timber | wood species | epoxy | glue wood | fibre glass | finish | time and money
Timber from the lumberyard is normally not good enough for boatbuilding. Traditional building methods uses quarter sawn planks, with the grain looking like this:
Planks from the yard are cut more economically and looks like this:
For strip building these are just fine. When cut they become almost perfect quarter sawn strips:
In some species (fx fir), knots are unavoidable. Chose planks with as few and as small as possible. Knots does not compromise the strength in the finished hulls, but they complicate the building process. Most important is low weight. Balance the boards in your hand and chose the lightest. Fir varies in density from 0,3 to 0,45 corresponding to 4-5 kg in the finished kayak or canoe. Built-in ballast is no fun, on the water nor on land.
The most important quality in the wood is tension and compression stability. Those are the loads the strips are subjected to in a finished hull. Bending properties, rot resistance etc is not that important, since strips are not supposed to bend once the hull is finished and the epoxy/glass seal protects the wood from water. The overall structural strength comes primarily from the two stiff layers of glass separated by a light wooden core – thus the thicker core the stiffer hull. Apart from that the wooden fibres contributes to longitudinal strength, while the glass gives transversal strength.
Chose fir or Western Red Cedar for almost everything - other wood species only in small details.
Pine. Possible but with a weight disadvantage.
Larch. Beautiful but heavy – 4-5 kg extra compared to fir or cedar.
Ash. Heavy but strong and bends beautifully. Perfect for solid cockpit rims, outer stems, crossbeams and thwarts in canoes.
Oak. Heavy and hard to glue and finish due to large acid content – avoid all together in strip construction.
Hardwood as maple, elm, alder, birch, linden, walnut, apple, pear, cherry can be used for small details such as deck hardware.
Sitka. Light and strong but expensive and hard to find.
Light mahogany: lauan, gabon, okume etc are possible to use but I see no advantages – often an unattractive pink color in need of staining.
“Real” mahogany (honduras, sapeli) is dark strong and beautiful but far to heavy to of any use
Teak is heavy and hard to glue and finish due to oil content. It dulls tools due to mud content in the pores.
Station molds can be made of particle board or plywood.
Epoxy is a two pot hard plastic - thermosetting epoxide polymer that cures (polymerizes and crosslinks) when mixed with a catalyzing agent or "hardener". Epoxy has a tremendous holding power on almost any material (low surface tension and minimal dimensional changes with temperature) except mold release and polyethene. The resin is easy to modify with additives, to suit most strip building uses: gluing, filling, laminating, molding etc - one of the most watertight of all plastics (almost no water absorbtion compared to polyester), heat- and chemical-resistant and durable.
The epoxy used here is a low-viscous, low molecular laminating epoxy without any additives or solvent. It means that it is thin enough to be brushed on, molecules small enough to penetrate into the wood and without fillers. The same epoxy is used for everything.
Epoxy cures down to almost zero degrees 0° C, but for practical use the limit should be approx 10° C. Chemical reactions double their reaction speed for every ten degrees.
In industrial applications epoxy is cured at raised temperatures (50-100° C). At room temperature the reaction is almost entirely linear, i e very few crosslinks. The higher the temperature the more crosslinks and the stronger the laminate. Adhesion improves when surface tension and viscosity decreases.
First heat, then epoxy
The best way to use heat is to warm the entire hull to 30-40° C, apply the epoxy and let the temperature go down slowly. The heat decreases the viscosity of the epoxy and when the temp goes down the epoxy is sucked into the wood. Doing the opposite - applying epoxy and raising the temp - will have the air in the wood expanding and pushing out the epoxy, causing blisters.
Some manufacturers have different hardeners: fast and slow. Normally the slow hardeners gives the best result. The epoxy is workable approx 15 minutes with the fast and 30 minutes with the slow in room temperature. Always mix small batches - not more than a couple of deciliters. Larger batches, especially in high narrow cans risk a chain reaction, turning burning hot and sets in seconds.
Epoxy has a very low surface tension and penetrate wood easily. This might lead to epoxy starvation in a joint. Some manufacturers recommend using thickened epoxy to avoid this, but my experience is that epoxy still seeps into the wood, leaving mostly filler in the joint. A better method is to saturate the wood first with pure epoxy, then use thickened epoxy for gluing.
Most epoxy for strip building contains no solvent to solve oily deposits on the glueing surfaces - be very careful not to touch the before applying the epoxy. Clean with acetone if in doubt.
Filling with epoxy may be done in three ways:
Wait until halfway cured and use as filler compound. Of course the strengths is decreased - use only for cosmetic filling.
Mixed with sanding dust, microfiber, microlight, silica or similar to adequate consistency. Visible but strong filling.
A molding form of paper or tape to hold the thin epoxi in place until cured. Strong and pretty.
The cloth has three tasks: lateral strength replacing ribs in traditional boatbuilding, protecting the soft wood against wear and tear and it is part of the tight seal on the wooden surface. Use light (150-165 gr/m2), preferably twill weaved cloth, compatible with epoxy - cloth is compatible either with solvent based polyester or solventless epoxy. One layer, doubled in a narrow strip along the keel, in the cockpit and along the hull-deck joint is sufficient.
Epoxy must be protected against UV. Best is solid paint - any kind is OK, but expensive marine coatings saves a lot on upkeep. Want to show off the woodwork? Well, you are not alone. A bright two pot marine urethane finish is best. Make sure that the chosen brand contains UV-inhibitors. Follow the instructions on the can.
Time and money
Time and money may be in short supply with many boatbuilders - at least one of them at any given time.
The time it takes to build a kayak is of course very individual, depending on competence, tools, efficiency, ambitions etc - but 100-150 hours is a reasonable count for a first time builder, following the manual. A professional may reach 40 hours, a highly efficient amateur aiming for a functional kayak 60 hours - and sometimes a formidable masterpiece is launched after 500 hours. I believe a reasonable level of ambition is generally preferable. The perfectionist runs the risk of losing the enthusiasm halfway - suddenly it is summer and the paddling pals go paddling. Better to get the kayak launched and go paddling. The once-in-a-lifetime-masterpiece may be kayak number two or three.
The cost is as hard to define, depending on your sources for materials, qualities, your ability to find good deals etc. The main cost is epoxy and fibre glass - approx 75%. You can buy small pots expensively at the boat shop, or large cans from a supplier or importer. You can make group purchases together with other builders.
The largest investment is your own work. It may be balanced against getting a beautiful and functional kayak, built for you and with better prestanda than anything commercially available. And do not forget the therapeutic value in working with your hands in an old craft tradition - or the unavoidable meetings at the launching beaches - "no, you can´t buy one of these. You have to build your own...".