Foiled Again  
By Gary Blankenship & Helen Snell - Tallahassee, Florida - USA

Egads, foiled again.

That irreverent thought kept flashing through my brain as I ground, sanded, puttied, and faired my way through a revision of Oaracle’s leeboard and rudder. In my mind, of course, were two recent articles in Duckworks that had addressed getting airfoil shapes on leeboards, centerboards and rudders. Based on those examples, my goal was to achieve a foil shape to improve performance – and maybe durability – without too much pain and aggravation, but taking a different route than those suggested by the other two worthy articles.

Some background. Oaracle is a Jim Michalak-designed Frolic2. Like all the small boats I have built, its foils are a bit rounded at the leading edge, a bit tapered at the trailing edge, and slab sided in between. Sure airfoil shapes are supposed to be more efficient, but exactly how much and whether it would justify a lot of potentially nit-picky (i.e., stuff I’m not that good at) work was uncertain.

click to enlargeHere’s Oaracle back before I made the modifications to the rudder and leeboard.

Four factors combined to lever me out of slab-sided complacency.
The first was Tom Hamernik’s excellent foil programs available free through Duckworks. Tom has released two versions and either works. All you have to do is specify what NACA section foil shape you want and the width and thickness of your board and, voila, you get a graphed cross section. Neat.

Second was Warren Messer’s recent article on how he made NACA 10 section foils for one of his boats using 1/8-inch plywood and thickened epoxy. For the first time, doing a proper foil shape seemed doable. But I saw two problems. One is 1/8 waterproof ply is nonexistent where I live and shipping in a piece or two doubles the price. The other is I already had a leeboard and rudder and it seemed wasteful to start from scratch on new ones instead of shaving down the existing ones. (When Pat Patteson’s excellent article appeared about making hollow foil-shaped boards, I was already well along on my modifications. Besides, there still was the problem of wanting to use the existing boards. . . .)

The third impetus was some conversations with noted small boat designer and sailor Matt Layden at this year’s Everglades Challenge and Cedar Key events. I had observed that the leeboard and rudder on Matt’s Sand Flea, an innovative 8-foot punt, had foil shapes. In response to my queries, Matt said doing a foil would not produce a huge improvement, but in his opinion was worth the effort. He also agreed that it was practical to shave down the existing leeboard and rudder, rather than start from scratch with new ones. Since there are several varieties of NACA foils, I asked for his recommendation. Matt said a NACA 9 section for the leeboard and a NACA 6 section for the rudder would be his choice. His final advice was not to obsess about getting micromillimeter precision, but just to get close. (Such accuracy is beyond my abilities anyway.)

The fourth reason has nothing to do with a foil shape but rather protecting the leading edges of the fins. Tom Lathrop, who helped Graham Byrnes build his winning design for the 2007 Everglades Challenge, wrote (on the forum section of about epoxying a resin-saturated Dacron rope on the leading edge of the centerboard. He said he had been doing it for 20 years to the leading edges of centerboard, rudders, and the like and never had to replace one. (You can find his account here) It sounded worth trying. (John Guzzwell, in his book on cold-molded boat building, recommended sheathing the hulls with Dynel cloth, which is polyester, not fiberglass. He reported the Dynel, while adding little strength, could take an amazing amount of abrasion.)

The project was begun with some trepidation but it was much easier from start to finish than I anticipated. To be fair, I neither tried nor achieved micromillimeter precision. But by my standards, it did come out much better than expected.

The first part was to call up Tom Hamernik’s programs. This was sheer simplicity. Simply pick the NACA section you want, then plug in the thickness and width of the leeboard or rudder. You instantly get a graph of the half cross section of the foil, that exaggerates the thickness to better show the curve. The graph used to make a full size template, using either thick cardboard, balsa wood, or thin model-making basswood ply.

click to enlargeTom Hamernik’s foil program prints out a graphed cross section of the chosen foil shape, with the thickness exaggerated. Here’s the graph and the ply template that was made from it. One was done for the leeboard and another for the rudder.


click to enlargeHere’s the template on a flat surface...


click to enlarge... and on the original, slab sided leeboard. If you look closely at this shot, you can see it’s quite a ways off the foil shape.

