Propeller Preparation

By Paul Williams.
Article posted 03rd November 2008.

Metal propellers from Octura are cast in beryllium copper. Other materials are used to make propellers, for example stainless steel, but the vast majority of metal propellers are beryllium copper. Before a metal propeller can be used, it needs to be sharpened and balanced.

Surface piercing propeller diagram.
Surface piercing propeller. Thrust is produced by the concave face, and you must not modify the shape of this face when preparing the propeller or you risk altering the propeller's characteristics. Material is removed evenly from the convex face to sharpen and balance.

I often get asked if I sell ready sharpened propellers, and my answer is always a firm "no". I hate sharpening propellers, and am only prepared to do my own!

It is possible to buy propellers ready sharpened and balanced for you. However, in my experience propeller preparation takes a long time, perhaps as much as two or three hours per propeller. People who sell sharpened and balanced propellers usually do the bare minimum to make a propeller usable, but "usable" is not the same as "perfect". We're after perfection, or as near as it's possible to get, and this means doing it yourself. This is not meant to denigrate those vendors who sell propellers and charge for sharpening and balancing. The blunt economic reality is that time is money, and if you want a near-perfect propeller, then it's going to cost a lot more than one that is merely usable. Ask yourself what price you would put on three hours of your time?

So, what is the difference between a "usable" propeller, and a near-perfect propeller? The difference can be remarkable. The propeller determines motor rpm, run time and speed, and a usable propeller will be down on all three. If you want to win, you need to have decent propellers for your boats - it's that simple.

A propeller with razor sharp, thinned blades does not waste as much energy on each rotation. Thinned, sharp blades enter the water with less impact and therefore create less spray, with more of the motor energy being converted to useful thrust.

Safety

Before going any further, it is imperative that before you start working with beryllium copper, you fully understand the health and safety aspects of this material. Beryllium copper ("BeCu") is toxic, and in its solid form presents no great hazard. The danger lies in the dust that is produced when BeCu is worked or machined. BeCu dust causes serious lung damage and is carcinogenic. For this reason, it is strongly recommended you do not use power tools to prepare BeCu propellers, as the last thing you want in your workshop is a fine mist of particles in the air that you will end up breathing in, and that will persist as dust for months or years.

I keep a seperate set of files for propeller preparation, and do most of the work on my propellers using various grades of wet-and-dry abrasive paper used wet, as the water flushes the wet-and-dry clear of dust and also traps the dust in the water. Also, files tend to clog easily when working BeCu, and can cause deep scratches. If I can avoid using a file, I do. However, to remove a large amount of material from the blades of a propeller you need to use a file, swapping to wet-and-dry for final finishing and balancing.

Preparation

The steps required to take a propeller from the new, as-cast state to ready-to-race are simple. First, you need to thin the blades by removing material from the convex face (see the diagram to explain convex and concave faces) using a file and wet-and-dry abrasive paper used wet. Some propellers have blades that are nearly the same weight as-cast, others can be radically different, perhaps even with one blade visibly thicker than the other. You may wish to check the initial as-cast balance to see which blade is going to need more material removed as you progress.

Rear view of a surface drive propeller.

Propeller Sections.
Propeller section "A" as shown above. In this diagram, 1 is as-cast. 2 is the sort of shape you can buy, and is usable but far from optimal. 3 is the kind of profile we're looking for; thinnned and razor sharp, this propeller will be a screamer. A razor sharp propeller with thinned blades has a lower impact force as the blade enters the water on each rotation, which reduces spray and increases propeller efficiency.

The key to producing a good propeller is to work over the entire face of each blade. What you are seeking to achieve is a propeller with thin blades, razor sharp edges and which is well balanced. This last point brings us onto balancers.

Propeller Preparation.
The trailing edges must be kept sharp to allow water to shed cleanly from the blades. Do not alter the concave face of the propeller - you risk reprofiling the propeller and altering the pitch or cupping.

Propeller Cupping.
Cupping on the trailing edge of the propeller blade alters the shape of the propeller's thrust cone. More cupping results in a narrower thrust cone which gives better acceleration and less vertical lift - the propeller "grabs" the water harder, producing much better acceleration from rest and out of corners.

Balancers

The only type of balancer that is sensitive enough to balance small propellers is the magnetic type. A magnetic balancer consists of a shaft which is suspended between two magnets. The propeller is threaded over the shaft, and when mounted between the magnets, the heavy blade will drop to the bottom.

There is only one problem with the magnetic balancer: unless the balancer is a quality item, it simply will not be accurate. The most important part of the balancer is the shaft, which must be dead straight and have ends which are precision ground. If the shaft is even slightly bent or curved, or if the tips are not machined or ground concentric to the shaft OD, then the shaft itself will be way, way out of balance and it will not accurately tell the balance state of a propeller.

Surface Finish

There is a myth that a highly polished propeller works better - it doesn't. Indeed, the reverse seems to be the case, that a mirror finish actually degrades performance. My personal preference is to finish a propeller with 600 grit wet-and-dry paper, leaving a satin finish. However, if you feel the need for a highly polished propeller, go ahead - but rememeber that BeCu tarnishes over time, and a mirror finish won't last once you get it wet.

Propeller Modification

This article about propeller preparation assumes you want to use a propeller as designed by the manufacturer, and for the vast majority of people (myself included) a standard propeller works fine. It is, however, possible to modify the size and shape of a propeller to achieve various effects. For example, increasing or decreasing the cupping to change the amount of lift generated or to increase motor rpm, or altering the blade area and/or diameter to reduce motor loading, or to fine tune boat handling.

Changing the shape of a propeller is a techique more often used in IC boating where the small, high-revving piston engines used have a very narrow power band. Subtly altering the pitch or cupping can allow an IC boat to run "on the pipe", or in the rpm range where the exhaust pipe is working in its tuned range. Electric motors produce maximum torque from zero rpm, and do not need this kind of propeller tuning, other than to reduce current draw or to alter boat handling.

Modifying propellers is a bit of a black art, and beyond the scope of this article. To achieve any kind of successful modification to a stock propeller requires plenty of time for testing, plenty of money to buy propellers, a comprehensive knowledge of propeller theory and a selection of specialist tools like a pitch gauge and propeller anvil. For most of us, a stock propeller finished to a high standard will give perfectly acceptable results.

Conclusion

Metal propellers require a fair amount of work to get the best results, and time spent learning how to correctly prepare a metal prop will bring rewards in the shape of a faster boat that runs with a lower current drain and higher motor rpm.

However, if all this mucking about with files, balancers and toxic metals seems like a lot of hard work, you can always opt for carbon fibre propellers. These work very well indeed, and require no preparation at all.

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Last modified: 08th July 2010 @ 09:06