Propeller selection for slow robot boat

Discussion in 'Props' started by Nikarus, May 14, 2020.

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NikarusJunior Member

Hello everyone,

I am designing an automatic robot boat that will swim around and clean surface of the water.

Currently no hardware is built, so every component is flexible. I need to select proper prop and motor for the boat.

The boat is really slow. I need to get 200-300g (2-3 Newtons) pull force from the motor+prop, and the boat should reach speed of 0.5-1 m/s. The boat is battery-powered, so I am looking for the most power effective solution.

I've been searching the internet for several days now and unfortunately can't get my head around of what approach should I take to select the prop.

I have a clear understanding of what pitch should the prop have. But how do I select its diameter?

Basically what I fail to calculate is: given selected prop, what torque is needed from the motor for it to rotate at needed RPM?

So if I want to get 1 m/s speed, I can take a prop with 40mm pitch and then will need 1/0.04*60 = 1500 RPM to reach this speed (taking out slip for simplicity).

But what torque will I need? Also, what role the prop diameter plays here?

Will be glad if anyone can help!

Last edited: May 14, 2020
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gonzoSenior Member

How did you calculate the power requirements?

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NikarusJunior Member

At this stage it is just a force that needed to overcome the water drag at the speed of 0.5 m/s.
So, knowing the formula for the drag:

If the drag coefficient is 0.8, boat drag area is 15x30 cm2 (0.045 m2) and the water density is 1000 kg/m3, then it will be:
1/2 * 0.8 * 0.045 * 1000 * 0.5^2 = 4.5 N

I plan to have two engines, so 4.5/2 = 2.25 N for each engine. Although number of engines is flexible as well.

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gonzoSenior Member

OK, but how did you arrive to the drag coefficient?

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NikarusJunior Member

At this point I just took a really bad drag coefficient, since boat shape is not decided yet. And it might be far from ideal at first prototypes.
So I thought since it will be angled on the front, I'll select "angled cube":

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gonzoSenior Member

You can't simply pick a random number. You have to do the hydrodynamic analysis to predict the drag, or make a model and tow it to measure the drag. Otherwise, you might as well get a random propeller and install it.

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jehardimanSenior Member

If that is your requirement, you don't want a prop at all, but wave powered powered propulsion. It will be more efficient that a prop and have greater thrust available to combat wind loading. So unless this is going in a swimming pool, I'd go wave powered.
The Wave Glider | How It Works https://www.liquid-robotics.com/wave-glider/how-it-works/

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NikarusJunior Member

Yes, this was my intention - to design everything in some CAD later to get more precise values. At this stage I want to get my head around general things - create some general model of the robot that I will be able to tweak after.
So, stuck on this propeller thing now...

Thanks for the proposal, that's an awesome tech!
But in my application it is swimming-pool size exactly - the water is still and the depth is several meters only.

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gonzoSenior Member

The propeller is one of the last things to calculate in the design. There are more important parameters. For example, "clean the surface of the water" needs to be defined and restricted to clear values. Are you cleaning oil spills? If you are collecting surface solid debris, what are the dimensions, weight and shape of the debris? Since that is the fundamental function of the boat, the design can't start until "to clean the surface of the water" is clearly defined and its limits or constraints decided upon.

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NikarusJunior Member

No, the robot is not collecting anything. It cleans the water in a different way - without mechanical interaction with the water.
Sorry, the robot is being created for industrial application and I am under NDA here. So, can't disclose all the details. But basically what it should do - it should swim in a pool of size around 10x10 meters, depth of 3-4 meters, and cover all the area. Like Roomba vacuum cleaners would cover a room.
It does not collect anything, so it has a constant mass. Basically it is a very simple, small boat with automatic program. The only thing is that it should be power-effective as it is battery-powered.

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gonzoSenior Member

If you don't disclose the basic parameters of the design, nobody can give you any relevant advice.

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NikarusJunior Member

I have no problem disclosing basic parameters. Things I can't disclose only affect hydrastatic stability.

Current basic parameters are (all flexible):
Length: 60cm;
Width: 30cm;
Height: 30cm;
Weight: 15-20kg (because of the batteries);

Shape: Boat-like, but not decided at this stage.

What I need now is to understand what approach should I take to calculate motor+prop combination for the boat to move with speed of 0.5-1m/s.

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gonzoSenior Member

The resistance from the water (friction, wavemaking, etc.) and whatever else uses power will determine the battery requirements. In turn, that will also determine the load the hull needs to carry. The dimensions you posted may or may not be adequate. Nobody can help you without a disclosure. Further "boat-like" shape does not mean much for calculations.

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hoytedowCarbon Based Life Form

I assume some battery requirement will be to power the cleaning apparatus.

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NikarusJunior Member

Yes of course it will require some battery consumption and will affect battery size.

I spent few more days digging and reading on this. Now I am using JavaProp app to model the parameters I need. I can use it to calculate shaft power, Torque and RPM to get needed Thrust from specific propeller.
So for propeller of size 80x40mm, to reach 2.25 N thrust and 0.5m/s speed I will need a motor that will make me 1700 RPM and 0.02Nm Torque. I think I will use it as a starting point.

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