Next is the nerve-wracking part. Using the tool of your choice – hand plane, power plane, grinder, drawknife, etc. – carve down the foil to match the template. Even though the leeboard and rudder were covered with a layer of fiberglass, I planned to used my old power planer, having learned it can go through a layer of glass with little problem. But the planer chose this moment to wear out, or at least indicate it needs a new pair of brushes, which weren’t on hand. So it was down to the local discount tool emporium. I picked up a small angle grinder with a sanding disc attachment for much less than the cost of a replacement planer. The 24-grit sandpaper proved more than a match for the glass and laminated ply of the foils.

click to enlargeAfter only a few minutes with the angle grinder, quite a lot of wood has been removed.


click to enlargeThe board is nearly ready for glassing on both sides.

The hardest part of shaping the foils was to remember to stop every couple minutes to check with the template to make sure I didn’t go too far, too fast – that grinder really removed the wood!

click to enlargeUse the template to check the developing shape frequently. Also take come care to ensure you’re not skewing the shape by leaving it too thick or thin at the front or back edge. That shouldn’t be too hard if you’re working with a new, unpainted boat as you can see the centerline of the glued up ply layers (at least if you have an even number of layers). This board had been painted, so some care was needed to make sure the two sides came out symmetrical. In this shot, the shape is getting pretty close.

(If you do take off too much, you can fair over the error with thickened epoxy.) It took about 90 minutes using the grinder and finishing with a belt sander with 60-grit paper to shape the leeboard and about half that time on the rudder. And that was making a point to frequently check the shape with the templates.

Because a significant amount of wood was removed from the leeboard, I decided to reglass it with two layers instead of one. No scientific measurements were taken, but it was interesting to see how, as each layer was laminated to each side, there was appreciable improvement in the board’s stiffness (and, hopefully, strength).

Also I decided to vary the finishing details of the leeboard and rudder out of curiosity. On the leeboard, I did all the glassing, and then cut back the leading edge and attached the Dacron rope. On the rudder, I attached the rope first and then glassed the foil, trimming the fiberglass even with the rope/board joint.

click to enlargeOn the rudder, I used a hand plane and belt sander to put a flat face on the leading edge, ready to take the epoxy saturated rope.


click to enlargeThe leeboard was glassed before the leading edge rope was attached. The lines from the layers in the ply show up better here, and help guide the shaping. These lines should have been a little straighter, and I did some filling with thickened epoxy after the board was glassed.

As long as you’re available to trim the glass with a sharp knife in its “green” stage (not sticky, but not at its final hardness) I don’t think it matters which way it’s done. If you’re not going to be around to trim the glass when its green, I’d do the glassing and then the rope. On the leeboard, I used a circular saw to cut a flat face on the leading edge of the board, and a power sander to even up the cut and extend the flat face around the curved edge on the bottom of the board. On the rudder, I used a hand plane and finished with the belt sander, and liked that method better.

In his writeup, Tom Lathrop said he used 3/8-inch rope. He didn’t mention the thickness of the board, but it looked about an inch and a half. Oaracle’s leeboard is one-inch thick and I spent an unnecessary amount of time obsessing which size rope I should use, finally settling on 1/4-inch. Unfortunately, I cut a bit more off the leeboard’s front edge than anticipated, and the 1/4-inch seemed a bit too small, so I grabbed a scrap of 3/8-inch and that seemed like a good fit. It then dawned on me that the size wasn’t that critical. Anything from 1/4-inch to1/2-inch probably would have worked well, just so it matches the size of the flat face cut into the leading edge.

Tom recommended using soft, braided Dacron (he said nylon would also work but he prefers the polyester), and I’m assuming he meant the single braided kind that has about 12-strands. I missed that detail until after I had attached the ropes. The leeboard got 3/8 inch double braided rope, the cheap stuff now commonly available at hardware stores or the big box home improvement outlets. Make sure you check, some of these ropes are polypropylene which might not saturate as well. The rudder got a scrap of 1/4-inch yacht-grade double braided rope that was lying around. The double braids are harder ropes, and probably didn’t mold as well to the leeboards (see below) as the softer rope would.

I used a somewhat simpler method of attaching the rope; you can go to Tom’s write-up to see how he did it. First before any epoxying is done, the ropes were cut to length. Then a flat headed nail (not a finish nail) was pushed through the center at the top of the rope.

click to enlargeUse a nail with a flat head to hold the rope in place for a dry fit.

