| PROPELLER GUIDE Author, Andy Kunz. e-mail Andy at: andy@rc-hydros.com Website: rc-hydros.com The propeller fills a unique position in a boat in that it can be used to adjust several characteristics of the boat. To compare a boat to a car, the prop would be your transmission, wheels, gear ratio, and tires. Quite a bit depends upon having the right prop, but finding the best prop for your boat is something of an art. There are several different types of propellers for models, just as there are for full-size boats. Almost all racing props are surface-piercers, and run about half in the water and half out. Running in this manner solves several problems when compared to fully submerged props, enough so that even some new merchant vessels are being designed to use surface props. Octura makes several different classes of props to fit the different needs. The all-digit props (1435, 1932, etc.) are designed to provide lift and are therefore suitable only in hydros, while the X, Y, and P series props are multi-purpose props designed to be more efficient and provide minimal lift but maximum thrust. On scale hydros, I have found that the X- and Y-series props give better performance than others. These props provide excellent thrust while angled relative to the direction of travel, and that makes them equally suitable for both solid and flexible driveshafts. Because of their higher efficiency compared to lifting props, they provide the competitive edge a racer needs. Lifting props do not work well when there is much of an angle to the prop shaft, so they are far more limited in their applications. There are some boats which just won’t run without one (usually riggers), so you should have one or two in your prop box. As we will see later, there are certain reasons to choose one particular type over another. PROPELLER BALANCING I’ve found one thing over the years that has improved my performance, extended the life of my equipment, cut down on noise, and earned me “attaboys” from my peers. The answer: prop work. A good propeller, because it performs so many functions on a boat, is essential. My props are all balanced as described below, then polished and measured. They are kept stored in soft foam in a box, ready and waiting for just the right time. If you ever meet me at a race and are in need of a prop, just stop over at my table. Proper Balancing “Your prop has to be balanced,” you often hear. But what is balancing, really? What most guys mean when they say their prop is balanced is that they’ve put it on a prop balancer and found that both blades weigh the same amount. This is static balancing, and is a good first step in the process of increasing performance. It has taken quite a few years and quite a few props, plus several good books, to find the things covered in the next few pages. For balancing you need a good balancer. My personal favorite is the Top Flight magnetic balancer. This balancer can be so accurate that you can actually see the effect of finger oil when static balancing your props. It can also be used for a variety of model balancing jobs, not just boat props. It is the best for small boats, though, because it is the only one sensitive enough to really show when a tiny, low-mass propeller is balanced. It’s worth every penny of the $20 or so it’ll cost you. Precision Equipment Notice that I said above that the Top Flight balancer “can be” so accurate that you can see the effects of finger oil. To achieve that level, though, you need to make a change. My experience is that you will need to replace the horizontal shaft with a precision piece of steel. My recommendation is to obtain a “drill blank” of 1/8” diameter from a tool maker or tool supply shop. Have a machinist grind a fine point on it which is dead on the center of the rod. Even better would be to have him make you a shaft which goes from slightly undersize to slightly oversize for the propeller shaft size(s) you will be using, but most machinists do not have the tools to accomplish that. Be happy with a straight shaft with a single good point on it. (Note - it is only necessary to have a point on one end. The other end can remain square, or could have the sharp edges chamfered or rounded slightly). After you have obtained a good, perfectly straight shaft, the next thing you need to do is balance it. It can be done, but it takes patience. If you balance the shaft properly, you will be able to balance your propellers properly. If you cheat, you’ll lose the benefit of having such a good balancer. Don’t be afraid to take the time to do the job right. Use 320 or higher grit wet-or-dry sandpaper with light oil (I use haircutter oil from my barber for all oil sanding) to remove weight from the heavy side of the shaft. Wipe the shaft dry each time you check to see if the shaft is balanced. Remember, this is a very precise tool you are working on, and you want it to be perfect. The weight of cutting oil will fool you! The next tool you need is a handle for holding the prop while you work it. I use a 4” piece of 1/8” driveshaft with a drive dog near one end, and a 1/8” wheel collar (from the airplane department at the hobby shop) to hold the prop on. It is best if you file a small, square notch into the side of the shaft so that the set screws do not allow slippage. If you are working with props for other shaft diameters, use an appropriately sized tool. |
