Propeller Sharpening and Balancing for New Modellers
By Chris Breen
Our metal props need to be both sharp and balanced. Balance is needed to stop the boat vibrating itself apart and to allow the RPM to reach their maximum. Remember our drive lines are (hopefully) spinning at around 20,000 to 30,000 RPM. Tell that to any mechanical engineer and they will generally shake their heads. The turbine in a Boeing 767 engine turns around 10,000 rpm for example, and your car engine turns a leisurely 2500 rpm or so at highway speed.
A few props can be purchased already balanced and sharpened, but most need work before use. Recently CNC machined alloy props have appeared which seem to be pretty well balanced and sharp via the manufacture process, particularly in the small diameters (up to say 35 mm). These can probably be used immediately, provided a visual inspection shows a nice sharp leading edge.
The leading edge is the edge that cuts the water as the prop rotates. See picture. A blunt prop will cavitate badly and lack drive. When we describe the leading edge as sharp, ideally we mean sharp like a chef’s meat carving knife rather than sharp like a chisel. We want a nice, finely tapered and sharp leading edge. As cast, the propeller blades can be quite thick and the edge blunt, so to create this “meat carving knife” can mean a lot of filing work. The finest taper is not absolutely essential, but the finer you can make it the better. What is most important is the actual edge, which must be as sharp as possible.
The trailing edge is not sharpened but left square and should be generally left as manufactured apart from possibly some minor work to tidy up any mould imperfections. Note that some props have a distinct cup on the concave surface at the trailing edge, which is deliberate. Do not remove this cup. (see sketch). Some props also do not have an obvious point where the leading edge finishes and the trailing edge starts. These props are probably best sharpened to some degree at least, around the curve until the straight part of the trailing edge is reached.
Now, every prop blade has two sides. The side facing the stern which is concave (or slightly dished), and the back face which faces the bow and is convex. The contours and shape of the dished face is what does the work and we should only remove metal from the rear, convex side of the blade. Other than perhaps a gentle rub with fine wet&dry paper to polish the concave side or to remove some corrosion, DON’T remove metal from the concave surface of the blade.
Some people recommend balancing first and then sharpening, but it doesn’t really matter. The objective is to achieve both in due course. You will need a prop balancer. These can be bought or home-made. The bought magnetic balancers are much more sensitive, but a serviceable home-made unit can be made with some bits of wood, two fresh Stanley Knife blades, some CA glue, and a length of prop shaft material (see photo). To ensure the two knife blades are parallel, hold then against a pane of glass as you apply the CA glue. Place the balancer on the work bench (needs to be sturdy) and shim the base block with bits of paper or card until the prop shaft spindle will NOT roll either way. The balancer is now ready to use. To hold the prop for sharpening I use a 150 mm long piece of about ¾ inch dowel into the end of which is glued a short length of threaded bolt of the right diameter. Drop the prop onto the bolt and fix firmly with a nut and it is easy to hold while you file.
Finely ground metal dust can be quite toxic so wear a face mask. I begin by roughly sharpening both blades and then test to determine which blade is the heavier (heaviest for three bladders). Mark it with a felt tip. Work on the heavier blade(s) until neither seems heavier. On the balancer the heavier blade will swing down and the prop will rock to and fro before coming to rest. The centre of mass of the prop will come to rest directly below the prop shaft so try to concentrate metal removal from the parts of the blade nearest to directly under the spindle. For example if the blade once at rest hangs vertically down, you can remove metal all along the blade but removal near the tip will be more productive.
Your prop is balanced once you can place it on the balancer in any orientation and it will not swing and rock to and fro and doesn’t come to rest in the same position every time. This perfectly balanced state may be quite difficult to achieve, but don’t lose heart. Just get it as close as you can be bothered doing, and it should be Ok.
Once you are happy with balance and sharpness, rub out all the file marks with wet&dry paper under a dripping tap. You want the worked surface to be nice and smooth and nicely blended into the rest of the convex side of the blade.
Once sharp, the prop should stay pretty sharp for a good period (years) provided it doesn’t strike debris or gravel or another boat (or other props in the prop box!) Generally a light rub over with fine wet&dry to remove any corrosion is all you will need to do. Some people polish their finished props to a mirror-like finish, while others allow the dull copper appearance to develop naturally. Which is better is a subject of debate, but if some rough greenish corrosion appears this should be removed with a gentle rub with wet&dry.
Now all you need to watch is that you don’t cut yourself while fitting or removing the prop.