Avoiding excess weight
Meeting builders on symposia, kayak meetings or on tour, the topic of kayak weight often pops up. Wooden kayaks have a potential for very low weights without sacrificing strength or durability. A normal-sized sea kayak with standard outfitting may tip the scale at 15-16 kg, but it does not happen by itself. The majority of first-time builders kayaks turn out at somewhere around 18-22 kg. I have also seen two kayaks that for reasons unknown, was close to 30 kg, in spite of good intentions. Returning builders frequently shave off approx 1,5 to 2 kg for the next kayak, and if persevering, close the gap to the theoretical ideal asymptotically for the following kayaks.
Epoxy and fiberglass
A common cause of excess weight is a liberal use of epoxy A common cause of excess weight is a liberal use of epoxy. Epoxy in itself is not very strong in relation to its weight (hardened epoxy left-over in a cup is easy to break with your hand). The strength is in the wood and fiberglass, and epoxy is just what unites those two. The less you use the better result.
- A first minimal layer before starting the lamination will seal the surface and prevent excess epoxy from seeping into the wood when laminating the fiberglass. Pour a small amount of epoxy (approx 1 dl) on the hull and spread it quickly with a rubber squeegee. The less you use the better.
- Don’t use heavier cloth than recommended – and preferably a twill weave wich is slightly thinner and thus require slightly less epoxy to wet out.
- Remove all excess epoxy that pools on or in the hull.
- Work the squeegee hard to make sure the fiberglass is completely wetted out and in contact with the wood. Cloth that floats up in epoxy will increase weight and risk getting sanded through later.
- Some building manuals recommend end pours – massive lumps of epoxy in the extreme ends. In my opinion, a very questionable practice, with dead weight in bad positions and requiring some degree of acrobatics with the kayak on its end and spilling epoxy from 2 meters height. Don’t, unless you intend to sink ships with that reinforced ram! If you need a filler (fx for a rope through the stem), carve a filler in light wood.
- If you buy woven strips of fiberglass for the sheer, chose the same weight as the rest of the fiberglass.
- Epoxy fillets between parts should be kept as small as possible – they will be strong anyway. Avoid Silica and other heavy additives. ”Microfiber” or sanding dust is lighter and strong enough for structural fillets while ”Microlight” can be used for visual touch-ups – extremely light and easily sanded.
- Take some care in fitting the strips. Gaps must be filled with epoxy which is three times heavier than wood. Of course, the choice of wood also is important – even if wood is much lighter than epoxy or glass. Spruce, Cedar (WCR) and Paulownia are light and strong enough. Heavier fir species or hardwood are heavier without any functional advantages.
Of course, the choice of wood also is important – even if the wood is just a small part of the total weight and generally much lighter than epoxy or glass. Spruce, Cedar (WCR) and Paulownia are light and strong enough. Heavier fir species or hardwood are just heavier without any functional advantages.
- When ripping strips keep the thickness down. With Spruce, rather 4,7 mm than 5,3 (slightly more with Cedar or Paulownia). No better-safe-than-sorry margin – the kayak will be strong enough anyway.
- When selecting boards, look for the light ones. There are surprising differences going through a pile of boards at the wood supplier. Of course, if you focus on looks, go for the most beautiful boards – but don’t blame me if your kayak exceeds my suggested weight range.
- Hardwood decoration may be nice – but heavy.
- Be choosy when it comes to store-bought hardware.
- Build the rim and hatch recesses in carbon fiber or solid wood – lighter than plywood.
- A foot support in light wood, integrated with the forward bulkhead is much lighter than a store-bought in aluminium and plastic, bolted to the hull.
- The deck line can be used as a super light handle – a hole through the stem and crossed lines, maybe with a piece of plastic tubing, is a highly efficient solution.
- Bulkheads in foam in much lighter than plywood – and does not cause damages to the hull if the kayak is dropped or thrown into a rock in waves.
- Avoid screws, bolts and other metal hardware if possible.
- A foam seat is better than one in wood or epoxy/fiberglass.
- Hatches are problematic. Plastic/rubber hatches are simple to use and attach, but heavy and expensive (though some of the newer ones are relatively lightweight). Wooden hatches cut from the deck are light and nice but somewhat tricky to get reliably tight. Plastic deck plates are quite light and reliable, but available in only small sizes (up to 8”).
- Deck line fittings in turned oak with a stainless steel bar is far heavier than a laminated line channel – multiply the difference by 16-20 fittings.
- Commercial rudders are often quite heavy – an adjustable skeg in wood or carbon fibre weights less (which of course is not much help if your plans call for a rudder – but some builders put rudders even on kayaks designed for skegs, in the mistaken belief that it is better!)