Care & Feeding: Wood Cored Foils

 by Phil Locker, Competition Composites

There was a time, not that long ago, when the typical sailboat foil was shaped from a plank of mahogany or similar wood, varnished, and then you went sailing with it. If you were lucky the plank was ripped into strips and glued back together with quarter-sawn (vertical) grain, each alternate strip being flipped end to end to resist warping of the overall structure.

[For the sake of simplicity, the term 'foils' in this article refers to rudders, centerboards, and daggerboards]

The foil would swell or shrink a bit with changes in moisture content; it was expected, and nobody got too fussed about it. These dimensional changes were fairly uniform across the foil as there were few materials used in its construction (just the wood, glue, and varnish).

Few serious racers today that would use such a simply constructed foil unless forced to by class rules. Most foils today are a composite of a laminate (fiberglass and/or carbon fiber and/or aramid) over a core (most commonly foam or wood). Composite construction gives a potentially lighter, stiffer, and stronger product that’s low maintenance and dimensionally stable.

To steal a phrase, wood is now an exotic material. Good clear planks of tight grain are rapidly increasing in price as supplies dwindle. It is more time consuming to work with (the blanks must be strip laminated prior to machining, and can require machining at a slower rate than a softer foam product). And a wood core encapsulated within laminated skins must be treated with care to ensure that no moisture can get to it.

 

So why would anyone in this day and age build foils with wood cores when it is so much simpler to use foam instead? There are a few reasons, the two big ones being:

  1. Weight: there are many sailboat classes that specify minimum foil weights. Some of these date back to the days of varnished mahogany and have never been updated. The most reasonable way of getting these foils up to minimum weight is by using a wood core (something heavy like Mahogany or Douglas Fir). The fiberglass skins can be fairly thin then as they’re primarily used for water-proofing rather than contributing significant strength.
  2. Mechanical properties:
    • Stiffness: the stiffness of a beam goes up with the cube of its thickness; if you are restricted to a thin foil, then it is hard to beat the stiffness offered by a wood core.
    • Flexibility: seemingly at odds with stiffness, wood is also quite flexible (within reason). More to the point, wood cores can be flexed over and over and over without fatigue.
    • Compressive strength: the most common place for a foil to fail is in point load on the compressed side (such as a daggerboard entering the hull, or at the lower fitting of a transom-hung rudder). If the point load is high enough to compress the core, the skin will buckle and the unit will immediately fail. Wood has amazing compressive strength for its weight. This is the number one reason why some of Phil’s Foils’ highest performance products will have at least some wood in the core.

Now that we know WHY wood is still used, what’s to be done to make sure your wood cored foil(s) stays in shape over time? You must be a stickler for detail when first installing your new foils, and later with maintenance down the road, or BAD THINGS WILL HAPPEN.

Really, the worst thing you can do for your foils is put them in the water. But since that’s going to happen, we have to stop the water from getting into the core. Every single hole you drill into your new foil must be drilled oversized, filled with epoxy, and then re-drilled to the proper size once the epoxy has cured. To do it right, first coat the inside of the (oversize) hole with unthickened epoxy, let that soak in, and then fill with thickened epoxy. Yes, even if you’re just putting a pull-up rope into the handle, or screwing on a fitting, you MUST do this. Or BAD THINGS WILL HAPPEN. Regularly inspect your foils for nicks and dings that might have compromised the skins, and fix them right away. A casual look at the centerboard during a capsize is not good enough! Also remember – wood is an organic material. It moves, especially with changes in humidity and temperature. Don’t bake your centerboard in the trunk of your car, and don’t leave it to freeze in the centerboard trunk over the winter. If keeping the boat in the water, use a proper barrier coat before applying antifouling. A bit of care goes a long way.

What are these BAD THINGS? Composite products are exactly what their name implies – a composite of different materials, each of which reacts differently to changes in moisture content – a worse situation than in the simple days of varnished mahogany. Odds are that the humidity in your boat park (or worse, in your lake) is not the same as the humidity in my workshop on the day that I laminated your new centerboard. So even though the wood was stacked in the shop for several months and quite stable in moisture content at the time the centerboard was constructed, any exposed end grain is going to act like a sponge and soak up moisture if at all possible. Wood strips are now bonded with epoxy. This soaks a small distance into the wood at the bond, resulting in stable glue lines. But the wood in between will swell. The result of moisture ingress will be distinct waves along the sides of the foil. While the fiberglass laminate is happy to “go along for the ride” on the flat port and starboard sides of the foil, at the edges it is being asked to stretch. Fiberglass doesn’t stretch, it breaks.Don’t underestimate the power of a swelling wood strip, it CAN break the laminate. Then more water gets in and the whole process snowballs.

Or if you’re in an extremely dry environment (or cooking the centerboard in the trunk of your car) moisture can be driven out of the core instead and you’ll end up with shrinking (cupping) of the wood strips.

The fix involves sanding it down to bare laminate, giving it several months for moisture content to stabilize (preferably in your air conditioned basement), then resealing the core and performing any necessary repairs to the skins. Then re-fair and paint again. A slow process and costly if you have to pay to have it done. It is much easier to ensure that the problem never happens in the first place.

One last thing – in the long term you may find that you have some minor cupping or swelling of the wood strips no matter how well you’ve maintained the board. It does happen, and it happens to every builder of these things. Contrary to popular belief, epoxy is not 100% waterproof. Over time water vapor can migrate through the skins and get into (or out of) the core. Then its time for a light sanding and repaint – usually by then the foil will have stabilized and will not change further.

Exhibit A: the customer filled the centerboard pivot hole, but the mix was “hot” and cracked through to the core. That’s all it took for bad things to happen.Look at the swelling around the pivot hole.
Look at the swelling around the pivot hole.
Exhibit B: hole drilled for a pull up line, but not sealed. More swelling of the core here.

Exhibit B: hole drilled for a pull up line, but not sealed. More swelling of the core here.

 

 

Competition Composites Inc. is a fabricator of custom sailboat boards and rudders.