Here is a project where I made a NACA 0012 foil shaped rudder for a Picnic Cat sailboat. The stock rudder for the Picnic Cat is a 1/4th inch flat aluminum plate that is 12 inches wide. Not very hydrodynamic. The NACA foil shapes are standard mathematical calculations for creating various foil shapes. The NACA 0012 is commonly used for sailboat rudders. The thickest point is 12% of the chord length (12 inches in this case) and occurs at about 30% of the chord length.
Doing a little math, I thought that the easiest way to get a foil shape was to screed on polyester resin (aka Bondo) using a basic technique I used with plaster when building my house. I came up with the plot points, subtracting 1/2 the thickness of the aluminum plate, as I was going to use the original rudder as the center part of the foil. That has the benefit of a metal leading and bottom edge cast right into the rudder. A good thing to have on a boat that is often brought in to a shore covered with rocks, barnacles, and oysters.
I converted all of the points to 1/60th of an inch, because that was the smallest gradation on my old engineering ruler. I then put in thumbtacks at those points. The screed board I am making is a plastic/sawdust mixture commonly used as decking material.
Once the thumbtacks were all in, I used a piece of thin spring steel as a batten for marking the curve.
Instead of scribing the line with a pencil, I just shot it with some spray paint and then faired in the line with a pencil.
I then cut out the shape with a band saw. Here is the screed board set on the flat aluminum rudder plate.
You can see how I will need to have the blunt leading edge faired into the foil shape.
On the back edge of the rudder, I will need to remove almost 1/8th inch of aluminum in order to have a sharp trailing edge. I didn't go down to a knife edge. Removal and shaping of the plate aluminum was fairly fast and easy with 80 grit on the belt sander.
Here is the screed sitting on the rudder. You can see that the trailing edge has been shaped already. I thought it would be easier to do the bulk of the shaping before adding the polyester resin.
The first coat is just slathered on, making sure that the material is pressed on to the surface. 
I ran the screed over it to make sure that I didn't have any high spots. Then you wait a few minutes until the material enters its "cheddar cheese" consistency. At that point, a cheese grater type of wood rasp can knock down the surface and get it ready for the next layer. If you miss this point in time, you'll have to sand cured Bondo. So it's either an easy minute with the cheese grater or +10 minutes with the belt sander. The choice is yours.
I didn't worry too much about extending the foil shape all the way to the bottom of the rudder. I wanted the solid aluminum "keel" on the rudder to be the only part that hit bottom (which happens).
This picture shows the red glazing compound sanded down over the polyester resin. That gets rid of the bubbles and sanding lines. It is the same process as used for auto body repair.
This is the rudder all faired down and ready for painting. You can see the aluminum showing in the trailing edge.
It then gets a coat of spray paint. I had a quarter of a can of "appliance white epoxy paint" left over from some project I don't even remember. Looked good on the rudder. The plate rudder had anti-fouling paint on it from the prior owner. I wasn't sure if it was the right type of paint for aluminum boats (i.e., not a copper based paint) and was happy to be rid of it. The shiny new rudder will not be left in the water when moored, so anti-fouling paint isn't necessary.
You can see that the trailing edge isn't a knife blade. From what I read, that isn't too important. It's the curvature along the chord that stops the eddy formation which causes drag. With the eddies reduced, the rudder operates with more turning force and less braking force when tacking. Weather helm is reduced to one finger on the tiller. A very nice improvement.
Here's the front edge.
I can't believe that I didn't take a full side picture. It's now on the boat and out on its mooring, so pictures are a little hard to get. The project was probably 4-5 hours and over a gallon of resin. For some reason, auto body putty comes in gallon cans, but they are only 75% full. Although it would only take 1 gallon and 1 quart if you don't have any waste, I'd recommend just buying two gallons. It takes a surprising amount of material, and having enough material to fatten the rudder into a foil shape is what it's all about.
I used a piece of Trex plastic wood for my screed and it worked great. If I forgot to clean off the Bondo, it could be knocked off when it was in the cheddar cheese phase. After that, it got more difficult, but not as difficult as removing polyester resin putty from wood.
I won't go into the benefits of a foil-shaped rudder as that is covered in many other places. I'll just say that it's a wonderful improvement for $60 in materials and a few hours of work.
I have read in a couple of places that there is some concern about whether regular Bondo or automobile body polyester resin will hold up under water. There are products that are made specifically for underwater applications (Marine Tex is one of them). As per usual, because it is for boats, it costs 5 times as much as regular polyester resin. I have used both on boats and honestly can't tell the difference. That would definitely be the case where, as is the situation here, the rudder is generally pulled up when the boat is moored. Also, this boat spends a lot of time on the trailer. The rudder is only submerged during the several hours of a sailing trip. Auto body putting for this application is just fine.
