Read part 1 here

Choosing a propeller and motor

Now we know how much force the boat requires to move along, we can choose a propeller that produces this force, however manufacturers don’t include this information, at least not for small hobbyist size props. So what propeller to choose, well first, a little propeller theory…

Propellers produce more force based upon the number of blades, the pitch of blades and the length. More blades produce more force, but it also creates more drag in the water, therefore the least number of blades would be most efficient. However, as a one blade propeller would be unbalanced it would cause a lot of vibration and turbulence. Therefore increasing the number of blades will reduce vibrations and create a smoother force. This is probably why a submarine propeller has many blades, for quiet running.

So where does that leave us? 3/4 blade props are commonly sold for model boats so I’ll start with a 3 blade. It’s important to remember that we’ll have limited power, so we want everything to be as efficient as possible. However, having a large 2 blade propeller would not produce as much force as a 4 blade propeller and there could be circumstances where we would want to sail at high speeds where it is less efficient, but necessary to avoid other objects. Therefore when choosing a propeller it must both be efficient but be capable of producing a large force.

Different propellers can also produce the same forces. A small propeller that runs fast, can produce the same force as a large propeller that runs slow. That being said, larger propellers are generally more efficient.

Based on this, I’m looking for a 3 blade propeller that is fairly large, but as we don’t know the forces these propellers produce, we’ll have to do some more experiments.

Experiment Time!

So far we’ve only discussed propellers, but they can’t be examined without looking at the motor as well. The motor will have to have enough power and torque to turn the prop, and the larger the prop the more torque is required. Therefore we can’t just select a massive prop as the motor might not be able to turn it.

I set up in the lab (also known as the bath) a floating tupperware container with a motor and propeller, and a hook to measure the force produced. At the same time, the speed (RPM) of the motor and current was measured. This was repeated for a range of propellers of different sizes, although it quickly became apparent that my tupperware was no match for the choppy waters. Plus with 1kg of force being produced it was a little difficult to hold with one hand and measure and operate the controls without water going everywhere. The solution, 3D printing!

Measuring forces with the ‘Tupperware’ boat.
Checking to see if the motor is working by using my drone as a test to power it.

So I printed out a small boat with it’s own dock that would be weighted to the bottom of the bath. This would allow me to operate the motor and take measurements without having to worry about the boat and water flying around. It was my first time 3D printing, so I’m sure I’ll cover that in another article.

There were a few problems whilst taking the measurements, the first being that the motors were occasionally jittery and some times would not spin at all, especially for the larger prop. So to troubleshoot, I did some technology CPR and got my drone, and connected it up to the motor. Using the drones controls, the motor ran at a smooth, full speed, so the motor was completely fine. After checking my setup I realised the issue was the wires connecting to the motors. I used crocodile clips and when I looked at the connections they were mostly crimped with one strand of wire. No wonder the motor was jittery and not functioning! Lesson learnt, don’t buy cheap Chinese wires!

When the motors did run, there was so much force produced that the propeller would unwind itself off the shaft. This was solved by installing a locking nut up against the propeller.

With the problems fixed, and data collected, I took all the measurements. Now, let’s look at the results! 

The Results

The above graph shows that for each propeller as it spins faster more thrust is produced, and even though our data is slightly noisy, you could say it’s close to a linear relationship. This is as expected and as the propeller becomes larger, it produces greater forces at lower RPMs. If we kept spinning the propellers faster and faster, they would reach a point where they do not produce more thrust, however we are far from that point.

Notice how the last 3 points for the 65mm prop stay around the same place. This is because as I was increasing the throttle it reached a point where the prop wouldn’t spin any faster and the motor also became very hot. The motor didn’t have enough torque to turn the pop faster. This means that the motor was operating beyond it’s maximum and further use would damage the motor.

Looking at the relationship between current and thrust, we can see that as the propeller size increases less current is used. For 600g of thrust the current used is 2.9A (35mm), 2.6A (45mm) and 2.1A (65mm). This confirms that the large propeller is more efficient.

For the 35mm prop the maximum thrust produced is 730g (because it’s a tiny prop), and the  45mm prop produces 1250g of force, which is considerably larger. However the 65mm prop only shows a maximum of 800g. As mentioned previously, it’s because the motor doesn’t have enough torque to spin the prop faster, however if we used a more powerful motor we could expect the line to continue and produce more thrust than the 45mm.

So with the current motor, we can produce the most thrust with the 45mm prop. However the most efficient thrust is produced with the 65mm prop but it’s maximum thrust is limited due to the motor. As the boat will be running on minimal power we’ll opt for the larger prop, but if we decide to go even larger or require more thrust we’ll have to change the motor.

Now we’ve chosen what prop to use, combined with the motor, what’s its most efficient operating point.

For the 35mm prop it looks like its most efficient point is anywhere after 40%, for 45mm it’s after 30%, although this is a little tricky to interpret due to the fluctuation in the readings. For 65mm the most efficient point is at 15%, after this, there is a decrease in efficiency.

To conclude, we know that the 65mm prop is most efficient, but it’s maximum thrust is limited due to the motor. The 45mm prop should be used if we want maximum thrust on the current motor, and the most efficient point to run the 65mm prop is at 15%.


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