Before being assembled in a R/C boat, the prop must be balanced and sharpened.
Out of balance causes
- reduction on the vibration that reduces engine, drive train and radio life and, in extreme cases, may even causes loose of boat control by strong interference on the radio system;
- performance increase (more speed);
- less prop walk (the tendency the boat has to deviate to the right as a reaction of the prop turning left);
- the tolerances on props fabrication are far bellow our necessities props from the manufacturers are not well balanced;
- props suffer wear and tear from the normal use of the boat;
- repairing damage props almost certainly will cause an out of balance situation;
- modifications made to the prop (cupping, sharpening) may render it out of balance.
there are two types of props frequently used on our gas boats: stainless steel and cooper-beryllium. SS props are more difficult to work on, because of the material hardness. The second type, although more soft, leaves a harmful residue. Be careful when working with them (by the way, the more frequently used). Work in a well ventilated area, wear a protection mask and use a vacuum cleaner to remove all the dust.
during all the balancing and sharpening process we will be talking about the prop front side and back side, leading edge, trailing edge, etc.
To prevent any doubts about what are we talking, mainly because front side and back side is not a universal concept and adopted by everyone, the pics bellow try to fix those concepts.
As we use here, front side is the side where the fixing nut screws on the shaft, back side is the drive dog side. This is an important understanding, as another papers may use just the opposite to name the sides and both will be correct. But can't be misunderstanding about the way and the side where to take material to balance the prop, be it named front or back.
Balancing and sharpening
- fine file;
- an assortment of wet sand paper;
- a Dremel (if you are used to use this tool).
If you are using a magnetic balancer (the one where the prop shaft is suspended between 2 magnets) check if the prop you are going to balance is not attracted by the magnets. Cooper-beryllium props are not, some stainless steel could be. If this is the case, better use another balancer as the magnets can disturb the balancing.
- with a #250 wet sand paper (or a Dremel with the correspondent sand paper) clean the rough production marks (keep the paper sand wet with fine oil during the operation);
- with a smooth file, sharpen the prop leading edge - the sharper, the best. As a RC saying: if it is not cutting the finger, it is not sharp enough. Attention: sharpen the prop tip to maintain a smooth angle from the leading edge trough the trailing edge. The transition from the tip to the prop center must be smooth and practically not visible;
- use the file to maintain a 90 deg angle between the trailing edge and the blade surface;
- the trailing edge is thicker near the hub and thinner at the leading edge - this shape is to be kept;
- fix the prop to the balancer and make it turn freely. The heavier blade will come down. Mark this blade, so you may identify it easily;
- with the file, take off material from the heavier blade. Attention: take material only from the back side, the drive dog side;
- check the balance frequently, until the prop stops in any position, meaning that the blades have the same weight;
- again with the # 250 sand paper smooth the file marks;
- continue with even more fine sand papers, smoothing even more the prop;
- at the end of each step, back to the balancer;
- when finished with a # 400 sand paper (or #600 if you are in the mood to) and the prop is perfectly balanced, use a polishing product, the Dremel and polish the prop. There are controversies about polishing both sides. Anyway, at least the front side (nut side) has to be polished.
That's all. Your prop is sharp, polished and balanced (at least statically). A few experts take another steps and make the dynamic balance of the turning prop, like the tire of your car. Here is an article by Andy Kunz about the issue:
After you have finished static balancing, you need to make sure your prop will stay balanced when running at top speed. This is a lot more difficult to do, and it's where I sometimes cheat myself. If you've taken the time to balance your prop as described above, you're already doing better than 95% of the rest of the guys out there. Here's how to get that extra 5%.
Spin balancing is what they are doing to your car tires when you have them balanced at a garage. The tire is spun, and a computer inside the tire balancer tells the operator where to place a weight and how big it should be. He will spin the tire multiple times, until the computer tells him it's done. Since there aren't any such tools available for us, we need to make do with what we can. This is another one of the other reasons I like my Top Flight balancer.
Basically, we will be spinning the propeller while using the stroboscope to make it appear as if it isn't moving. If it isn't running true, we remove weight from the heavy blade until it is balanced. Here's how to do it:
First, make a small mark with a permanent marker on the tip of one blade. Next, you spin the prop on the balancer shaft. While it spins, shine the strobe on the prop, watching carefully to see if the shaft is rotating perfectly. By watching carefully, you will be able to see if the shaft is running true or moving in a circle. The ink mark will allow you to identify which blade is heavier (the heavy blade will appear to be on the outside of the wobble). Lightly oil sand the heavy blade to adjust it. Repeat this until the prop is moving in a perfect circle, with no movement of the shaft. If you have done a perfect job of this, your prop will still be perfectly static balanced. If it isn't, you will have some vibration at certain speeds but not at others. The goal is to make it perfect at all speeds, but if you can't do that, just make sure it's perfect at operational speed. The ideal spin balancer would spin the prop at the speed you would be turning it on the water. I don't have a tool, which would allow me to do that, so I just spin the shaft with my fingertips. If I ever figure out how to get 30K RPM on the balancer, it will be a good day at the races!
Mount the prop on the drive shaft in your boat, and run the motor up to the expected operational speed, using the optical tach to find that speed. It will probably be between 50% and 75% of the maximum speed your motor turns, so you can work it from that angle as well. (This is where stick radios with the spring return taken out can be very handy.) Check to make sure the prop runs true at operational speed. If it doesn't, you can find the heavy blade as you did on the balancer, but it might not be the prop that's out of balance! Try making sure your drive shaft is balanced as well - it should run true at even full RPM. This is a very fine detail, but it will help keep you from wasting power. One little hint - always mount the prop the same way on the drive shaft. I like to make a very tiny notch, which identifies how the prop mounts to the drive dog. This will help you keep your drive system balanced, not just the prop.
Author, Andy Kunz.