SP Guide To Wooden Foils
by Adrian Baker
Illustrations : David Boyes
Centreboards and rudders, otherwise known as foils, are items of high performance equipment. Attention to their design and construction should be aimed at producing foils which are as stiff and as light as possible, features which can be directly translated into 'boatspeed'.
There are a number of excellent builders who can provide ready-built foils with proven performance and a high standard of construction and finish. However foil construction is not beyond the scope of most amateurs with the correct knowledge. Although the design stages require some appreciation of hydrodynamics, particularly for those classes where there are few design restrictions, many classes have foils which require the minimum of shaping and are hence much easier. For a moderately skilled amateur, tackling this work can be very rewarding, especially if he uses the correct materials and has a few basic techniques at his command.
Wood is still the most popular material for foil construction since it is readily available, relatively inexpensive, can be easily worked and takes adhesives and finishes well. Moreover, being a unidirectional fibrous material wood has a good stiffness-to-weight ratio and can be adequately protected from moisture by high performance coating systems.
2. Choice of Timber
Choice of wood depends on one's exact requirements. The simplest form of construction is to use a dense, stiff timber such as Honduras mahogany (Kyaha) which is often simply coated with epoxy to provide a waterproof 'plastic' coating - often termed 'wood epoxy saturation
Considerable weight savings and stiffness gains can be made by using a lightweight wood, such as Western Red Cedar, to serve simply as a 'core' and sheathing the surfaces with a suitable fibre reinforcement in an epoxy resin matrix. In this case, the main strength and stiffness in bending is being provided by the skins and is far greater than that contributed by the wood. These foils have the highest stiffness-to-weight ratio and are used in most of the high performance racing classes.
Wood Stiffness and Density
Table 1 compares the stiffness (given by Youngs Modulus (E) parallel to the grain), with density, for various wood species.
|Wood Species||Youngs ModulusE, |
parallel to grain (N/mm.sq)
(stiffness per unit weight)
|Western Red Cedar||8403||320||26,3|
3. Preparation of the Board
For any foil, once the wood has been selected, it must be cut to size using a jigsaw. A single piece of wood cannot be used as a foil blank as it will warp. For high performance and long term durability, the foil should always be of laminated wood construction. This takes the form of narrow (approx 38-50mm) strips fastened together with adhesive. This type of construction avoids the warping normally associated with wood but particular attention should be given to the correct orientation of the strips.
Briefly the procedure is as follows:
- Cut the timber plank into strips of the required width and ensure that they are kept together in the same sequence and orientation as when cut. Mark a series of arrows on each strip in felt tip pen all in the same direction down its length at regular intervals.
- Cut the strips to the required length corresponding to the foil length and lay edge to edge alongside each other loosely in the clamping system with all marked surfaces face up.
Then perform the following operations:
- Turn each alternate strip 180° so that adjacent strips have arrows pointing in opposite directions, but with every arrow still on the uppermost surface (fig. 1)
- Then turn each alternate strip upside down.
Each face of the foil should have alternating blank and arrow-marked strips, with all the arrows on one face pointing the same way. The strips can now be glued together.
4. Recommended Adhesives
Because of the considerable gap-filling properties available when using an epoxy adhesive, strips may be clamped together and glued without the need to plane the edges. The adhesive is sufficiently cured after 48 hours to proceed to the next stage. After cleaning off the excess adhesive which will have squeezed out, the laminated board is passed through a "thicknesser' at the required setting.
Epoxy adhesives are far superior to other types for this type of construction. One of the following SP resin sytems suitably thickened to the required consistency are commonly used:
SP106 wood epoxy system with SP209 Fast Hardener
SP320 High performance wood epoxy system with fast hardener SP320 Fast
SP110 epoxy laminating system with SP210 Fast hardener
SP120 epoxy adhesive with SP220 Fast hardener
After the resin and hardener have been mixed in the correct ratio (usually by volume) SP Microfibres are added at the rate of approximately 30-50% by volume to give additional gap filling properties. The exact percentage is not critical and will vary depending on a number of factors including the degree of absorbancy of the timber. However, the final mix on the wood should be of a 'thin slurry' consistency.
If the foil is to have additional reinforcement, the thickness of the reinforcement must be allowed for. For centreboards particularly, class rules should be consulted as it is beneficial to use the thickest board possible which will fit the internal dimensions of the centreboard case.
The plan shape must now be established. Centreboards usually have a tapered plan shape and in order to keep the chord width to thickness ratio relatively constant, it is worth tapering the thickness of the board towards the tip, proportional with the taper of the plan shape, using a plane (hand or electric) (fig. 2). This operation saves weight without compromising stiffness. After ensuring that the edges make a perfect right angle to the faces, a centreline can now be drawn along the edges around the periphery of the board. Scribing from both faces with a gauge and averaging the centre will compensate for any thickness variation over the length of foil (fig. 3). This scribing technique can also be used on boards of constant thickness since there is usually some slight thickness variation.
