This building guide will show the basics of how to build a folding trimaran using foam sandwich or plywood composite panels. We will give you hints on how to make your own fillers, what tools to buy and some thoughts on how to organise your workspace.
It is best to have a dedicated space to build the boat and organisation is important. It saves time if everything is organised and easy to find. It is a health hazard to have electrical cords and tools where they can cause injury. It is a good idea to sketch out the space you have available and decide where everything will be fabricated.
Price out your materials and collect the necessary tools.

TOOLS

The tools we use are basic handyman tools and some you can make yourself. It is always a good idea to buy the best quality tools you can afford.

• Battery powered drill with a set of drill bits and a Phillips head screw driver bit. The drill is used to screw dry wall screws through foam to hold it in place until the resin hardens. This is not hard work for the drill so a less expensive one works fine.
• Electric sanders. Random orbital to sand large areas and a flat orbital for edges and corners. Best to buy quality; ones with a velcro pad save time when changing disks often. Building a boat is especially hard on sanders. We found that tradesman quality sanders can stand up to the heat generated on filled laminated panels.
• Jig saw - a good quality one is recommended, a cheap one will last about half a day. Fibre glass and resin panels are very hard on jig saws and you will need special blades.
• Small electric angle grinder fitted with a rubber backing pad and sanding disk. We use a grinder to sand the hardest fillers and areas where a strong secondary bond is necessary when laminating.
• Filleting tools can be made from cake icing tools or any other metal implement shaped with a grinder. Grinders are also good for cleaning filler off tools when you forget to clean them properly.
• Measuring tools (tapes), string line, squares, bevel gauge, rulers, chisels, clamps (lots), spirit level, and straight edges.
• For resin and hardener measuring use metering pumps available from resin suppliers. For small amounts use medicine measuring cups or syringes.
• Compactors (parsley chopper). Any fibreglass material supply store should be able supply a couple of these. Look for those that are easy to clean.
• Staples 12mm/0.5 in and large stapler for tacking plywood to frame and scrap copper wire to join chines until the glue sets. (Copper wire is available from electric motor rewind shops). This is for plywood construction.
• Metal strapping, about 20mm wide (used in house building and available from hardware stores) to hold foam panels together until resin hardens for foam construction.
• Vacuum cleaner
• Scissors (for cutting fibre glass).
• Knife with replaceable blades.
• Mixing cups and jugs. Use cheap plastic ones and throw away when the resin becomes too hard to remove.
• Mixing sticks. Large ice cream sticks or tongue depressors available from craft stores.
• Paint brushes-cheap ones are best.
• Foam paint rollers and roller frame.
• Safety gear-disposable gloves, goggles and masks when grinding
• Electric planer for making scarf joints for plywood construction. Also an attachment called a Scarfer can be used on an electrical circular saw from ATL for cutting scarfs in plywood.
• A number of clamps.
• Spray guns and compressor if you are going to spray paint the boat.

Apart from these tools you will need screw drivers and wrenches (spanners).

WORK SPACE

A well thought out work space will save time and make the job much easier and safer. You can avoid moving things unnecessarily by planning ahead. Below is a plan that we use to build a small trimaran. The long tables can be used to laminate panels or scarf the sheets together.

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLES

We built our tables using ¾” (17mm) plywood with wall studs for legs. The table is covered in plastic to stop the resin from sticking to the plywood. Below is a drawing of one of these tables. The boat panels are made full length. We use as many tables as it takes to make the length. The tables can be joined using clamps so when the panels are made the tables can be separated and used for various jobs like laminating and painting.

 

 

 

 

 

 

 

 

 

 

MATERIALS

FOAM vs PLYWOOD

The principle behind using a foam panel is to use the strength of an “I” beam construction with minimum weight. The glass skins are the flanges and the foam core acts as the web. This makes the laminate light weight with great strength and stiffness.The foam can be glued together to the length required before laminating with glass.
When building a boat the hulls are the one component that is hard to change later on. You might be short of cash and decide to buy a few sheets of plywood at a time or you may consider resale value is important to you and decide to build in foam sandwich. It is a sad fact that many boats have to be sold for various reasons after they are built and you should plan accordingly.
Building a boat should be a labour of love and we recommend you build it using whatever materials you feel confident with. However we do want to dispel the myths of not using polyester resin because it is old fashion or foam because you have to vacuum bag or use resin infusion. Making your own panels can be a little tricky at first but you can save money and make them any length you need instead of the length that fits on a palette.

PLYWOOD

All plywood used on the boat should be marine grade. First grade marine plywood is expensive because the core veneers are knot free, void free and even thickness throughout. The face veneers must have a clean grain pattern. Gaboon or Pacific Maple is a good choice. Exterior grade plywood is only good for making the temporary frames, don't be swayed into using exterior grade plywood for the construction of the boat, it will have a waterproof glue line but the core may contain voids (which can lead to rot) and it is generally heavier than Gaboon or Pacific Maple..
Plywood is relatively easy to find and most people feel comfortable using this material. It is easy to cut and sand. Epoxy resin should be used to laminate the plywood panels. Many people develop an allergy to epoxy so your body must be protected with gloves when handling the resin and masks when sanding.
Plywood sheets will have to be joined by using an 8 to 1 scarf joint to make the panels the correct length. A scarfing tool can be purchased from ATL that attaches to a electric circular saw that will cut the correct angle.

