Repairing composite kayaks

These instructions are split into the following pages:

• Composite kayak construction
• Working with repair materials
• Filling gouges and scratches
• Finishing the repair

[edit] Composite Kayak Construction

Figure 1 - Basic Hull Lay-up
Gray – the hull mold
Red – release agent – typically PVA (polyvinyl alcohol)
Green – Pigmented gelcoat
Blue – Fiberglass impregnated with resin

Figure 2 - Hull lay-up with core
Gray – the hull mold
Red – release agent – typically PVA (polyvinyl alcohol)
Green – Pigmented gelcoat
Blue – Fiberglass impregnated with resin
Yellow – Core material between layers of fiberglass and resin.

Composite kayaks are made from layers of reinforcing materials saturated with a resin or epoxy. The reinforcing materials are usually fiberglass or Kevlar, though carbon fiber, Spectra and other high-tech materials may be used. As well, these materials can be blended, either woven together in a single fabric or layered on top of each other. In addition to these materials, core materials such as Spheretex and Coremat may be used. The outside of a composite kayak is usually covered with a layer of gelcoat – an opaque resin or epoxy material that is used to give colour and protect the underlying layers from abrasion and UV radiation.

There are several resins and epoxies that can be used. However, epoxies are quite expensive, so they are not often found in commercially produced kayaks. The most common resin is probably polyester, though vinylester resins are also used. It is easy to identify a resin by its styrene smell when new; modern epoxies are usually odourless. When repairing kayaks, it is important to know whether you’re dealing with resin or epoxy. A resin repair will not stick well to epoxy, whereas an epoxy repair will usually adhere to anything. If you have an epoxy kayak, you will not be able to use a resin gelcoat – you’ll have to search for an epoxy gelcoat.

An excellent source that describes the characteristics of resins and epoxies is on the web at Site not working - to be replaced when I find a good one - sorry

Figure 1 shows a typical lay-up. In most modern kayaks, the reinforcing material is woven into a fabric. Some manufacturers, notably in Britain, also use a layer of randomly oriented chopped strands referred to as mat, which is easier to form around odd shapes but does not produce as strong or light a composite as a good woven fabric. In British kayaks, mat is often found in the deck.

Figure 2 shows a lay-up with a core material. The core material increases the thickness of the lay-up without as much weight as would occur if more reinforcing layers were used. The increase in thickness results in an increase in strength and stiffness. Core materials are usually used for keels and wide, flat deck areas. These core materials are usually a fabric that is filled with tiny glass spheres or a lightweight, low-density material.

In construction, two female molds are made, one for the deck and one for the hull. The molds are covered with a coating of mold release - polyvinyl alcohol, then a layer of gelcoat, which, by virtue of it being a gel, sticks to the mold and doesn’t run down to the bottom (at least, not too much). On top of the gelcoat, layers of reinforcing material are added, each being saturated with resin.

Some makers will vacuum bag the lay-up at this point. Once all layers are in place, the entire mold is covered with a thin plastic film and the film is sealed around the edges of the mold. The edges of the mold are designed to hold the film and still allow air and resin to flow under a vacuum. The mold is then subject to a vacuum and the plastic film is sucked down, compressing the layers together and squeezing out the excess resin. This process enhances the saturation of the fabric with resin, reduces the amount of resin to a minimum and results in a kayak that is light and strong. It is cured in this condition.

Other makers will not vacuum bag. Very good quality composites can have the resin impregnated fabric sqeegeed until there is a very fine layer of resin over the fabric and no dry fabric or bubbles are present.  Done well, this can result is an exceptional composite. Many makers will not do this, as it requires a higher level of skill among the workers.  If not done, excess resin is present. The impregnated fabric will not have a tight, compact finish and will be heavier and less strong for the weight.

Once cured, the resulting hull and deck are cleaned up, drilled for fittings and cut to final shape and joined together. In some kayaks, the joint consists of a plastic extrusion, H-shaped, into which an adhesive is placed (Figure 3). The deck and hull are placed into the seam and the inside of the seam is covered with a fibreglass tape to seal and reinforce the extrusion. Fittings are then added to the kayak and the craft is complete. Those without plastic extrusions use a fibreglass tape directly (Figure 4). Some cover the outside of the seam with gelcoat.

Some manufacturers will use an H section that has a hole in the horizontal bar of the H. This hole is used to feed the rudder control cables to the rear of the kayak without creating a hole in the hull or deck. Current Designs’ Solstice series is an example.

Figure 3 - Seam constructed with H channel Blue – Composite deck and hull Black – Plastic H section Red – Adhesive or sealant Green – fiberglass tape over seam

Figure 4 - Seam construction without channel Blue – Composite deck and hull Dark Blue – resin and fiberglass tape over seam

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