For this reason, the pot life of a resin is much shorter than the time taken for the resin to gel on the mould.

   When resin cures in a mixing bowl, the exothermic heat can be so violent, that the resin will smoke and crack and burn if touched.   Therefore, do not mix more resin than you can use in a reasonable time.

   It is not advisable to reduce the amount of catalyst to slow gel time because of the risk of under cure.   Sufficient gel time control can be had by varying accelerator content.   The disadvantage of pre-accelerated resin is that this control is removed.

   The shelf life of polyester resin is greatly improved if it is kept in a cool place away from light and it can vary from one week to three years depending upon storage conditions.

   Un-waxed polyester resin; where it is anticipated that a period of time will elapse between starting and completion of a particular section being laminated, an un-waxed resin should be used.   Un-waxed resins can take up to several days to achieve full cure thus facilitating the bonding of the subsequent layers. When cured, the surface, using un-waxed resin is difficult to sand so a coat or waxed resin or gelcoat will be required.

Polyester resin may be thinned by adding a MAXIMUM of 15 parts of Styrene Monomer to 100 parts polyester. Check with your supplier.   The first coat of resin applied to wood can be thinned for deeper penetration.   It should not be necessary to thin laminating resin as this weakens the cured laminate.  Thinning will lengthen the surface cure time and will require more catalyst.

   For health reasons, there are now some "Low Styrene Emission Resins". They have a substitute for Styrene Monomer or a reduced quantity in their make up. These resins are quite different from high viscosity resins which can be thinned with Styrene Monomer.

Gel coats are designed as a protective coating for structural laminates. They are available in brush and spray versions and are best applied at a thickness of 0.5mm. Most exterior gel coats are based on isothalic resins with low styrene emission and are available in both brush and spray forms. Nowadays, they are blister resistant and usually approved by marine authorities.  

   There are several types of gelcoat, each having its own particular function.   The most commonly used, is the one for female moulding.   This gelcoat comes in various colours and is unwaxed.   It is usually pre-promoted and, as with resins, will need catalyst added before being applied to the mould surface.  Clear gel coats have an increased resistance to water permeation because they contain no pigments and when backed up by a vinyl ester resin laminate virtually eliminate any possibility of surface blistering known as Osmosis. 

    The type of gelcoat you choose, and the way you apply it, will certainly affect your finished boat. If you are building a male moulded boat, you may well replace the exterior gelcoat with a urethane or epoxy based paint system.

Polyurethane and epoxy paints applied correctly perform well on male moulded boats and even enhance some of the older gelcoat systems. Development is fast and effective so consult your paint supplier for the latest technology.

Cobalt Naphthenate is the common accelerator (or promoter) in most polyester resins and should never be brought into direct contact with catalyst (MEKP), outside of the resin mix, as an explosion could result. For safety reasons, general laminating resins are usually supplied pre-promoted and extra accelerator can be added if you require a quicker setting time although, as they are pre-promoted, we usually have to adjust the catalyst level. In this instance, we should be using un-promoted resin and adjusting the amount of accelerator to suit the conditions.  Never add excessive amounts of the accelerator to any resin.

Catalyst – MEKP (Methyl Ethyl Ketone Peroxide) is normally a clear liquid and must be handled with extreme care. Polyester resin will not harden without catalyst. The amount of catalyst added to the resin is critical and it is normally used in a ratio of 1-2% by weight of the total polyester resin. As a rule of thumb, 20mls of catalyst is usually needed for 1kg of resin. Accurate measurement is important because a small increase or decrease of the amount of catalyst can have a large effect on the working time of the resin.

   The catalysts used with polyester resins are almost invariably organic peroxides.   These are unstable and should be treated with the greatest caution.  They are all irritating to the skin and cause burns unless washed off immediately.   Injury can be more serious if catalyst is splashed into the eyes.   Immediate treatment in such cases is to wash out the eyes continuously with plain water or weak bicarbonate solution.

This do-it-yourself material can be made for a fraction of what you would pay if you bought it, made up, from your local supplier.   You will use sizeable quantities of filler (commonly referred to as “BOG”) during construction of any fibreglass boat.   There are several materials that can form the dry ingredients of the resin putty mixture.   These include industrial talcum powder, Q-Cells and micro balloons.  When mixed with waxed polyester resin and a small amount of additional accelerator they make and excellent and economic filler. This material, if stored in a covered container, will keep for up to two or three weeks. When you want to use the filler, you simply dig out a quantity and place it on a mixing board.   You then add a dash of catalyst.  This does not have to be measured, as you will soon gauge the mount required to make the bog set in the desired time. Check with your local fibreglass supplier as to the recommended materials.

Designed for either general laminating or gel-coating, these resins are a benefit in areas where there is a higher than usual fire risk.   As fire retardant resins are generally more expensive than regular laminating resins, most builders tend to only use them where necessary.

Acetone is a highly volatile material, used as a general purpose solvent and cleaner. Used for cleaning brushes and rollers after laminating.   Acetone should be stored in a sealed metal container and measured out in small quantities; say 2” [50mm] in the bottom of a plastic container in which you should thoroughly wash the brushes and tools. You can store brushes and rollers in clean acetone overnight, make sure you use a sealed container, as acetone has a high evaporation rate.

Release agents are liquid or pastes which are applied to mould surfaces to form a barrier skin and prevent sticking by the resin or gelcoat.   Polyesters stick very well to most materials and surfaces and if no release agent is used, it is impossible to remove the lay-up or casting from the mould.

In some instances the skin can be irritated by polyester resin in which case it is wise to use a barrier cream in conjunction with gloves. Low cost disposable gloves are available and specialist barrier creams should be available from you fiberglass supplier. It is always recommended to use rubber gloves when working with epoxy resins.