This is a "freeze-dried" summary of everything I've managed to learn researching boats and foil shapes for rudders, leeboards, etc. As a result it glosses over a lot. That's OK. This is supposed to be just enough to let any Joe Average or Jane Typical, someone like me, confidently make decisions about whether or not to shape daggerboards, rudders, and such like an airfoil. I've deliberately left out bibiliographical references.

If you're making a wingsail or a wingmast, the same coordinates apply, by the way. Since air is less dense than water you can use a very fat section profile (0018, 0025, 0030, maybe greater) on a wingsail or wingmast.

The basic NACA section profiles are "symmetrical" so they are good for rudders, especially, but also leeboards, daggerboards and centerboards. These are called the "four-digit" series and are identified by the name "NACA 00XX".

Look at your board end-on (in other words, a fish-eye view from underwater). The Y dimension is measured in both directions from an imaginary centerline through your board. It gives you both thickness and curvature. Connect the dots with a flexible batten (for example, 1/2-inch PVC tubing works for large foils. On small ones try piano wire, or if you have musician friends, a bass guitar G-string).

The chord is the "width" or front-to-back dimension of your board(s). For example, on certain Bolger boats the rudder and leeboard are both 10 inches wide. That's the chord. The thickness of a NACA section depends entirely on the chord of your board.

Finally, don't cheat. You'll discover that foil sections are thicker than you imagine. That's OK, because a proper foil shape has much less drag than you think looking at it. The thicker the foil, the more lift it will give you. The drag also increases as the foils get thicker, but you can't really separate drag and lift. Foils for boats seem to generally encompass 0010-0015 for centerboards and 0015-0025 for rudders.

Symmetrical foils are by far the easiest to make by hand. If you have access to CAD-CAM machinery by all means go nuts.

These coordinates were taken from a standard reference text [Riegels, Aerofoil Sections]. He gives standard NACA profiles in this series as 0003, 0006, 0009, 0012, 0015, 0018, 0021, 0025, 0030. There is nothing magic about this. An 0010 or 0017 profile is perfectly legitimate but you'll have to generate the coordinates yourself. There is a formula but I'm not talking . After all, this is the Cheap Foil FAQ.

The main difference between a "thin flat plate" and a NACA foil is that the foil will generate lift, i.e. power, at a higher angle versus the boat's direction. Where a plain flat rudder will stall and simply slow you down somewhere around +/- 5 to 10 degrees from center, a NACA foil will be effective over maybe twice that range. Your boat's rudder will work better. Tacking will be easier. A little bit of weather helm translates into more sideways 'push' upwind.

As a leeboard, centerboard or daggerboard the difference between the plate and foil would be much less pronounced because the boat is usually moving "crabwise" through the water at a much smaller angle than you'd use at the helm when tacking. As long as it doesn't vibrate, a flat plate is probably as effective and a lot less hassle than a shaped foil section. Using aluminum, brass (big in the 1880s and 1890s) or even iron eliminates having to weight a board to keep it down, and allows the plate to be very thin, which is good (3/32 to 1/8-inch, perhaps).

Remember, the smaller your boat the more the drag of a flat plate affects you. (This is why skipjacks don't seem to care that their centerboards are about 4 inches thick and dead straight across the leading and trailing edges, and why keels on big sailing vessels are often flat at the stem and it doesn't matter that much. Loads of sail area = power). So thinner is better.

So on a small boat keep the plate as thin as possible and as light as possible unless you need extra ballast weight. Aluminum, anyone?

Let's say we have a 10-inch-wide (or 10-cm-wide) rudder to shape. How thick is the foil shape at the thickest? Remember, the thickness at each point depends on the chord of the board.

Looking at this, don't expect to make a NACA 0009 section on a 10-inch leeboard laminated of two 1/8-inch pieces of plywood. Laminate something that's just a shade under 1-inch thick. Pragmatically speaking, 0012 looks pretty good (five 1/4-inch pieces laminated or 1/8 on either side of a 1-inch core). 0015 also looks good (three 1/2-inch pieces or two 3/4-inch pieces).

I did alright shaping a cedar rudder foil (0012) using a Surform rasp, a large mill bastard metal file, a hand sander, and a sanding block. It _was_ tedious.

How "tall" the rudder or board is defined by the aspect ratio. [ Literally it's (Span*Span)/Area ]. According to sources, AR>1 and <2 is perfectly effective for small boats. You don't have to have a stiletto for a rudder or board. You can if you want.

Our hypothetical 10-unit board, above, would have an AR of 1 if there is 10-inches (cm) of board immersed when it's all the way down (10X10/100). With 20 units of board immersed, it would have an AR of (20*20/200 = 400/200 = 2).

Strictly speaking this isn't necessary. It also complicates the process of fairing the foil. If you'd like some aesthetic freedom, or a way to help potwarp slide off the rudder, use a small angle, not a large one.

In general a foil is happiest when the tip is rounded to an elliptical plan shape. (The common example is the RAF Spitfire wing, looking down from above). If an ellipse buffaloes you, just use a compass to make a circle with Diameter = Chord, or maybe a shade greater, and fair it by eye.

An end plate on a rudder gives it more power for its area; it's a cheat to make the board or rudder act as if it has a higher aspect ratio. The finished assembly looks like an upside down T. The winglet/plate should probably be at least 1.5 inches (units) on either side of the board.

(1) The values listed beneath the NACA numbers are the distances + and - to the foil surface from the centerline in % of chord.

(2) Cut values for 0012 and 0018 in half to get 0006 and 0009. Double 0015 to get 0030. Halve 0006 to get 0003.

(3) It's conventional to make the chord 5% longer than you really want, and then slice off the last 5% to a blunt edge after laying out the foil profile. Example: you want a 20-inch chord rudder. Calculate profile based on 21 inches, leaving off last 5% or 1-inch when laying out.

(4) The nose of a NACA 00-- foil is always circular. Fair this in by eye as best you can using a compass or template or some sort (bottom of McDonald's coffee cup). This radius can be generated by the formula referred to above. (I'm still not talking.)

(5) The thickness of a NACA foil is always greatest at 30% of the chord from the nose.

I'd rather be sailing.

Macintosh users should find Foils 1.2 for a freeware foil-drawing application. PC users can find NACAGEN, a shareware program for doing the same thing.