Hammer the nail into the board at the top of the flat area on the leading edge just enough to hold it in place, being careful to get it centered. Stretch the rope down the face and put in another nail at the bottom, again centering the nail and the rope on the flat edge. The tension will hold the rope in place; it’s not necessary to put a groove in the flat face. It’s also not necessary to stretch it bar tight, just do it enough to hold the rope straight and in place.

Now remove the rope from the board, leaving the nails in the rope. The holes in the board will mark where they need to be replaced. Cut some plastic sheeting that is longer than the rope, and wide enough to drape over the leading edge of the board and several inches down each side. Use duct tape to fasten one edge to the board, and then fold it back, exposing the flat leading edge.

click to enlargeHere the rope has been dry fitted with a nail at each end holding it in position, and the plastic has been taped on one side, ready to drape over the soon-to-be-epoxy-saturated rope.

Now mix up some epoxy and dunk the rope in it. I used about 8 ounces for the 4-foot long 3/8 inch piece for the leeboard, and probably could have got by with 4 to 6 ounces. Poke the rope with a stick for a couple minutes to make sure air bubbles are disbursed and the rope is thoroughly saturated. Take the rope out (wearing gloves!) and place it, like you did before, on the leading edge. Make sure you don’t hammer the nails all the way down – you’ll want to remove those when the glue dries (well, I suppose you could use a Monel and bronze ring nails, but I prefer to take them out). Now drape the plastic over the saturated rope, pull it down firmly, and tape it in place. It helps to have precut the pieces of tape; trying to tear strips of duct tape with glue-covered gloves is a frustration best left to others. You’ll need pretty long pieces to fit over the bottom of the board where the rope goes around the curved leading edge.

click to enlargeThe rope has been removed, saturated with epoxy, and replaced. The plastic is draped over the rope, pulled tightly down, and taped firmly in place. Note the long pieces of tape at the bottom end to anchor that section. What to do with the ends of the rope is up to the builder. Tom Lathorp notched the bottom to fit the rope and at the top anchored it in a hole. I wanted until the epoxy had partially cured and then flattened the ends, later going back and sanding the hardened rope and using some thickened epoxy to fair the ends.

While the glue is still wet, run your hand over the plastic covered rope. You can easily feel if it has slipped off center anywhere and you can simply move it back. There will (or should be) enough pressure from the plastic to hold it in place. Now walk away and let the epoxy cure.

An unrelated tip: If you regard epoxy as liquid gold, try to have some other use lined up for the left over epoxy used to saturate the rope. I didn’t plan ahead when I did the leeboard. When I got to the rudder a few days later, I made sure the excess was used to fill the weave of the fiberglass cloth on the leeboard.

Also, to state the obvious, you can use Dacron rope to protect the leading edges of your boards and rudders even if you don’t go to the trouble of shaping them like foils.

click to enlargeThe joint between the rope and board has to be filled with thickened epoxy


click to enlargeHere’s the protective rope as it curves around the bottom edge of the leeboard . . .


click to enlarge... and the bottom edge of the rudder. Notice the nice curve on the leading edge provided by the ropes. I whacked the hardened rope with some miscellaneous heavy objects and didn’t managed to dent or mark it. Hopefully it will stand up to rocks and oyster shells for many years.

Anyway, once the epoxy is dry, you’re about done. You’ll have a groove between the rope and board (bigger if you use hard rope like I did, smaller if you use softer rope like Tom recommends) to fill with thickened epoxy – no big deal. After that fill the weave of the glass cloth and fair the boards to whatever standards you want or have the patience for. I was pleasantly surprised when I put the templates back on the leeboard and rudder to find out the rope on the leading edges matched exactly the curves on the templates. Sometimes you gotta be lucky.

click to enlargeGlassing the rudder, and filling the fiberglass weave on the leeboard. All that’s left is sanding, fairing, and painting.

Because of a minor shoulder ailment, it will be a couple months before Oaracle is back in the water and we get to try the revamped foils. But when I stood back to look at the finished product, I couldn’t help thinking, “Egads, I’ve been foiled. And I’ll probably do it again. (When I build another boat . . .)”

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