# Thread: Electric propulsion design guide

1. ## Electric propulsion design guide

This video tutorial is the latest in a series that I am working on. It is a continuation of my own learning curve that started with building a solar drive for my kayak. The more I explain the basics to others, the better I end up understanding them myself.
It was made to assist other DIY enthusiasts in understanding motor specifications without going into any theory. Just simple, practical equations and ratios.
I work through two examples - a 12V chordless drill motor as well as the 18V motor that I used in my own homemade trolling motor. In the latter example I show how you can calculate a long list of useful motor characteristics from only a handful of known specifications.

2. Senior Member
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## Re: Electric propulsion design guide

Well I'll be following with interest. Thanks for posting it.

3. ## Re: Electric propulsion design guide

For brushed motors with good efficiency the following can be used as a quick reference:

If you know the following:
stall torque

Then maximum power = (max rpm x stall torque) x pi/120
if torque units is Nm then the answer will be in Watts.

This assumes "full throttle", i.e. maximum rated Voltage is being applied.
Maximum power will be put out by the motor at half the no-load rpm.
Torque will be half the stall torque.

The motor will operate at maximum efficiency if the load is such that the motor runs at +/-87% of no-load rpm.
Power will be around 40-45% of maximum power.

4. ## Re: Electric propulsion design guide

There are many aspects of the power train that still need exploring and testing.
I hope to make tutorials on topics like brushless motors, propellers, more on solar panels and charge controllers etc. Feel free to raise questions on or make suggestions on what else could be included in future videos, or for discussion here on the forum.

5. ## Re: Electric propulsion design guide

I have built a dyno to test a punchy little brushless motor.

The motor is a Keda 195kv. Actual kv tested is 215 (no-load rpm/voltage).
Max current is rated at 80A, nominal voltage is 22V (6-cell Lipo).
I did tests (i.e. varied the torque load on the shaft) at 13V and 26V using LiFePo batteries. These tests all happened a the max power setting (i.e. "full throttle") on the ESC.
The ESC was controlled with a servo tester. The servo tester connects directly to the ESC with the standard 3-wire servo cables used on rc models. No need for radio transmitter and receiver. Just a word of caution when using this approach: dodgy 3-wire plug connections can result in rogue throttle signals to the ESC.

Efficiency for both voltages peaked at around 92%.
Max efficiency rpm is around 90% of the no-load speed.
Shaft power at peak efficiency was 150W and 300W respectively.
13V gave efficiencies greater than 88% between 120W and 210W.
For 26V this "efficiency band" ranged from 240-420W.
Both voltages yielded peak efficiencies over a very similar torque band and peaked at almost exactly the same torque.
Rpm at peak efficiency was 2500 and 5000 respectively.

I have been promised a double surf-ski hull (46cm x 7.4m!) and will hopefully start building my next solar-race boat soon. No, it is not wood, sorry.

6. ## Re: Electric propulsion design guide

I also did a test at 26V and partial "throttle" on the ESC.

The Short version -

I selected point on the 13V curve of around 100W with an efficiency of 87%.
At that same power, the 26V "full throttle" curve showed an efficiency of only 65%.
I played around with the ESC throttle until I had the same output rpm and torque. Motor-ESC combined efficiency worked out at 77%.
I chose to test at a specific output torque and rpm so that it can be assumed that a similar propeller is used as might be used in the 13V case. This implies similar propeller efficiencies.

To appreciate the differences in efficiency, consider the power required to get 100W at the shaft. At 87% efficiency you need 115W, at 77% efficiency you need 100/0.77 = 130W and at 65% efficiency you need 100W/0.65 = 154W from the battery.

The 26V full throttle case also suffers from lower propeller efficiency due to high rpm. So 100W on the shaft will not result in the same propulsive power as 100W on a slower shaft.
Last edited by whiskeyfox; 10-26-2019 at 03:30 AM.

7. ## Re: Electric propulsion design guide

The long version - To explain I will use an example:

At 13V full throttle, the motor puts out 100W at around 2660rpm. I am assuming a 3.7:1 reduction gear drive so the prop rpm is 719.
From hull drag simulations and many fudge factors later, lets say my kayak requires 77W propulsive power to cruise at 4kts.
If motor efficiency is 87% as tested, I will need 100W/0.87 = 115W coming in from the battery to get 100W on the motor shaft. Assume the gear reduction is 97% efficient.
Power on the prop shaft is therefore 97W.

My calculations show that a 12" x 8" prop will absorb 97W and run at 79% efficiency at that boat speed and rpm. 97W x 0.79 = 77W. So I can maintain 4kts on 115W.

At 26V full throttle the motor puts out 100W shaft power at 5373 rpm (1452 rpm on prop shaft). This greatly limits the size of propeller if it is to absorb only 100W at such a high rpm.
Propeller pitch is determined largely by the rpm and boat speed, and in this case it is 4". The largest 4" prop that can turn at this rpm and only absorb 100W is around 9". This propeller would have an efficiency of only 75%. To produce 77W of propulsive power the propeller will need 77W/0.75/0.97=106W from the motor. The motor is only running at 65% efficiency so battery power required to maintain 4kts will be 106W/0.65 = 163W.

