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Technical Manual

Building tougher Wooden Boats

by Adrian Baker, Product Manager. SP Svstems

"Epoxy Systems" used with wood have now become the norm in modern wood racing dinghies whether they are amateur or professionally built. Although epoxy formulations were well known in the aerospace industry in particular for high strength bonding and as a matrix for laminating, to the average boat builder they were a completely unknown entity.

The revolution in the UK started in 1978 when the first epoxy system for wood boat building was introduced and made under license and marketed by, what is now Structural Polymer Systems Ltd. The new epoxy was of sufficiently low viscosity to achieve effective penetration into wood and specially formulated to achieve excellent adhesion and good coating and laminating performance. In addition, glass sheathing was now offered as an additional technique to the repertoire of the wood boat builder. The technique of "fillet bonding" using the various fillers available in combination with the resin system, gave the builder new scope in fastening panels whilst the well established boat building technique of cold moulding using wood veneers was made easier and more effective. For the first time we had a product which could exclude wood's old enemy moisture. Levels of moisture content above approximately 20% lead to fungal growth and rot. It has been found that raising the moisture content in kiln dried wood leads to a progressive deterioration in wood's natural stiffness. Excluding moisture by coating with epoxy meant that the properties of dried timber could be stabilised and the virtues of wood as a unidirectional natural fibre could be exploited. This stabilisation of properties was very important because it meant that wood could be regarded in the same way as other structural materials eligible to be engineered. The idea of using wood as an engineering material was essential to the "new" boat building philosophy and it was not surprising therefore that the more academically minded were quick to appreciate the virtues of the new boat building epoxy.

Many dinghy builders at the time were, however, sceptical of the claims of the "wonder" epoxy system and completely puzzled by its apparent complexity. As a dinghy builder myself I considered it needlessly expensive without really getting to know what it was all about, a reaction which, I am assured, was shared by others building boats at the time. However, my scepticism was soon to turn to enthusiasm. At the time I was having trouble introducing the maximum allowed athwartships panel curvature to the main bottom panels of my Graduates which I was building. As soon as the screws holding the glued panel in place were withdrawn the next day in preparation for planing the centre of the boat ready for the keel, the glue would invariable crack and the panel lift off. The glue I was using was a well known acid-catalysed urea formaldehyde type like Aerolite or Cascamite which, whilst being quite satisfactory for most operations, just did not have the tensile strength or toughness for my application. The problem was exacerbated by the fact that often the hog was less than perfectly faired and the glue line was undeniably thick in places. Changing to the new wood epoxy system (later to be called SP106 System) solved the problems and with its gap filling properties proved perfect for the job. With more experience using the system I thought of other ways to exploit the epoxy's amazing strength and toughness.

As a coating, Wood Epoxy Systems have played an important role in both the protection of new boats and renovating old ones. With water-proofing as one of the main objectives, three coats applied to the bare wood as a primer will considerably extend the lifespan of paint finishes and can make the boat virtually maintenance free. Even small stones which impact against the hull whilst trailing the boat on public roads, whilst causing the paint to sometimes chip, will invariably not penetrate the epoxy coating if it is sufficiently thick. Since epoxies are heavier than oil based paints, care must be taken not to increase weight of the boat too much by applying too great a thickness which is why three coats each of approximately ~ microns has been selected as the optimum coating scheme. The added stiffness the coating imparts when applied to both surfaces of plywood panels is particularly noticeable on older plywood where additional penetration can easily be achieved and where the outer ply veneer has fatigued. Significant increases in stiffness can be achieved on panels of 4, 5 and 6mm thickness with the greatest gains being made on the thinner panels where the contribution of the coating is a greater proportion of the total thickness. The end result is that not only do we end up with a boat that remains down to weight, but we have one which 135 stronger and stiffer and may not need repainting for ten years. It is not difficult to see therefore than an extra £100 spent on the boat when new can save that amount in maintenance' alone.

