Toroidal propellers: A noise-killing game changer in air and water

General Discussion
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Saw this article a couple of weeks ago and thought this might be a good place to post. Would love to see something similar from BlueRobotics.

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Hi @roboFisherman,

I came across Sharrow’s propellers about 6 months ago, and we’ve discussed them a bit internally since then - definitely some interesting ideas involved.

As I understand it

  1. Rotors with “split blades” were originally patented in the early 2000s in Europe
    • Patent expired in 2020
  2. They were (successfully) used in some computer fan designs in 2012 by the German manufacturer Blacknoise
  3. Sharrow invented their propeller in 2012 in the US, and seemingly started patenting it in 2017 (with blades that specifically loop back to a central hub)
  4. MIT started patenting the use of toroidal propellers in 2017, whereby each blade has a toroidal shape that loops back and connects with a previous blade

In all cases it seems a focused-on benefit is the reduction of tip vortices, which reduces unwanted noise (and cavitation in water) and helps improve flow. The Sharrow design also highlights the reduced slip (which further improves efficiency), and the MIT design highlights the rigidity and safety benefits of blades forming a connected structure without sharp protrusions.

Me too, if the hydrodynamics are viable for the size and rotation rates of the motors on our vehicles. In terms of potential challenges, we would need to

  1. license Sharrow’s patent(s) to do so
    • which could be expensive - reliant on negotiation from both sides
  2. likely do some extensive design and validation work (to find appropriate geometry and materials for the size and torque/speed capabilities of our motors)
    • which could be both time consuming and resource intensive
  3. find some cost-effective way of manufacturing them in large quantities
    • which could be challenging, because the looped and rotated geometry means multi-axis machining (Sharrow’s approach), several-part injection molds, or 3D printing would be required, none of which tend to be cheap for mass-manufacture

It’s also worth noting that some of the tip vortex reduction is more relevant for un-shrouded propellers (like are typically used on boats, planes, and drones), so the efficiency gain for ROV thrusters may not be as significant as it is for other applications. There are seemingly some modern computer fan designs that are more efficient than the Blacknoise ones, which is at least some indication that shrouded applications may not have quite as significant benefits.

I think it would at least be interesting to have an investigation of how effective they could be, but we’ll have to see whether that’s something we (Blue Robotics) decide to prioritise and devote resources to, or if we decide the other product iterations and advancements on our roadmap are more important, and wait until there’s some more compelling evidence they’d be useful and cost-effective on our thrusters and/or motors :slight_smile:

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Hi -

We did some investigations in the early 2000’s on this exact topic, determining the efficiency gains of shrouded or un-shrouded designs. It was assumed the ‘Kort’ designs are freely scalable, but they are not, as the velocity of the tip vortices are the predominant concern. If the RPM of the propeller is very slow, the advantages are quite minimal if the throat length of the nozzle or shroud is too short.

In that investigation, we found the three largest contributing factors - for us in our tests - was gap between the propeller tip and the nozzle ID, the chord of the propeller at that interface, and RPM. Blade pitch was also a factor, but reliant on the target RPM and resultant ‘boundary layer adhesion’ requirement across the throat of the nozzle.

That said, an un-shrouded design such as the Sharrow are optimized to limit tip cavitation without that interface.

In an ROV, un-shrouded could create a lot of different problems.

I think the next big step in ROV thruster design will be a vectored Coanda Nozzle…

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I did some bucket testing with 3D printed shrouded toroidal propellers. I found that they produce far more thrust than regular propellers, and therefore require slower motors to avoid overpowering the ROV. They seem to be not as efficient as regular propellers, though. I think they could be justified when there are space constrains, and it is needed to get the most thrust with as small propeller as possible.

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This appeared in my feed last night,

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Yeah, I saw that video a little while ago. Unfortunately (as they mention at the end) it’s not indicative of whether the propellers are more or less efficient than traditional ones, because the propellers they used weren’t optimised for use with those motors or for use in water.

Definitely a bit of fun to see people trying out the new design options though :slight_smile: