While this instructable is specifically for this Bayliner sailboat with a tiller-style rudder, the instructions should be general enough for you to modify it to work for many sailboats. With that said, there are many many nuances to fiberglass/composite marine construction, so this type of build will require more research beyond what is covered here.
Step 1: Previous Rudder
The rudder was foam-core/fiberglass sandwich. Think of it as a Big Mac; the three buns of the Big Mac were layers of fiberglass, and the meat was the foam (the yellow stuff). The only difference was that the buns would have all been connected and fully enclose the meat.
First, I cut apart the rudder along its perimeter with an oscillating saw, so that I could use the pieces as templates for the build.
In the fifth image you are seeing a piece of balsa (I think) at the edge of the rudder where the mounting hardware was located so as to provide compressive stability for the tightened hardware.
In the last image, if you look at the top of the image you can see where the previous owner had chopped off the top of the rudder. There was a rudimentary wooden cap on that, so you can see how easy it would have been for water to get in.
Step 2: Rebuild
In the first image below, you can see the old pieces of the rudder all stripped of foam next to the new plywood pieces. In the background you can see the middle fiberglass 'bun' of the whole kit 'n caboodle. I scraped away all of the foam because I had originally wanted to save the exterior pieces and reuse them, but the Big Mac style construction made it more difficult to reuse them.
Simply place the old pieces on your sheet of plywood, trace, then cut out with a jigsaw.
If for some reason, you only have one template to work with, and you are using two pieces of ply that will later get glued together, be sure to flip the template over before tracing, so you have mirrored pieces.
The customer asked for a little more material at the top of the rudder, as you will see in the last images of the Instructable. It ended up making it look a little strange, however.
Step 3: Sanding
As I mentioned, it is good to sand the two pieces separately, although this picture is of the two already glued together. A handy trick is to imagine your surface and the lines of ply as the lines on a topography map. The curved edge of the rudder closest to us in the image is the narrowest edge, from the little notch all the way down the side to the very bottom of the rudder. This is because it is the edge of the rudder that points forward when it is on the boat.
I started by using a disc sander, but it was too slow, so I switched to a grinder. The grinder worked well, but it was a bit too fast, so if you decide to use one, be very judicious in your use of it, otherwise you will end up with big divots.
Step 4: Fiberglass Layup
In the back is a finned roller that you use after you mix and start pouring the resin to remove the air bubbles from under the chopped strand mat and to spread the resin around. After a while the roller gets all gummed up, and I ended up using just my gloved hand to push out the bubbles, and I found that a simple plastic spreader worked best for spreading.
Don't worry about the stuff that hangs over the sides. Originally I wanted to have it fold over and seal the edges at the same time, but this was near impossible, as we will see soon, and I just let it hang and harden from any of the spilled over resin. I dealt with it later with a lot of sanding.
Step 5: First Layer and Sanding
Glass one side of the rudder, let cure.
Cut off excess edge stuff and rough sand/grind.
Glass other side of rudder, let cure.
Cut off excess, sand until flush.
Glass edges based upon which were generally 'up' when clamped in a mostly horizontal way (images 4 and 5).
Glass the remaining edges.
Sand the nasty edges until flush.
The first image is after the glass on the faces have cured, showing the excess. The third image was after sanding the excess from the faces. The following images were taken doing first layers of the edges, after the faces.
Step 6: Additional Layers and Difficult Spots
There are some really difficult spots that you need to pay attention to. Generally, corners are the spots you need to look out for. It's like trying to wrap a piece of paper over a 3D form without letting any edges lift. They will tend to lift up one end of your saturated cloth and allow air to get right in there, which means you'll have to sand that air bubble out and re-do it later. The very bottom tip of the rudder was one of them. Although the second picture is after I had drilled the holes for the hardware, it's useful to see the method for tackling those difficult spots.
Visibile at the tip of the rudder is a bit of blue painter's tape. For that spot and others, which I will mention later, I basically taped the heck out of it, making a small well, and poured in enough resin to cover it. You can also see in this picture, how it has started to get thick/bulky. That's normal as layers build, you just need to sand it down flush later. Sometimes the tape gets sealed in there, so I just left it in.
Step 7: Notes of Caution
After you think you've sealed the whole rudder and you go to sand it smooth, you may uncover more white patches or air bubbles. It's extremely frustrating to think you're almost done and find another one of those, but it pays off to patch them properly.
If there are some air bubbles or pockets that just don't seem to patch up and keeps reappearing after you sand this product is really helpful: http://www.marinetex.com/marinetexepoxyputty.html. It's a putty-like marine epoxy, so it serves the same purpose as regular epoxy, but it is much more workable and can be packed into a hole to completely seal it. The is the best product for repairs of deep scratches or small punctures in a fiberglass surface.
The notch at the top of the first image was one spot that I taped significantly in order seal every spot with epoxy. This is the point where I switched to epoxy from resin, as I had run out. The purple is the natural color of the epoxy after it hardens.
Step 8: Hardware Holes
Use this tutorial to help you get the holes right: http://www.boat-project.com/tutorials/drill.htm.
Basically, you need to drill your holes bigger (1.5x, I think. The tutorial with specify this.) than the hardware needs. You then fill the hole with epoxy and let cure. Then you drill your holes again with a bit sized for your hardware. After painting, get some sealant (specified in the tutorial) and coat the bolts, holes and the inside face of the bracket immediately before placing them on the rudder.
It's really important to drill your holes square through the rudder. If you don't, you'll find when putting the bolts through, that they won't meet with the bracket holes. If you're slightly off (like I was), you can just enlarge the hole at the problem end. If you're really off, you'll have to sand the paint away, drill the new hole, fill with epoxy again, re drill, then paint.
Step 9: Painting
Before painting, you must 'cut' the surface (a light sanding), so there is surface for the paint to bond to, and you will need to remove any oils or chemicals that are on the surface with acetone or a similar product.
The paint will usually specify a total thickness of paint required to be considered sealed, and will allow you to calculate the number of coats from the average thickness per coat.
Next use a top-side paint (I think we used an auto-body paint) to cover the surfaces above the waterline to make it look nice.
Lastly, apply a bottom paint (also called anti-fouling paint) below the waterline of the rudder. Bottom paints, especially, vary greatly by geographical location, type of water, EPA legal restrictions, etc. Their purpose is to prevent organisms from attaching to the submerged surfaces, so naturally, they will contain certain chemicals and/or metals. Copper is a common ingredient in anti-fouling paint, as it slowly leeches from the paint, preventing any organisms from attaching permanently.