For this issue of building a 36/600 Class model yacht, the topic will be on the construction of the rudder. Of all of the components of a model yacht, the rudder is the one that offers a wide range of options on how to make it. As simple as it is, there is more to a rudder than meets the eye. In this article I will briefly describe the principles of rudder design. In the upcoming October issue of the MMYC's Newsletter I will go in to more detail on rudder design. For now, I'll just jump into how I made my rudder and the materials I used.
As we all know a rudder steers the yacht on the water. With out it, it would be nearly impossible to sail a yacht around a race course. At this point I must state, when designing and building a rudder, there are a few concepts that was be followed when constructing a model yacht rudder.
First, a rudder is a foil just like the keel fin is, and because it is moving through a liquid environment, it creates lift just like the keel fin does. This lift is important to the yacht because it helps counter act the heeling effect of the sails.
Second, because a rudder creates drag, it slows the boat down. However, if the rudder is designed right, the drag disadvantage is not too much of a hindrance.
The third item is the shape of the rudder. By being too long or too short, too narrow, or too wide all has impact of the performance of the rudder. One good thing about shape is it provides opportunity for design improvements. There are several shapes that are better than others, so it is up to the builder to find the best. It is important to keep these points in mind when building a rudder. So lets get started.
Weight is a constant worry and I wanted the rudder as light as possible. Since the lightest material is balsa wood, I decided to give it a try. With past rudders I have built, the one area that has been a problem is warping. When warping occurs, the rudder's foil distorts and the rudder is not straight and may affect sailing. To avoid this problem I used a flat aircraft airfoil shaped 3/16 x 3 x 36 inch balsa sheet. With a flat balsa foil sheet this avoids any warping problem.
As for the shape of the rudder, it is up the builder. The dimensions of the rudder I built are 7 7/8 inches long, 2 ¾ inches at the top, and 2 inches at the bottom. The top of the rudder is angled to match the angle of the boat hull and the leading tip is rounded, (see Example 2). You don't want the rudder too wide because it adds drag and may not add any extra lift. Having a rudder too long also adds drag and no extra lift (lift really occurs at the area of the rudder close to the hull). The main idea is to have the most lift for the least area. If you think having a small area rudder is a better idea, think again. You need lift and enough control area to be able to turn the yacht.
The down side to using balsa as a main building material is, that the material is soft and not very strong. To add some strength, glue and clamp two halves together to form the rudder. This is all very good but there are some design items to consider. As already stated, use straight wood, anything less is a mistake. Next, mark and saw out each half of the rudder shape that matches with the foil shape of the airfoil shape balsa sheet. By using aircraft balsa airfoil sheet for the initial rudder, it provides a foil shape when shaping the rudder.
Once the two halves have been sawed out, the next step is to place the rudder post into position in the two halves. I like to place the rudder post at the 20 percent point from the leading edge. Calculate and mark a line at the 20 percent point on the inside of each half. Next draw a line parallel to the leading edge for the length of at least three inches. Then make a grove on each line for the rudder post to lay in. The easiest way I have found to do this is to use a brass tube that is the same diameter as the rudder post and gouge out the slot. To get started, use a ruler to guide the first few passes until the grove becomes deep enough. From then on, keep gouging the slot until it is half the depth of the rod. Do this for both halves. Don't over gouge the slot, because if you do, the rudder post will have more room than it needs.
You may be thinking, "What happens if the post becomes unglued and slips out?". To take care of this, put a bend in the post. Just carve out the bend in the wood for both rudder halves. Before we get to gluing, work needs to be done on the rudder post. For most model yachts, brass is used for the rudder post and tube. Brass is strong but a little heavy. For the rudder of a 36/600 model yacht, the diameter is 1/8 inch. The downside to brass is it is heavy. To get around some of the weight issue, here is a great idea to get around the problem.
Instead of using a solid brass rod or a solid aluminum rod (aluminum is too soft for a rudder post) why not use both metals? For this to work, purchase a brass tube that has an outside diameter of 1/8 inch. Next, purchase a solid aluminum rod that has an outside diameter of 3/32 of an inch. The aluminum rod should slid into the brass tube without any gap between the two. To get the aluminum rod to stay in the brass tube, as you slide it in apply Epoxy glue, (see Example 1). With the aluminum rod and brass tube glued together, there is another step to do to make sure the two won't separate. At the end that goes into the balsa wood, make a bend in the post. Do not put too much of a bend in the post, just enough to do the job (too much and the post will break). To make really sure nothing goes anywhere, take a hammer and flatten the bend tip. With the bend carved in the wood at the end of the slot and with the bend at the end of the post, you are ready to glue the rudder halves together.
To glue, just apply a lot of Epoxy to both rudder post slots and to one rudder half. Lay the post in the slot and put the other rudder half down the glued half and press together. You should have Epoxy leaking out all around the outside edge. To keep the two halves together, put wood slats on the outside and apply paper clamps all the way around. Check your work and set it aside for at least eight hours.
Once the two glued halves have dried, it is just a simple matter of shaping the rudder's foil into the correct shape. If too thin (not enough camber) the rudder won't create enough lift. If too thick (too much camber) the rudder will create too much drag and lift won't be maximized. Since there is a basic foil shape to the rough rudder, it is a good starting point. To shape my rudder I use a wood rasp to get the basic shape down what I want. The rudder I made had a camber of ¼ of an inch. I just rasp a foil shape then I use a finer rasp to finish off. Once the foil shape is where I want, I start sanding the rudder with a medium-heavy grade sandpaper, (see Example 3).
All during the rasping/sanding process I make visual checks of the developing shape, and make any needed adjustments. To make sure the rudder's edges are straight, I use the glue line of the two halves. If the wood is straight the glue line will be straight. I rasp/sand down to the line, checking with my eyes and fingers as I go. Even though balsa is soft it still takes time and effort to shape the foil, but don't rasp/sand too hard and take too much off.
To finish the final shape, I switch to medium sandpaper. Never sand with just your fingers, use a block of wood with the sandpaper covering it. When shaping a rudder make sure the edges are sharp, the end of the rudder is thinner than the top, and the foil shape is the same. To make sure this is the case, keep checking with your eyes, and feel with your fingers. You don't want ridges, holes, or off-shape areas.
When done sanding, there are parts of the rudder that will be thin and weak. To add more strength to the rudder's wood, apply a coating of CA glue. First dust off the rudder, and by using your finger, apply a few drops of CA glue and smear it around. Keep doing this on both sides until all wood is covered. You can do this a couple of times if you like it, it won't hurt. The CA glue soaks into the wood making it harder as well as sealing up the wood pores. If you like, you can very lightly sand the rudder with fine grade sandpaper.
At this point you can coat the two sides with fiberglass resin (without the cloth if you like) or as I did, apply several coats of Epoxy glue. Either way it will waterproof the wood and add strength. With each coat of fiberglass or Epoxy, sand the coating to remove any blemishes. When I apply Epoxy glue, I mix up enough for one coating. I apply the Epoxy in thin coats with sanding (wet sanding) between coats. By the time I'm done, I have applied four coats.
To see how much the rudder weighed, I weighed it on a US Mail letter scale, the weight was a little over ½ ounce. At this point the rudder is strong and lightweight. To finish, I apply a light coat of paint and the rudder is ready.
The good thing about making rudders is, if you don't like your rudder, or want to try something different, rudders are easy to change. Not only is rudders cheep to make, the materials are easy to find at the local hobby store. So far I have made two rudders like I have described and both turned out fine. Next issue I will go over how to install the rudder tube into the hull. Until then, happy building!