 

 

 

FOAM

Foam provides good heat and sound insulation. It is easy to cut with a knife (until it is laminated) and bends more easily than plywood (before it is laminated). Foam can be laminated using epoxy, polyester or vinyl ester resins.
Foam is more expensive than plywood but if you can use polyester resin in your area then the difference is less when you consider the difference in weight and resale value of the finished boat.
Attaching fittings to a foam boat can be a bit tricky. The foam must be removed where bolts pass through and replaced with strong filler to prevent crushing the foam, this can be achieved by drilling the bolt hole in the appropriate place and using a bent nail or cut down allen key in a electric drill. Insert the tool in the foam and carefully spin, this will form a void which is then backfilled with filler. Any exposed foam around lockers etc. must be removed to a depth of about 10mm and backfilled with filler.

FIBREGLASS

Biaxial fibreglass cloth is used to cover outside of the hull (plywood construction) or the foam panels prior to cutting.
Double bias cloth or tape is used where the fibreglass needs to go around contours or where diagonal strength is required.
Uni-directional glass is used where strength is required in one direction.
Triaxial is used where strength is needed in three directions. This glass is difficult to bend around corners and wet out and is best used on a flat surface.
Glass can be wetted out using a brush, roller or spreader depending on the area to be covered. A compacting roller is used to consolidate the glass in the resin and to ensure there are no air bubbles. We use peel ply on most of the areas we glass. It ensures a smooth finish that cuts down on sanding later (we believe the extra cost is well worth it). Some epoxy resins leave wax on the surface (amine blush) which has to be washed off before laminating or painting. Some polyester resins leaves wax as well. The peel ply will remove this waxy layer needing only a light sanding before finishing.

RESIN

Epoxy resin is used to build plywood hulls. It can also be used with foam panels. It is mixed with hardener before it is used. It is best not to mix large quantities (we find 200-400ml is enough for one person to comfortably handle in one go). You can order different hardeners to allow for temperature differences. Follow the manufacturer's directions. There is a lot of info about the W.E.S.T System. This is wood epoxy saturation technique. All wood and plywood is completely sealed with epoxy resin.
Epoxy can cause allergic reactions in many people. It can range from mild skin irritation to carbuncles and lung and eye irritation. Always wear protective clothing and be careful around children.
Polyester resin has been around for a long time and there are still many old boats that have been built using polyester out there. With polyester and vinyl ester people do not develop the allergy as with epoxy but you need to check if EPA rules apply where you live, the stryene smell can be quite potent. Polyester resin can be either waxed or unwaxed. The unwaxed resin can be used whenever laminating and when the job can't be finished at one time. It will stay sticky and doesn't need to be sanded before the next layer is added. The low styrene emission polyester resins help lower the styrene odour but care is needed when working inside the hull where the fumes can build up. Care should be taken when using MEKP (Methyl Ethyl Ketone Peroxide) the catalyst. You should wear gloves and eye protection and definitely keep out of the reach of children.
Vinylester resin also uses MEKP as a catalyst and acts in a similar way to polyester. It is more waterproof than polyester.

Always refer to the resin manufacturer's recommendations.

FILLERS

Resin and hardener (or catalyst with polyester) should be mixed thoroughly before adding any filler.
Fillers can be added to resin to use for either gluing applications where strength is needed or for fairing areas before painting. Material suppliers offer filler for particular purposes when you buy the resin.
We make our own fillers using different combinations depending on the job. If you want to make your own you will need to experiment with combinations until you feel comfortable with the result

AEROSIL-CABOSIL

This is milled silica designed to be added to resin. It will help stop sagging of filler. It makes the filler hard to sand. We use it in combination with cotton fibres to make strong glue.

Q-CELL LIGHTWEIGHT FILLER

Q-CELL consists of hollow inorganic microspheres with exceptional whiteness and low bulk density. It is inexpensive and light weight filler for interiors and fairing the hull above the waterline.

3M GLASS BUBBLES TYPE K20

3M Glass Bubbles are very fine hollow glass spheres of consistent shape, wall thickness and size distribution, suitable for use as a cost effective volume filler. They make are a good filler for areas below the water line.

COTTON FIBRE

Cotton fibre is sometimes called flox, cotton flox or cotton linters. We mix cotton fibre with aerosil to make a strong glue.

TALC

Talc can be mixed with resin and used as a filler. It is easy to sand although a little heavy. It is good to have around when you get an attack of fiberglass itch.
None of these fillers have a particular ratio, use what you need to get the job done. Q cells bulk out the mixture, cotton fibre adds strength and aerosil stops filler from sagging.

FILLETING AND GLASSING

Resin is mixed with filler to form glue or filleting compound. The glass tape is laid over the fillet to make the joint stronger. Glass will not conform to a sharp edge so all edges must be rounded and all angles must be filled with a radius of filler (a fillet). The resin and hardener is mixed first and the appropriate filler is added until the mixture is like the consistency of peanut butter. Add the filler to the angle in your favourite manner. Some people use a plastic bag with a hole cut in the corner and apply the filler as if icing a cake. Some use a spatula and roughly fill the angle. Use a round tool to make a smooth radius to the appropriate size (large enough that the glass will bend without pulling away). The tool you use can be anything that resembles a cake decorating tool. A putty spatula shaped with a grinder works well.
If the fillet hardens before you have time to apply glass tape it must be sanded to ensure a good bond. It works well to apply glass tape either pre-wetted or dry over a wet fillet that has hardened enough to hold its shape. This saves time and you don't have to clean up any dags that you left the day before.
Pre-wetting the glass tape is a good idea if you are working in a confined space and it would be difficult to wet out the tape easily. Pre-wetting glass works well when making beams where several layers of glass must be added. After glass is applied be sure to compact it well using a compacter.
Strips of peel ply can be added if the area is to be finished later on.