Propellers For Fast Electrics

By Paul Williams.
Article posted 24th October 2007.

The propeller is one of, if not the, most important components on a fast electric powerboat. To put it bluntly, everything else you do on your boat - all the money you may spend on high performance cells, motors, hull and hardware, and all the time and effort you expend on building - is wasted if the propeller you fit is either the wrong size or type. Similarly, if the prop chosen is not carefully sharpened and balanced, not only will you never see anywhere near the top speed the boat is potentially capable of, you will also cause a lot of damaging vibration. Your boat will also be very noisy, and people might mistake it for an IC boat, especially if it's very slow.

The propeller is the final link in the chain. It's the point at which all the power the motor and cells produce is turned into thrust. Get it right, and your boat will fly. Get it wrong, and you will have a very expensive, very inefficient water heater in the shape of a powerboat.

How does the prop produce thrust? Put simply, the propeller throws some water backwards on each revolution and, true to Newton's Third Law Of Motion, imparts a reaction force to the boat pushing it forwards through the water.

Types Of Propeller

You can buy propellers from Octura, Graupner and a variety of other suppliers. Octura in particular make a vast range of props which, at first glance, can seem bewilderingly confusing. Props can be made from beryllium copper, plastic, carbon fibre and stainless steel and can be broadly divided into those suitable for hydroplanes and those suitable for cats and monos. This distinction is down to whether the prop is a "lifting" design or not.

Octura propellers. Investment cast in beryllium copper. Top: P747. Left to right: X432, 1732 and X632.

A hydroplane can use a high-pitch lifting-type prop as the hull is designed to ride on two forward planing surfaces and the prop (hence "prop-rider"), whereas a mono or a cat would use a propeller that provides little or no lift. It is generally true that hydroplanes can use both high-lift and low-lift propellers, but cats and monos tend to stick to low-lift propellers primarily because low-lift props tend to be slightly more efficient, do not slip as much, and provide better acceleration from rest and out of corners.

Choosing A Propeller

Factors which affect choosing a suitable prop include:

• hull type
• motor characteristics
• boat usage

If you have an outrigger hydroplane, you will have the choice of running props such as Octura's 17 series that are designed to provide lift. However, you could also fit an M, Y or X series, perhaps to reduce motor load or to fine-tune the boat's handling characteristics.

It's very unlikely you would choose to fit a high-lift high-pitch hydroplane propeller to a catamaran or monohull. The high slippage and lift of these props tend to make them unsuitable for hull types other than hydroplanes. Lifting the transom of a cat or mono will introduce all manner of undesirable handling problems.

Home-made magnetic propeller balancer. I made this from some scraps of delrin, a length of 1/2"∅ aluminium bar and a couple of neodymium magnets. The balancing shaft that the propeller is mounted on is made from silver steel that has had 60° points precision ground on each end.

In the days before cheap brushless motors, 7 and 12 cell models used 540 modified buggy motors mounted to a reduction gearbox. The gearbox was necessary because the 540 simply doesn't have the torque or efficiency to directly drive a sensibly sized propeller. The only way to get the 540 to perform was to allow it to spin at very high rpm on a gearbox, using the greatly increased torque to turn a much bigger propeller.

Today's highly efficient brushless motors can directly drive bigger propellers than a 540, without the added weight, complexity and power losses inherent in using a gearbox. The accepted wisdom of years past was that high speeds needed big propellers. While it is true that a large propeller is theoretically more efficient than a smaller propeller, in practice high speeds are achieved nowadays with smaller propellers turned at high rpm.

If you are building a boat powered by a Speed 700 BB motor, you will need large propellers in the 45-50mm range, as the 700 BB is quite a slow-revving motor and needs a large propeller to provide useful thrust. Unlike a brushless motor, the 700 BB can only draw so much current before it simply burns up.

Choice of propeller is also dictated by what you want to use the boat for. If you are racing, you want to use every last drop of power your cells (nimh or lipo) can deliver, whilst achieving the necessary run time for the class. If you want to have a crack at setting speed records, or simply want the ultimate speed for fun-running, you need the propeller that gives the highest speed without forcing the motor to draw destructive levels of current (destructive to the motor, the speed controller or the cell pack, or all three!). If you want to long run times for fun-running, choose a smaller propeller to reduce motor current drain and increase run time.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

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Last modified: 08th July 2010 @ 09:06