For the partial throttle case I now keep the ESC powered by a 26V battery but use the 12" x 8" prop. By advancing the throttle until the motor rpm is at 2660, the output power and torque will match that of the 13V full throttle case. As before, the propeller will be turning at 719rpm, 79% efficiency and pushing the boat along at 4kts.
The only difference is the efficiency of the motor-ESC combination which is down to 77%. Power drawn from the battery will be 100W/0.77 = 130W.

Overall propulsive efficiencies are therefore -

13V, full throttle: 77W/115W = 67%

26V, full throttle: 77W/163W = 47%

26V, partial throttle: 77W/130W = 59%
Last edited by whiskeyfox; 10-26-2019 at 03:57 AM.

8. ## Re: Electric propulsion design guide

To get an idea of how much the gear reduction contributes to overall efficiency I recalculated the cases discussed above but without the gearbox.
I used 100W on the motor shaft as the common reference. The prop is now direct drive, so no gear losses apply.

My kayak requires 61W propulsive power (i.e. thrust x boat speed) at 3.7kts.
At 13V full throttle the prop would turn at 2660 rpm. It can turn a 7" x 2" which would have an efficiency of 61% so it would be just enough to maintain 3.7kts boat speed.
Motor efficiency would not have changed (87%) so battery power required is 115W as before.

At 26V full throttle the prop will be turning at 5373 rpm. The motor would be at 65% efficiency so it would need 154W from the battery to put out 100W at the shaft.
A 5" x 1" prop will absorb 100W shaft power at this rpm but only put out 45W of propulsive power. This would only be enough to push the kayak along at 3.4kts.

Overall efficiencies -

13V full throttle: 61W/115W = 53%

26V full throttle: 45W/154W = 29%

26V partial throttle efficiency will again lie somewhere in-between these values.

Please note that all propeller efficiencies used in these calculations were obtained from theory. Actual values might differ due to assumptions made during the calculation process.
I try to keep these comparative calculations as realistic as possible but errors do occur and sometimes it is necessary to fudge a number to simplify the explanation. With so many variables at play it is not always practical to compare "apples with apples". Use the results for what they are, a rough indication of performance trends as key variables change.
Last edited by whiskeyfox; 10-26-2019 at 05:43 AM.

9. Junior Member
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## Re: Electric propulsion design guide

Wow, friend! This looks absolutely amazing and there is so much material here that I hope I’ll be able to capitalize on when I start looking at my own boat. What has been the most difficult thing about this process? Are there things that you wish you’d done differently, or that you want to change in the future with this build?

10. ## Re: Electric propulsion design guide

Wow, friend! This looks absolutely amazing and there is so much material here that I hope I’ll be able to capitalize on when I start looking at my own boat. What has been the most difficult thing about this process? Are there things that you wish you’d done differently, or that you want to change in the future with this build?
The design process is not difficult in principle. The theory can be reduced to a few simple ratios and rules of thumb to get pretty decent performance. My own practical experience is limited to the solar kayak I built a year ago and the more recent dyno tests. There might yet lurk a variety of practical pitfalls to a solution that looks good on paper. I intend to throw the "full house" at the next build as far as hardware is concerned and will report on the more elaborate power-train as I learn along.

I now have two 300W solar panels and an MPPT controller. The panels themselves are "smart", they have some built-in gimmickry that act as a type of MPPT. MPP is at 31V and 9.7A, but it will maintain the full 300W down to 24V and 12.5A. I hope the real MPPT knows how to handle this unusual power curve. The ESC-MPPT is another interface that needs to be tested.

The most technical aspect would have to be the spreadsheet program for calculating propeller performance. It is something I have developed over time, originally for aircraft props. I have never done any tests to validate the results.
Something about your post prompted me to check if there are published results for model aircraft props. I found that APC props have performance data on a vast range of propellers.

https://www.apcprop.com/technical-information/performance-data/

To their credit, they admit that the NASA analysis tool used to generate the data under-predicts drag at low speed. They provide this link to real test data performed by Prof Selig at UIUC:

https://m-selig.ae.illinois.edu/props/propDB.html

I was able to validate some of my spreadsheet results on propellers that fit my assumptions about blade geometry.

11. ## Re: Electric propulsion design guide

I finally got around to making the video on propeller performance.
It is still just a high level explanation without detailed equations.
The focus is on performance graphs to explain the differences and similarities between model aircraft props and marine propellers.
In the process I also demonstrated how variables such as pitch to diameter ratio, blade area ratio and blade count affect efficiency.

Data for the model props were taken from wind tunnel tests done at UIUC: UIUC PDB - Vol 1 https://m-selig.ae.illinois.edu/props/volume-1/propDB-volume-1.html
It happened to include the prop I had used on my solar kayak. This made for a convenient case study since I had first hand test data for performance in water.
Marine prop performance were based on the Wageningen B-series.
Reynolds number is also addressed and I show how it affects performance for model aircraft as well as marine propellers.

In a sequel to this I will look at the equations and show how to use them for different applications.

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new zealand
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## Re: Electric propulsion design guide

^ There used to be (15 years ago) a ton of really good info and active development on prop design, test and build, on the Human Powered Vehicle Association forums. That all seemed to implode at some point, but might be worth taking a look at. Those guys had made significant performance improvements over the model aircraft props, for HPV boating applications.

Pete

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