Building Foils

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This document is compiled by an amateur, not a professional. It has been compiled in good faith, but almost certainly contains errors and inaccuracies. “Best practice” also changes frequently with changes in technology and materials. None of the procedures listed are guaranteed to work, and some or all of them may be hazardous. If you feel unable to take responsibility for your own actions and errors and therefore may resort to litigation then you are advised not to read it, let alone build anything based on information here. In any case you are advised not to build a foil without someone experienced in the materials to contact for advice.

The best core material is probably high density PVC foam (200 kg/m³). If you're really keen it could be 130kg/m3 near the bottom. Failing that (and HD foam cores can be a hassle to work for the less experienced - you'll need very sharp tools), prepare an even thickness blank from timber strips (Western red cedar) approx. 50mm wide. Organise the strips so that alternate ones are turned end for end so that the grain runs in different directions in order to reduce the effect of any warping of the timber. This can even happen long after the foil is fully glass coated and complete, which is most unwelcome! Bond the timber strips with epoxy, and when its cured plane the blank to an even thickness. Cut the blade out to the required profile, but make it 5mm smaller than the intended finished size at trailing edge. (Apart from anything else this ensures that minor damage doesn't expose a wood core) Shape the foil, allowing about 1.5mm undersize for the laminate, fairing and painting. Make the trailing edge as sharp as you can, because the fibres will overlap here to create the true trailing edge.

Use skins of unidirectional carbon and woven carbon cloth, to give lowest weight and maximum stiffness. Start with layers of 200 g/m² unidirectional carbon, one layer over all of the foil, plus a second layer of 200 g/m² unidirectional carbon over the top half, and a third layer of unidirectional carbon, approx.150mm wide, over top part of the foil, extending approx.100mm past the bottom of hull when the foil is right down. Then add one layer of 200 g/m² woven carbon cloth, and finally a layer of 86 g/m² glass cloth, both covering all of the foil. If you wish you can use white pigment in the top layer to enable you to produce a white foil without painting it too. Resin choice inevitably varies with temperature and speed of working, and probably what you've got left over from the last job too. SP320 slow is fine, as is Ampreg 20, although with the latter you'll want to use approx. 50/50 fast and normal hardener in summer, more fast hardener in winter, probably near all standard on a hot Australian summer day! Consult the data sheets for information about recoat times and so on.

Suspend the blank with the leading edge horizontal by means of screws in timber supports at the head and tip of the board. Cut the carbon oversize, approx. 50mm wider than the board. Using a roller, wet out the board with resin, apply the UD carbon, aligning the fibres along the leading edge. Wet out the fibres with epoxy, leave a few minutes to soak in and then wet again. Next comes the layer of carbon cloth. Again wet it out off the foil, wait a few minutes, and the roll on more resin. Finally add the layer of glass in the same way. Squeegee the excess resin away, remove air from the laminate, then squeeze along the trailing edge overlap to remove air. Check for bubbles under the glass, and squeegee out. After approx. 1 hour its time to add filled epoxy which will be used to fair the foil. Make up a reasonably runny filled epoxy mix that will roll on nicely. If you're going for a pigmented foil then use glass bubbles and white pigment. If you're going to paint it anyway leave out the white pigment, and if you have ambitions for a clear finished foil (your laminating better have been really neat if you're attempting this) use silica in place of glass bubbles. Anyway roll on a coat of filled epoxy and repeat twice more ( 3 coats resin/glass bubbles). When the layup is part cured (still a little flexible, but no more than that) trim the excess glass from the head and tip with a sharp knife. Leave the foil to cure overnight Remove the supports and screws, and trim the trailing edge to the finished size, using a jigsaw with a carbide tipped blade. Trim and sand the head and tip areas. Now its time to glass coat them. Apply 2 layers x 200g/m² glass to the head edge, and 3 x 200 g/m² cut on bias at ±45° around the tip of the foil. Cure overnight. File / sand all the edges to shape. Apply 2 coats of resin/glass bubbles to the head and tip. Cure overnight, and then the next day post cure the foil at approx. 45°C for 3-4 hours (A wooden box & a fan heater does this nicely).

Now the hard work begins! Start by spraying on a guide coat ( car paint ) of some contrasting colour with the layup (bright red perhaps). Sand all the paint off! You should have enough filler to sand pretty aggressively (Andy Paterson uses a belt sander with 40 grit abrasive) without cutting (much) into the glass. On no account whatsoever cut through the glass, and cut into it as little as you possibly can. If it goes black stop immediately! Now suspend the foil with the screws and blocks as before, leading edge up. Finish the foil with 2 coats of neat epoxy (with white pigment if appropriate. Leave the first layer to cure for approx. 1 hour, then a 2nd coat. The epoxy will fill and flow over the big scratches. When this has cured fill suspension holes, and apply 2 coats of epoxy on the head edge. Apply another guide coat of paint and sand, sand, sand, the finish coats. Wet sand it starting with 100 grit, working through 180/280/400 and then finish off with 600 wet sanded to give smooth matt finish. Most people think this is all the finish required, but feel free to carry on with 800 grit and 1200 grit and then an abrasive polish to get a real mirror finish. It will last at least a day on the beach!

Based on Article by Jim Champ August 1999 (With considerable assistance from Andy Paterson of Bloodaxe Foils)