So much for the benefits but which products are we talking about?

In SP Systems' latest product range there are two wood epoxy systems, SP106 and SP320 (Spacote).

SP108 used with SP209 (fast) or SF207 (slow) hardeners is the standard resin system for most operations. It is not recommended for clear coating because of its relatively poor light stability and tendency to discolour when exposed to sunlight. This means that SF106 should not be considered as a candidate for coating decks where a clear finish is required but is perfectly all right for using inside buoyancy tanks. etc. SP106 is very popular with wood boatbuilders particularly in the 6 litre size where plastic mini-pumps can be used to conveniently dispense both the resin and the hardener. For 1.2 litre packs mini-pumps are inappropriate and a syringe method is much better in the interests of accuracy and economy.

SP320 Spacote is the most recent development in the wood epoxy line and is ideal as an all round adhesive, filler and coating system with the added facility of being excellent for clear coating or where clearer glue lines are required. Whilst being slightly more expensive than SP106, SP320 Spacote can be regarded as more highly developed system. The mix ratio of 5:2 by volume of resin to hardener means that the correct mix is more accurately achieved than with the 5:1 SP106 System. SP320 Spacote is also significantly lower in viscosity and higher in strength making it more suitable for use with reinforcement fibres especially carbon and Kevlar.

As a general rule it is important with both systems not to deviate from the recommended mix ratio for any reason as a 10% deviation either way can greatly affect the properties. If pumps are used they should be cleaned out with "standard" epoxy solvent every week and the volume they dispense monitored periodically.

The volume dispensed from the pumps fitted to the resin bottles is 30cc whilst the appropriate pumps fitted to the hardener container is 6cc for the SP106 System and l2cc for the SP320 Spacote System. Both 50cc and 10cc syringes are available if the user wishes to dispense small quantities whilst half litre plastic graduated mixing cups are available for the larger quantities that one would use in coating applications. Since SP320 Spacote was specially formulated for coating applications mixing cups are a convenient measuring device for this system and strongly recommended.

The development of a monocoque "kit" version of the National Graduate had been something I had been thinking about for a few years but previous attempts by others using polyester resin and glass tape had not given me any confidence that it could be made to work with the Graduate whilst still retaining its attractive lines. However, after familiarising myself with the basic technique building a Streaker and then seeking inspiration from building a couple of Fireball dinghies, I applied myself with renewed energy to the Graduate.

The rising cost of building the boat by conventional methods using a pine stringers framework over a system of permanent and temporary bulkheads had lead to a severe reduction in new wooden boat registrations and the boat was just too expensive if any profit was to be made. It was for this reason mainly that I was prompted to develop the new kit version. Borrowing ideas from both the Fireball and the new breed of 'stitch and tape' boats such as the Streaker, Mirror and Miracle, the panel outlines of the new boat were traced from a conventionally built proven design and stitched and glued together with plumbers copper wire and epoxy over an I-beam central spine structure holding the stem, bulkheads, centreboard case, thwart and transom in their correct relative positions. A feature of the new adopted building method was the novel bow panel construction. The bow panels covered the area bounded by the fore bulkhead, stem, chine and hog. The solution which was adopted could probably be applied to other similar chined designs like the OK. Dinghy. The panels were pre-formed in a female G.R.P. mould from two individual plywood laminates 3mm thick in such a way that each panel would retain its necessary curvature. The panels in each "sandwich" were glued with an epoxy/microfibre/collodial silica mix which was applied to each component before being consolidated then bent, clamped and bolted into position in the moulds (one for each side of the boat). When the adhesive mix had cured for approximately 48 hours in warm conditions, the panels were released and trimmed accurately to the pattern size. The panels were wired, glued and screwed to the boat as a pair in order to balance stresses on the central spine structure. The new technique ensured that a consistent shape was produced every time with the minimum of wastage in the shortest possible time. All the panels on the Kit Graduate were butt jointed using an epoxy/microfibre mix and reinforced internally with low density fillet joints. When the panel angle exceeded 120 or so, the fillet was always reinforced with glass tape 75mm wide using SP106 epoxy resin mix.