Section shape is one of the most important contributing factors in determining foil performance. The most commonly used sections for both centreboards and rudders are NACA sections of the 00-series where the maximum chord thickness occurs approximately 30-33% from the leading edge (fig. 4). Some have found that sections of other series work well and there seems to be no one universal section that has been found to be superior for all classes. In order to obtain the correct section throughout its length it is worth marking 'stations' down the length of the board and determining the thickness at each station. Making a 'Formica' template to fit each station certainly helps to keep each side symmetrical and maintains an accurate section. The templates are worth keeping for future use in order that the section may be checked during the life of the foil.
For centreboards, after determining accurately the position of the bolt hole, the keel line should be marked on both faces since it is below this line that the foil is shaped to a streamlined underwater section down its length.
Most professionals work to a series of construction lines marked on each face and down the edges planing off the excess wood to one set of lines from the edge. The board is then turned over and worked to the corresponding set of lines on the other side until the correct shape is achieved. It is worth noting here that the lighter woods such as cedar, are much easier to work than the harder more dense types such as Kyaha. The final shape is achieved with abrasive paper (60-80 grit aluminium oxide production paper) usually fitted to an electric flat-bed orbital finishing sander. Approximately 0.5-2.00mm can be left on the trailing edge if the board is to be sheathed, but up to 4mm should be left on other boards as it is this edge which is very vulnerable to breakage. Techniques for achieving additional strength over this vulnerable area are discussed later in this document.
6. Sheathed Foils
The simplest type of reinforcement used is woven E-glass fabric. In more precise terms this is bidirectional, woven E-glass fabric where the bundles of fibres are woven at 0° and 90° to each other. A plain weave style is usually chosen and is adequate for most general sheathing work. Whatever the additional stiffness required down the axis of the foil, at least some fibres should run across the chord at 90° to the main axis to give some stability to the fine trailing edge. It is common therefore to use the warp down the axis of the board with the weft running across the chord. Usually a balanced woven fabric is used where the number of warp and weft bundles is approximately the same therefore giving equal strength in both directions. It is quite common however, to have the warp slightly stronger than the weft and the fabric is therefore described as 'unbalanced'. Typical of this type is RE210 (210gm per sq.m. woven E-glass). Whilst one layer (ply) of such fabric provides little in the way of traditional stiffness, it does add to the compressive strength of the wood fibres and gives good protection against abrasion and damage when incorporated into an epoxy matrix.
Significant stiffness benefits can be readily obtained by using unidirectional fibres of glass (E or R glass) running along the length of the board. Table 2 compares the Youngs Modulus, E (bending), and density for these types of reinforcement.
|Reinforcement||Youngs Modulus E||Density Kg/Cu.m||Specific Stiffness Value|
Referring to table 2 it is immediately apparent how much stiffer than wood these fibres are (see Youngs Modulus,E). The penalty however is weight and hence it makes sense to use a low density, less stiff material such as wood on the neutral axis (functioning as a core) and employ skins of relatively dense, high modulus fibres on the exterior for stiffness - so forming a 'sandwich beam'.
7. Theory of Bending
(i) For Plain Wood Foils
Bending stiffness (N.mm.sq) = Ew x t³
Where Ew Youngs Modulus for the wood and t thickness in mm.
If the thickness t is constant, then the bending stiffness is a direct function of Ew. It therefore follows that the higher the stiffness 'E' of the timber, or the greater the thickness 't', then the stiffer the foil will be.
(ii) For Sheathed Foils Using Unidirectional Fibres
In the case where the two faces are sheathed using unidirectional fibres, the stiffness is largely determined by the type of reinforcement used since the contribution of the wood towards the total stiffness achieved is not significant -compare Youngs modulus E for wood (Ew) and reinforcement fibre (Ef).
If y = distance (mm) of the reinforcement from centreline
A = area (mm.sq) of 1 layer of reinforcement
Ef = Youngs Modulus for the fibre
Then bending stiffness Ef.l
Where l is approx = 2 x Ay2
For twice the sheathing thickness or twice the reinforcement weight, A becomes 2A and bending stiffness is therefore doubled. The distance 'y', of the reinforcement from the centreline is also important and the further apart the reinforcement is placed, the higher the stiffness value.
As a consequence unidirectional reinforcement is therefore generally located over the area of greatest chord thickness; between 50% - 75% is typical (fig. 5).
8. Using Unidirectional Fibres
The following fibres from the SP Astran 1 range are appropriate: -
E-glass: AE250 (500g/m.sq)
R-glass: AR250 (250g/m.sq)
R-glass: AR500 (500g/m.sq)
Also included is RE430 (430gm/m.sq) E-glass woven fabric, which is so heavily 'biased' (or unbalanced) in the warp direction (10:1) that it can be considered as a unidirectional material.
