Fabrication Overview (or, “Why These Things are Expensive”)
There’s no getting around it – well built foils for your sailboat are expensive. I’m doing my best to “work smart” and minimize the labor required, but there’s only so far you can go without cutting corners.
Here’s a quick overview of the fabrication process:
- For a wood core, rip the plank into strips and flip every-other-strip end-to-end and epoxy back together. This creates a blank that is more resistant to warping than a single plank would be. This also provides an opportunity to go from a flat-sawn plank to quarter-sawn strips (these terms indicate how the grain runs down the strip – quarter-sawn is stronger).
- Drill any through-hole locations oversize, and fill with thickened epoxy. This will stop any moisture from migrating into the core, and prevents crushing of the core. (For foam cores, this is step 1)
- After the epoxy cures, shape the blank into the desired shape of the foil core . You can use hand planes, power planes, grinders… to shape by hand while comparing to templates. Instead, I use a CNC milling machine, which produces a very accurate shape – and I can be doing something else while the machine runs in the background.
- Layup any fiberglass/KevlarTM/Carbon Fiber skins, using a proper laminating epoxy, and then wrap in peel-ply, a breather layer, and vacuum bag the whole thing. Vacuum bagging will:
- ensure that no bubbles form from the core off-gassing
- ensure the fabric bonds completely with the core
- draw off any excess resin, keeping the total layup as light and stiff as possible.
- Apply a top coat of epoxy, with a UV-resistant hardener** (usually mixed with an easily sandable thickener; the exception is foils that will be done in a clear finish).
- Sand the foil to the desired level of smoothness, and repeat step 5, until the foil is smooth and fair.
- Prime (high build primer followed by epoxy primer/sealer), wetsand, and paint.
- Post-cure the final product at an elevated temperature when appropriate. This increases the hardness and stiffness of the epoxy resin.
That’s about it. Well, except for carefully packing for shipping, invoicing, ordering raw materials,…. and the many other details required to keep a small business viable.
Do sailboat manufacturers go to all this effort in making their foils? Some do, but most don’t. And while a good foil _can_ be made in a mold, to do it right is another long involved process. There are no short cuts.
** a word about UV protection. Epoxy does not like ultra-violet light, and it will degrade over time if exposed to sunlight. I use a UV-resistant hardener in the final coat of epoxy that goes on the foil. This is fine for centerboards and rudders that are left under a boat cover when you aren’t out sailing. For foils that are going to be constantly exposed to sunlight, they should be painted or varnished.
The paint used in the shop at this time is Imron(TM) 2 part polyurethane. I keep white in stock, and will order other colours upon request (sorry, but you have to pay for the whole quart if you want a custom colour). Imron is for use above the waterline, but makes a fine paint for foils that aren’t constantly immersed. The immersed portion of keelboat rudders is primed with Awlgrip(TM) 545 epoxy primer and prepared for the customer to finish with their choice of bottom paint.
The goal is to provide a smooth and fair finish. We do not apply so much epoxy as to significantly impact the final shape, or to add needless weight. With a graphite finish, after sanding, you can expect the finish to be somewhat translucent and you will see glimpses of the underlying materials. An example is shown below.