I am currently building a mini boat-like robot to clean the water from floating trash. I used two servo motors to move the loader that I have installed in front of the boat, and two 3 V DC motors installed in the back to allow the boat to move across the water. I also used Arduino to program my servos and motors. When I tried to test my robot in water, the two DC motors did not push the boat forward. Why didn't the air push my boat? Here are the materials that I used to build my robot:

  • Two 1 L water bottles
  • A small plastic box to hold the electric circuits.
  • Another small plastic box to serve as a trash collector.
  • Two plastic propellers (taken from a mini fan).
  • A toy loader.

The total weight of the boat is around 700 g. The picture below is the robot I am working on:

enter image description here

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    $\begingroup$ Not an expert on this but I don't think these types of fans are not designed to produce high thrust, more a gentle flow of air. Compare them with the type you get on model aeroplanes etc. Also I would be surprised if 2 3V motors was enough power given how high drag your design looks. $\endgroup$
    – nivag
    Apr 23, 2015 at 14:12
  • $\begingroup$ @ChrisMueller - it's been removed from Physics, so I removed your comment. $\endgroup$
    – user16
    Apr 23, 2015 at 14:15
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    $\begingroup$ For future you should ask a mod to migrate it in that case. Particularly so we can see any answer's that have already been given (not so relevant in this case). $\endgroup$
    – nivag
    Apr 23, 2015 at 14:16

2 Answers 2


Your fans are too small. If you hold 1 and make it go max speed you will barely feel the force it applies.

Instead consider a drive-train going into the water and turning a screw. Because water is much more viscous and dense you get much better acceleration.

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    $\begingroup$ I agree that submerged propellers would work better. However, I'd argue that it's the density of the fluids that makes the difference (primarily). $\endgroup$
    – Dan
    Apr 23, 2015 at 15:26

Your air propellers did push your boat, but the delivered thrust was not enough! As ratchet freak an Dan said, the density of the medium in which the propeller is working is of great importance, as the delivered thrust $T$ is roughly calculated by:

$$ T = c_{th} \frac{\rho}{2}v^2A_{prop} $$

nevermind the factor $c_{th}$ but you easily see that

$$ T \sim \rho $$

and as the density $\rho$ of water is approximately 1000 times higher than the density of air, a propeller in water will generate significantly more Thrust given the same circumstances as for the same propeller in air.

  • $\begingroup$ Thank you for your elaborated answer. But when I put the propellers down manually (without using the drive-train thing) the speed of the propellers is reduced significantly. Isn't because my motors have low voltage? $\endgroup$ Apr 25, 2015 at 12:21
  • $\begingroup$ not only the thrust, but also the torque increases significantly, when working in a denser medium... $\endgroup$
    – Knigge46
    Apr 27, 2015 at 11:25

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