# How to design an electrical propulsion system?

Discussion in 'Electric Propulsion' started by Davezor, Apr 15, 2020.

1. Joined: Apr 2020
Posts: 5
Likes: 0, Points: 1
Location: Slovenia

### DavezorJunior Member

Hi there, my name is Dave and I'm currently studying electronics.

I and a few friends of mine decided to build an autonomous, solar powered boat quite some time ago and finally came together during the Covid-19 crisis to make plans and calculations so we can finally really start working on the project.

While we are all studying some type of electrical-mechanical engineering, nobody has any real experience with building boats and correctly sizing the propulsion system.

So far, we decided on the boat shape and an aproximate weight (including the batteries, motor, shafts etc.).

Boat length is 2m or aprox. 6.5f feet, width and height are both around 0.5m or 1.6 feet. We calculated the total weight to be around 100kg or 220lbs. Max. speed we are aiming for is 5 knots.

We designed a model and created it out of fiberglass and steel reinforcements. Right now, we are in the process of running simulations to determine required thrust (force) to move our boat at 5 knots in sea water with the help of CAD tools. The only real problem we have as of right now is correctly sizing the propulsion system.

I decided to take on this part of the project and determine the following:

- required electrical motor power
- determine the required propeller

I have no idea where to really start with the calculations since I'm more of a programmer than anything else. I get the results of the simulation from my friend - how much trust is required to move the boat at 5 knots and the maximum propeller size we can use in a few days. For the sake of it, let's say I already know these two numbers... how would I start with the calculations?

Currently, we are using a 12V DC battery supply to power the electronics and the motor we have not yet chosen. However, I bealieve that 12V won't be enough and current draw will be huge, that is why I need to make these calculations.

My plan:

From the max propeller diameter (implying max diameter is optimal for low velocities) and required thrust, find a few propellers that we could use. Depending on the propeller, determine the required RPM of the motor shaft and torque loaded on the shaft by the propeller. From the RPM and required torque, choose an adequate motor that can provide this torque at 12V/24V/48W DC and whatever current is required which results in the required motor power rating in watts [W]...

The problem with this approach is, where can I find these propellers and corresponding data I need?

Is there a better approach? If so, how?

2. Joined: May 2017
Posts: 1,612
Likes: 419, Points: 83

### BlueBell"Whatever..."

Tow the boat (at operating weight) at 5-knots and measure the drag with a fish scale or better.
That's how much thrust you'll need from the... propulsion system.

3. Joined: Apr 2020
Posts: 40
Likes: 8, Points: 8
Location: Slovenia

### sebasebaJunior Member

That boat would have a Freude number of 0.59, which for most hulls is hard to achieve due to wave drag.

Freude number is calculated as Fr = v / sqrt(g * L), g being 9.81 m/s^2 (acceleration of gravity), L length of waterline (2 m in your case), v being speed in m/s.

Up to Fr = 0.4 you can go normally, then at around Fr = 0.6 you have the "wave barrier", like the sound barrier but on water surface. That's very hard to beat, exponential amounts of power from Fr = 0.4 to Fr = 0.6, after you break it, you overtake the barrier. To achieve that, like jets that have special shapes, you have to make special shapes for boat, i.e. planning hulls. You raise from the surface and skim on it.

I suggest you either aim for lower speed, increase the length of the boat or research how to make those planning shapes.

(I could be wrong in what I wrote, I'm quite a newbie in boat building.)

BlueBell likes this.
4. Joined: Mar 2018
Posts: 720
Likes: 136, Points: 43
Location: Europe

### DejaySenior Newbie

5. Joined: Apr 2020
Posts: 5
Likes: 0, Points: 1
Location: Slovenia

### DavezorJunior Member

From hull simulation analysis in Ansys, the drag force at 5 knots is ~50N in water that is still.

If we take a safety factor of x2, the required thrust is 100N.

So we have...

- Required thrust: 100N
- Max. prop diameter: 12 inches

The objectives are:
- required RPM of a 12inch prop (pitch needs to be determined) to generate 100N thrust
- loaded torque on the motor shaft at RPM of the propeller
- required DC motor power and torque @ shaft

How to go on from here?

6. Joined: Mar 2018
Posts: 720
Likes: 136, Points: 43
Location: Europe

### DejaySenior Newbie

A quick search gave me this: DC Motor / Propeller Matching3 Mar 05Lab 5 Lecture Notes
You'll probably have to look at datasheets of propellers for the relevant data to match it. Or ask in RC boat forum where people might have more experience to recommend suitable propellers.

I have to learn all of this too - later. There is a book "The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers" by Gerr that I plan to buy and read. Later

7. Joined: Jul 2012
Posts: 281
Likes: 58, Points: 38, Legacy Rep: 14
Location: s.e. england

### alan craigSenior Member

As Sebaseba says, your boat is too short to do 5 knots, so any amount of modelling won't really help. As it is only 2 metres long may I suggest you build the actual boat and then use an onboard datalogger to measure power, rpm, speed etc., and simply change propellers until you get the best result. Model aeroplane propellers are commonly used for small electric (and human) powered boats.

EDIT I forgot to mention:
build the actual boat...because a 2 metre flat bottom boat is not a huge task and will give you a ton of practical experience and data.

Last edited: Apr 20, 2020
kerosene likes this.
8. Joined: Apr 2020
Posts: 5
Likes: 0, Points: 1
Location: Slovenia

### DavezorJunior Member

Thanks, I'll have a look at it.

I'll look into it, thanks.

9. Joined: Aug 2002
Posts: 15,189
Likes: 925, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

2X factor is really huge. Commercial motors, rated for continuous operation at full power are 110%.

10. Joined: Apr 2020
Posts: 5
Likes: 0, Points: 1
Location: Slovenia

### DavezorJunior Member

We took a 2x factor because not even simulation can predict every scenario. We will see how it goes and downgrade accordingly.

Oh and its not a flat bottom boat. Google for "sea axe hull". The submerged part is almost identical, the upper part that is not submerged is wider due to solar cell placement and not as high, very close to water. Imagine a curved aircraft carrier.

Simulated drag coefficient is 0.04

11. Joined: Mar 2018
Posts: 720
Likes: 136, Points: 43
Location: Europe

### DejaySenior Newbie

Theoretically 50N at 5 knots would be ~2.5m/s * 50N = 125 watts. But no clue how accurate that model in ansys is. I've played around with prelimina.com (similar method to michlet).

From what I've learned, efficiency and speed is determined to like 90% by length and weight(displacement) alone. So try saving weight or making it longer. But waves slamming into your deck will slow you down too.

Would love to see some pictures. I think these autonomous boats and vehicles could become really awesome with starlink global internet. Imagine being able to remote control it with little latency from your home

12. Joined: Apr 2020
Posts: 2
Likes: 1, Points: 1

### DrDaveNew Member

I notice your boat is shorter, thinner, and HEAVIER than I am. It's not going anywhere but to the bottom. Make it 16' long and at least 4 wide. Or not, what do I know. Make scale model or you'll need really big fish scales to measure it's resistance while towing.

You sure you didn't make a typo?

13. Joined: Apr 2020
Posts: 5
Likes: 0, Points: 1
Location: Slovenia

### DavezorJunior Member

We made a prototype a year ago, boat floats at the level we predicted and simulated. Even turns to its original position if flipped over.

We had no way at that time to test the drag.

14. Joined: Apr 2020
Posts: 2
Likes: 1, Points: 1