Selecting Propeller

by Colin Vauvert
(Wollongong - Australia)

I have just fitted out a 965mm (38") shallow V mono hull, transom is 360 (14") x 100 (4"). I fitted a 2550kv/rpm brushless motor, with a 180A programmable ESC, powered by 2 x 7.4v 5200mA LiPo's @ 14.8V. The servo and transmitter and receiver are Spectrum.

The shaft and fittings are all 4.76 (3/16") with a 60mm strut, surface mounted (flush) shaft is 100mm (4") long and dual 145mm L (5-1/2" x 25mm W (1") 76 support. 100mm (4") centers. It weighs 3.1 Kg with batteries installed.

Is there a way to calculate the right propeller for this configuration?

Kindest Regards Colin (Aussie Battler).

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To my knowledge - No, there is no formula going from a bare bones specification to a diameter-and-pitch answer. There may be a formula to extrapolate a similar, known boat/motor/prop combination to a usable starting point for final selection. Read on.

This is a fast boat you're building, and I have to admit I have limited experience optimizing RC speedboat propellers.

Typically, when choosing a propeller you need to get a handful and compare which gives the most desirable result, whether it be highest speed or longest run-time or a compromise.

Props can be expensive, so staring out with a narrow field is going to save some time and money.

Here are a few ideas that I would try:

• Motor Manufacturer Recommendations
  If the motor is a "Marine" motor, chances are there is a recommended range that they would recommend. You could shoot them an email and see what they come back with.


• Compare with a known kit
  Compare with commercial kits of similar design, size and weight. Unless they have propellers listed as spare parts, it may not be apparent what size comes with the kit. If that's the case, you can ask at a decent forum (RCgroups.com for instance) or your local hobby shop.
  
  I just did a quick look at Tower Hobbies and found a Traxxas Spartan Brushless deep vee. It's about the same length as your model boat, but weights less and deep vee (similar enough though).
  
  Even if this is not a perfect match, any info you can find about it's propeller can be helpful. See what size brushless motor it has. Because of it's smaller hull size and lower weight, chances are the motor is one or two sizes smaller. By finding out what the stock propeller size is, you know your propeller should be bigger.
  
  See if you can find a closer match. I'd put emphasis on finding a kit with close to the same weight and similar motor spec if at all possible.
  
  There may be a formula to take you from a known motor-propeller combination to determine what diameter propeller you should start with (presuming pitch stays the same). I know power and displacement (weight) scales to the cube, but swept area of the prop squared. I'd be guessing if I took this any further. You could ask a propeller manufacturer if they know of such a formula.


• Get Advise from a Propeller Manufacturer
  You'll probably have better luck getting a response the smaller the manufacturer. They want to sell you a product, so they may be able to give you an idea. Naturally, it makes sense to give them the business if you're happy with their answer. You could try the Prop Shop in the UK for instance.

If you find there is a formula as outlined above, please use the "comment" feature on this post to enlighten us or shoot me a message through the contact page. I'd appreciate it!

Once you have a few propellers to try, it's a matter of comparing clocked speed and run time per charge.

Best of luck!

Petter