| You will also need a metal file. I bought a 12” metal file from the local Home Depot. This has a fine cut for wood, but for metal it’s rather course. I like this myself, but you should get what works best for you. A set of jeweler’s files will be useful as well, for removing flashing on the hub and blades. You’ll also need a single, small can of Play-Doh® or similar putty. This will help us make the prop the right shape. A stroboscope will also be needed to spin balance the props, and possibly an optical tachometer such as is used for model airplanes. A stroboscope makes a bright, short flash which makes moving objects look like they aren’t moving, and the tach measures how fast something is rotating. Finally, two small wood blocks are handy tools. One should be about 1” x 2” x 3”, the other 1” x 6” x 12”. The larger one should be of a hard wood such as oak or maple, the smaller one should be of a softer wood such as pine. The hard block will be used for thinning the prop blades, the soft one for smoothing the lines. I have found that the larger block works better laying on top of my workbench, the smaller one in my hand. Balancing Technique Correctly balancing a propeller is accomplished by a tried-and-true engineering principle known as stepwise refinement. Begin by meeting easy parameters, then continue meeting harder and harder ones until you have achieved your goal. I have balanced hundreds of props over the years (I think I keep Octura in business sometimes), and what is described below is the process I use for my own equipment. Everything depends on the tools you use, how well you know to use them, and how much patience you have to achieve the result. Save your pennies and buy good equipment, take some time to learn how to use them, and then be patient. It takes me about an hour per prop from start to finish, and I usually work on about 4 at a time. I also use the process described below after I damage a prop in order to make it usable again. Never discard a dinged prop – you can almost always cut it and rebalance to have a usable one again. Step 1: Dimensional Balancing The first step to fully balancing your prop is to dimensionally balance it. The goal of dimensional balancing is to make sure that both blades are the same size. This will keep the load on the motor constant, which will help prevent power-robbing vibration. Begin by mounting your prop in the handle. Remove the flashing from the hub using the jeweler files. My next step with Octura props is to make sure that the prop is cut to the size of the mold. Many of these molds are apparently rather old, and sometimes I get props which are so different that you can see the differences between the blades without any special tools. Some blades are thick, some are thin, some have more flashing than others, etc. Next, place a clump of Play-Doh® on your work surface, then push the prop into it to leave an impression. Very carefully remove the prop, rotate it 180 degrees (for a two-blade prop), and check to make sure that the impression perfectly matches the other blade. Use a metal file or emery board with oil to remove any oversize portions. Repeat until both blades are exactly the same size and shape, each time refreshing the Play-Doh to get a new impression. Step 2: Sharpening The next thing you need to do after getting the blades shaped the same is to sharpen them. Right now, your blades most likely have squared edges. You job is the make the leading edges as sharp as you can, but keep the trailing edge square. First, make sure the trailing edge is square by lightly going across it with your file if you haven’t already. You don’t want to take your blade out of dimensional balance, so go easy. The trailing edge should be squared off relative to the face of the blade. Next, use your file to remove metal from the front of the blades, moving the blade from the trailing edge toward the leading edge. This will give you the nicest cut. You don’t want to cut a edge, you want to thin the entire blade down so that the whole thing is a gentle taper. After you are satisfied with the sharpness of the blade, use oiled 320 grit sandpaper to smooth the blade and remove any imperfections that the file may have created. I prefer to use a 1x6x12 piece of wood as a work bench of sorts, with the sandpaper on the top side, moving the propeller to work it. You can go to a finer sandpaper to make the prop smoother. I usually stop at 600 grit myself. Step 3: Static Balancing Once you have dimensionally balanced your prop and sharpened it, you need to make sure it is statically balanced. The goal of static balancing is to make sure the blades are the same weight, which will definitely help with later steps. We also gain the benefit that both blades will become very closely matched in thickness, helping to maintain a constant load on the motor when racing. Use your prop balancer to make sure both blades weigh the same. This is accomplished when your prop will stay level on the balancer, without a tendency for either blade to fall vertically. You need to remove thickness from the heavy blade, never changing the outline. Use the hard block of wood and 240 or 320 grit paper. Finished that? OK, now you are half done. Half? Yup, half. Your prop is balanced tip-to- tip, but probably not “top-to-bottom.” You also need to be able to make either blade be on either side of the balancer for it to be balanced in this manner. Most boaters are happy when they can get the prop to stay level, but they forgot to make it stay level in both directions. When your blade is only balanced for one direction, you will find that the prop will run much smoother but not as well as it could. |