To protect the unidirectional fibres against damage they are usually placed in addition to and under a bidirectional E-glass such as RE210. This layer will also give some additional stiffness across the thinnest part of the chord. If the unidirectional fibres do not extend 100% across the chord down the length of the board, some wood is usually removed down to a depth corresponding to their total thickness (perhaps only 0.2-0.5mm in depth) to form a shallow 'trough' in which they can be laminated on to the wood.
For tapered plan centreboards, up to two layers of unidirectional fibres are commonly used, each starting at the top but extending down and finishing at different points (fig. 6). The first may extend approximately 1/3 of the way and the second layer 2/3 of the way down the board. Stiffening the remainder is usually considered unnecessary as the loads at and near to the tip are relatively low.
When using either unidirectional or bidirectional reinforcement, a low viscosity epoxy laminating system will give the easiest handling characteristics and superior results.
9. SP Epoxy Laminating Systems and their use on Wooden Foils
SP320 with SP320 slow hardener
SP110 with SP210 standard hardener
Full details of using these epoxy resin systems with E-glass reinforcement fabrics are given in SP Systems "SP Sheathing Guide". Although this is principally a guide to using bidirectional E-glass similar principles apply when using unidirectional fibres.
We recommend that the unidirectional fibres are applied first and covered with SP peel ply over the entire surface. The peel ply serves to improve the surface finish through consolidation, economise on the use of resin and provide a clean textured surface for subsequent operations. When the resin has cured sufficiently to become hard, the peel ply can be ripped away and a very light coat of epoxy filler mix applied. For this, either of the above resin systems is appropriate as a resin base to which SP Microballoons should be added. An alternative is to use the ready mixed two-part product SP S-Fill. The use of filler is only appropriate to boards which are to have a pigmented or painted finish.
Once the surface of the board has been faired to the original profile, (check using the template) the bidirectional E-glass can be applied over the whole surface).
For laminating the unidirectional fibres it is preferable to operate on one side of the foil at a time and allow this to harden before attempting the other side. This operation is usually carried out with the board 'face down'. However, for laying the bidirectional reinforcement it is easiest to work with the toll clamped at one end (the handle end for centreboards) so that the leading edge is horizontal (a Black and Decker Workmate is ideal for this) (fig 7). In this attitude both sides can be sheathed simultaneously and the fabric simply draped over the leading edge and allowed to fall away to the trailing edge along which it can be trimmed. The board is laminated in this way for three reasons:
- To preserve the correct section shape by covering the leading edge
- To give the necessary reinforcement to the leading edge
- To enable a hard-wearing trailing edge to be easily achieved
On centreboards with straight leading edges, a plain weave fabric RE210, is ideal as the final layer. With an elliptical leading edge, wrapping a plain weave fabric over this may prove difficult and it is preferable in this situation to use a woven fabric of a similar weight which will more readily conform to this type of shape. Suitable fabrics are RE185 and RE166 (both 165gm/sq.m. E-glass) or RE292 (290 gm/sq.m) E-glass. Both the RE166 and RE292 have 4-hardness crowsfoot weave styles and RE165 is a Twill.
10. Leading and Trailing Edges
Additional protection can be given to the edges, whether foils are sheathed or not, by using a simple technique already being used by some builders.
When the plan shape has been established and the board has been cut out with a jigsaw, but before profiling has started, a groove can be incorporated around the periphery using an electric router set to a depth of approximately 5-8mm. A 3-6mm router bit will control the width. The groove is then filled with a thickened epoxy mix (usually epoxy resin system plus colloidal silica and graphite powder). This serves to give a very hard wearing and easily repairable edge. After leaving the mix to harden, the excess material can be removed with a Surform or plane to leave a smooth edge down which a centreline can be scribed before shaping. After shaping the foil to a streamlined section, the reinforced edges will become more apparent and the trailing edge can be shaped to a sharp point while still maintaining sufficient strength. The foil is then sheathed or coated.
Alternative systems usually concentrate on reinforcing the aft edge only, since this is the edge most vulnerable to damage. If the sheathing fabric is extended and trimmed off some 4-6mm beyond the edge of the wood on both sides, the enclosed space can be filled with an epoxy filler mix which can be finished in the usual way.
Sometimes this trailing edge is composed only of unreinforced resin which is often considered to be sufficiently strong, especially if the aft edge of the wood is left thicker (usually a 2-4mm flat section is chosen).
11. Finishing Systems
For foils requiring a clear finish the sequence of operations would be:
- "Flow" coating (instead of filler stage previously described) - SP320 Spacote with fast hardener.
- Polishing, or varnishing with 2-pack polyurethane varnish - SP Ultravar and SP Fastcote.
For pigmented finished, (usually white is chosen) the sequence would be:
- Application of Filler - either chosen SP resin system and SP Filler powder, or SP S-Fill.
- Application of epoxy Hibuild undercoat - SP302 Hibuild
- Polishing, or painting with 2-pack polyurethane paint.
Individual data sheets on all SP products and additional information is available from SP Systems.
SP products are available at chandlers and yards throughout the UK or direct from SP Systems.
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.