Building a kayak from fiberglass is challenging project with many advantages over building one in other materials. Fiberglass is often stiffer, easier to work with, and produces a kayak that is durable and easy to repair. Once you have the initial work of building a mold out of the way, subsequent boats are faster to build.
Select a Design
Select a kayak design. Almost any commercial plan can be selected and used as long as you account for the difference in the hull thickness between the material called for in the plan and the fiberglass. Instead of using an existing plan, design a new boat using kayak design software. Both Kayak Foundry and Delftship are easy-to-use programs that make designing kayaks easy.
Find or build a plug. A plug is a part that looks just like the kayak design that you're building. To make a plug from plans, you build a kayak, usually out of wood, finishing just the outside. But an existing kayak could be used as a plug as long as you obtain permission from the kayak's designer and manufacturer. Once the plug is finished and the surface is perfectly smooth, cover it with paste wax and a PVA mold release. The better the finished surface of the plug, the better the final DIY fiberglass kayak will be.
Build a mold. A fiberglass mold for a kayak is usually a female part that you'll build the kayak into. A kayak typically has two molds; one for the hull and one for the deck. Around the plug, build a three- to five-inch flange where the seam between the deck and hull will be. This flange provides a place to run a knife, producing a hull or deck that lines up exactly with the other side. To build the mold, cover each side of the plug with fiberglass. Using matt fiberglass is typically easier, because it conforms to the plug easier than some fabrics. Once dry, pull the plug from your mold and then sand and fill any imperfections in the mold. The mold is then prepped with multiple layers of wax and PVA.
Spray the mold with colored gelcoat. Gelcoat provides a smooth finish to the outside of the kayak. Consider using a bright color; it will be more visible on the water. For the hull consider using white, which hides scratches.
Lay fiberglass into the mold. The goal of the lay-up is to use just enough fiberglass to provide the desired stiffness and durability to the finished kayak. The more fiberglass used, the more durable and stiff the kayak will be, but more fiberglass means a heavier kayak. Using extra partial layers of glass in the sections of the kayak that will be exposed to more wear and tear is a good way to gain durability and save weight. Once the fiberglass is positioned correctly, wet out the glass using epoxy, vinylester or polyester. Epoxy is the strongest and most expensive, polyester the weakest and least expensive. Vinylester is a good mix of cost and characteristics. Once the fiberglass dries remove it from the mold. If you prepped the mold correctly, this should be easy.
Join the deck to the hull using fiberglass tape on the inside and outside. When fiberglassing over cured glass, rough the surface up with sandpaper before applying the new layer. This fiberglass seam holds the boat together, so make sure it reaches into the far ends of the kayak. Then install the remaining outfitting. Often it's easier to build a separate mold for the cockpit coaming and glue it to the kayak at this point. The bulkheads, foot braces and hatch covers should all be installed with epoxy or a marine sealant.
- Boat Builders Manual: Building Fiberglass Canoes and Kayaks For Whitewater; Charles Walbridge; 1982
- Canoe and Kayak Building the Light and Easy Way: How to Build Tough, Super-Safe Boats in Kevlar, Carbon, or Fiberglass; Sam Rizzetta; 2009