Motor Body Ratio to Propeller Ratio

Hello everyone,

I’m trying to build a plastic collection device and would like to use a thruster or a centrifugical pump with a underwater BLDC motor.

What could be the propeller or impeller ratio according to motor body ideally, in my thoughts it has to change according to design criterias of the propeller or impeller (like fin angle, fin quantity etc.) I’m new to this area and if you have a book or introduction that you can recommend for the design criterias it would be perfect.

Actually my main goal is to increase the ideal flow rate according to power consumption. For example most centrifugical pumps make maximum 10.000 liter per hour on 750-1000 watt power consumption. I’m planning to make more efficent rates for example 30.000 liter per hour on 600 watt consumption. My first focus is on the propeller design and the second is the water inlet outlet ratio. So if you have any recommendations i’m open to it.

Adding to these if you have a guide to determine the flow rate according to propeller and thruster design beside analysis softwares (since I’m not an mechanical engineer :slight_smile: ), it would be perfect point to start for me.


Aykut Arabacıoğlu

After my post i’ve seen the response of Elliot on this post

So this could be a great way for me to start :slight_smile:

Hi @alkunare,

As with many design tasks, the different components of a propeller/motor combination have various tradeoffs. From an intuition standpoint, the following may be worth considering:

  • larger area pushing against the water means more drag
  • moving mass takes energy (so try to reduce weight of moving components)
  • steeper propeller pitch angle
    • increases bite (but also drag), and
    • increases water movement (but too much just spins the water, instead of pushing it through)
  • more blades have a similar effect to increasing pitch angle, but also
    • increase balance (so may reduce noise), and
    • increase mass to move (also applies to larger blades)
  • torque is force x offset distance, so
    • motors have extra efficiency if their driving force is applied from further out
      • but may require extra material to do so, and
    • the larger the propeller radius the greater resistance it has to being spun
      • but also allows larger blade areas / pushing more water
  • water in = water out, so for inlet and outlet
    • size ratio affects pressure differential
      • the importance of which depends on the application
    • shape affects drag

While intuition can give a decent idea of general considerations and potential high-level improvements, there are enough variables to consider that the design and fine-tuning processes can be quite complicated (especially when also considering things like material properties, power usage, and manufacturability).

Hydrodynamic analysis tools can help quantify the performance of a particular design, and optimisation tools may help to determine design improvements within some set of criteria/constraints :slight_smile:

You may also wish to consider our recently released T500 Thruster, which is larger and more efficient than the T200. I’ve just made a very rough measurement of the 75%-radius propeller pitch angle as ~25°, but you can make a more accurate measurement on the CAD model.

Dear Eliot,

Thank you for your explanations and efforts. I’ll be considering all of these in my designs.

Beside these “Hydrodynamic analysis tools”, which one would you reccommend for me a “newbie” :slight_smile: to easily learn.

I will examine carefully the T500 thruster and consider using it after your recommendations :slight_smile: .



That’s unfortunately not something I’ve got much experience with, so I don’t have particular recommendations for one software tool over another. I have a decent intuition for mechanical considerations, but I tend to work more with software and electronics designs than detailed mechanical ones - especially hydrodynamics (which I’ve done very little of).