So much for the applications, in this next section I would like to deal with the products in more detail.

The wood Epoxy System is, by definition, a very versatile product. Using the base resin and the option of one of two hardeners (slow and fast) and a variety of fillers, a "system" can be created which can be used as an adhesive of varying viscosity and thixotropy and filler depending on requirements. In addition a Wood Epoxy System can act as a matrix resin system for fibre reinforcement or behave as a coating. This is what we mean by a "Wood Epoxy System" rather than just a wood "adhesive".

When using a Wood Epoxy System as an adhesive the addition of microfibres gives the strongest mix. Microfibres are finely chopped, bleached, cellulose fibres which give added strength to the glue line. Their main contribution is to absorb resin and control its absorption into the surrounding wood fibres to help to ensure that the glue line is not starved of adhesive. It is important however that the glue line is not too tightly clamped. Clamping is not strictly necessary in the interest of adhesive performance as shrinkage, which is present with most other types of glue, although present to a small degree, is certainly not noticeable with epoxy. If a very absorbent material is being glued or if the joint has vertical faces then the addition of collodial silica (silicon dioxide) will add thixotropy to avoid sag as well as limiting resin penetration to save weight while still being adequately strong. When gluing hard woods with a high grain strength such as mahogany, penetration should be encouraged and therefore silica is not normally used.

Microballoons and glass bubbles (the latter sometimes called microspheres) are hollow bubbles which are less dense than microfibres, the former being phenolic and the latter being glass. They are often used in an adhesive mix where the surface area of the adherends is higher to offset the lower bond strength. Typical applications being cold moulding of wood veneers or where softer, less dense woods are being used and weight saving is important. This type of filler is not recommended for use with the most dense, stronger types of woods such as mahogany, especially in high strength laminating. When forming laminated knees or ribs for instance, it is microfibres that are added to the resin mix and not the bubble type of filler. A common application of bubbles is in low density fillet joints and filler mixes which have to be sanded smooth. An important feature of microballoons is that, being phenolic, they are capable of catalysing the resin and hardener reaction and speed curing. It is worth noting that this also means reduction in pot life and working time.

The useful feature of a Wood ' Epoxy System is that the viscosity is sufficiently low for it to be used with fibre reinforcement such as E-glass, often for reinforcing low density fillet joints, sheathing hulls, or, more commonly, foils.

The sheathing process, particularly when used on a low density timber such as yellow pine or obeche, not only adds thickness and strength but also toughens up the surface so that it becomes far more resistant to surface damage. Unidirectional fibres of glass (E-glass or better still R-glass which is 20% stronger) laid up under a bi-directional woven E-glass will add significant stiffness and strength. Whilst on the subject of making foils a useful tip is to incorporate a hard epoxy forward and trailing edge which will effectively resist damage and can easily be repaired. This feature is particularly important when very fine trailing edges are required. The technique involves first making a groove around the periphery of the foil before the profiling procedure begins but after the plan shape has been established from the square block. A router is usually used for this purpose set to a depth of approximately 3/16" and perhaps 1/8" - 3/16" wide. Once the groove has been made it is packed with a graphite/collodial silica/microfibre epoxy mix and left to harden overnight before being planed flush with the square edges of the board. After the board has been profiled to a hydrodynamic shape, the trailing edge can be planed to a very sharp, cut off, square section which aids efficiency. The leading edge can be rounded off in the normal way and will resist minor impacts, and whilst not being as hard as brass, you will achieve an edge which is much lighter and considerably easier to shape to fit in with the board's section shape.


These three detailed technical articles are written by Adrian Baker, who won several national championships in one of the British two-person National classes. He worked for a major British distributor of epoxy resin products, and the OK class in Britain was grateful for his willingness to share his professional expertise.