The stock rudder for my Epic V10 surfski catches kelp and weeds so I decided to build my own "weedless" rudder as a replacement. Here's how I went about it. . .
Step 1: Materials
This is the second time I've made a surfski rudder. The first time I used marine plywood but this time I decided to use balsa since I thought it would be easier to shape and I would be glassing the outside for strength anyway.
The stock rudder is about 5/8 inch thick with a 3/8 inch diameter shaft. I bought a piece of 3/8 inch stainless steel rod from a metal supply store and balsa from a hobby shop. Two part epoxy resin and 4 oz. fiberglass cloth were left over from another boat building project so I used that.
Step 2: Rudder Shaft and Rough Shaping
I measured the overall dimensions of the stock rudder and carefully measured the shaft length, diameter, set back from the leading edge, hole diameter, etc. Obviously the new rudder has to fit the boat in the same was as the original so whatever measurements it takes to get that right. I decided on a rudder rake angle of 38 degrees from vertical based on measuring some other weedless rudders from other boats. The width and depth I kept the same, just had it swept back more.
These measurements were traced onto a piece of 3/8 inch thick balsa. The stainless steel shaft was bent and drilled then placed over the balsa and the outline of the shaft traced onto the wood. Balsa is soft enough that cutting out a channel for the shaft to fit into wasn't hard. I used my table saw to cut out the rough shape and a utility knife and Dremel tool to cut out the slot for the rudder shaft to fit into.
After the shaft was fitted into the balsa, I taped it off, mixed up some epoxy and poured it all around the shaft to lock it into the balsa. Let it cure.
Step 3: Laminating the Full Blank Together
After the epoxy cures locking the shaft to the core of the rudder I cut two pieces of 4oz fiberglass cloth and used a 1/8 inch piece of balsa on each side to lay up the complete blank. The 1/8 inch pieces brought the total thickness to the required 5/8 inch. More epoxy to wet out the glass and wood layers and then find some heavy stuff laying around to pile on top to provide some pressure for it all to set up. Now the full blank is done.
In hindsight, the glass layers inside the blank were probably unnecessary. By the time I had cut/ground/sanded the blank to it's final shape, the great majority of the glass had been removed. Also, the glass layer was very hard compared to the balsa on either side making shaping much more difficult. Next time I'll leave these layers of glass out.
Step 4: Shaping the Rudder Foil
The rudder needs to have a streamlined shape for least resistance. After some internet research I decided to use a NACA 0012 foil shape. This shape was originally developed for aircraft surfaces but works just fine for rudder shapes as well. This was the hardest part, trying to hand shape an aerodynamically efficient shape into the blank. I found a way to print out the cross sections I needed and then taped one to each end of the rudder as a guide. It's not perfect, but to the naked eye you'd be hard pressed to find the flaws.
After the blank is shaped, more 4oz fiberglass cloth was epoxied to the outside. This really stiffens it up and provides a hard outer shell to the balsa core. After the glass cures, more epoxy coats are added to fill the weave of the glass and provide thickness to sand to the final shape. Two fill coats were added and when they were hardedned, I sanded the coats down to the final surface. Too much sanding and you'll expose the fiberglass cloth, too little and the surface won't be fair. After the final sanding, the rudder is ready for a trial run on the boat.
Step 5: Fishished Rudder
It sheds weeds just fine (whew!). I then took it off the boat for final smooth sanding, then coating with a UV resistant varnish. The varnish provides a gloss coat for the surface and will keep the epoxy from degrading in the sunlight. I could have added pigment to the epoxy to avoid the varnish but I wanted to see the balsa core and it also adds a gloss coat.
That's about it. I'm currently working on another rudder just because I have the materials on hand and I really want to get a good shape this time but the first one is very good and the original stock rudder is now my spare.