Building Rudder/Sealing Marine Plywood
Pi is mostly right. I have ended up building a lot of rudders in my day including one for my old 25 foot Folkboat. 3/4" is no where near thick enough. The rudder should be laminated from two thicknesses of plywood to reduce the likelihood of warping. Minimally the blade should be 1 1/4" thick (two layers of 5/8" marine plywood) with 1 1/2" probably being a bit better.
The two thicknesses should be glues together using a high quality epoxy resin. My current favorite is MAS but any quality resin, WEST System or System 3 will do. To avoid laminating a warp into glued up blank, you will need to glue up the two halves with the bottom layer lying on a flat surface. My two current preferred methods of clamping the two halves together while curing is either to lay plastic below the bottom layer of plywood, apply glue to the two halves and then put the two pieces together. Then put plastic over the two pieces and then pile weight evenly over the top of the laminated piece.
My other favorite way is to drill 1/8" holes at 6 to 8 inches on center in the top layer. Lay plastic below the bottom layer of plywood, apply glue to the two halves and put them together. Using a drill with a Phillips head bit chucked up I clamp the two halves together with 1" or 1 1/4" drywall screws driven into the 1/8" holes. When the blank has cured I remove the screws and fill the holes.
A 25 footer is big enough that rudder shape does count. The blade should be 15% to 20% shallower than your keel or centerboard in the down position should have at least roughly a foil shape (teardrop shape). That means that in rough terms that the leading edge is a semi circle and that the trailing edge has a long taper over perhaps something less than half of the horizontal fore and aft length of the rudder. For structural reasons the top of the rudder above the waterline is traditionally left square but is tapered down to the foil shape by the time the rudder is at the waterline.
Plywood is pretty easy to shape. I typically have used a heavy-duty disk grinder with a 15 or 30 grit floor stripping heavy-duty sandpaper to rough cut the shape. The glue lines allow you to see if you have humps or hollows and the glue lines should be fair and true. The key here is to keep the grinder moving rather than to stay in one spot. When you are close to the right shape you can knock down any bigger humps with a sharp, low angle, block plane and do a final faring vertically with a sharp jointer plane if you have one.
I would test fit the rudder and drill any holes for hardware that is required. This last item is a bit of a pain in the neck but well worth the effort. Plywood rudders want to float up. This makes them want to self steer if there is any rake to your transom and it means that you need a downhaul on your rudder which sort of defeats the purpose of having a kick up rudder. I typically pour a puck or two of lead into the bottom of the rudder. To do so, I either get discarded balancing weights at a tire shop or spent rounds from a shooting range. I have melted the lead in an old iron put in a gas grille but that can be pretty slow. I would take a look at a traditional boat building book for safety precautions because you do need to be careful working with molten lead.
I typically drill the holes for the pucks with a 2" hole saw and then take a rasp and taper the hole outward on either side. I then screw stainless steel screws radially around the hole so that the screw heads are below the face plane of the plywood. I screw sheet metal (with a light plywood backer) over the bottom of the holes and then pour the holes full of lead. Once cooled lead is easy to shape with the grinder. Because the wood scorches I fill the void around the lead with unthickened epoxy. Lastly, I fair the face of the lead and plywood with thickened epoxy.
Once the rudder blank is faired and ready I would follow the manufacturers instructions and apply several coats of unthickened epoxy to seal the plywood and then fill any voids, screw holes etc. with thickened epoxy in a syringe. I would apply a single coat of fiberglass cloth in epoxy and then do a final fair and coating with epoxy. The rudder will need to be painted to protect the epoxy from UV damage. While this all sounds like a lot of work, it can actually be accomplished in two of three weekends.