Does an impeller need a driveshaft?

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Discussion Overview

The discussion revolves around the design and functionality of impellers in marine propulsion systems, particularly focusing on whether an impeller requires a driveshaft. Participants explore various design constraints, potential applications, and alternative propulsion methods, including the use of electric motors and magnetic fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants inquire about the specific design goals and constraints for their impeller applications, noting that current market options may not meet their needs.
  • There is a suggestion that electric motors could potentially drive impellers using coils, with some participants affirming that this is an area of ongoing research.
  • One participant questions the feasibility of using turbines for propulsion, drawing parallels with hydroelectric generation and seeking clarification on the differences between turbines and impellers.
  • Another participant emphasizes the distinction between impellers, which push water, and turbines, which are spun by water, while also discussing the potential for electric motors to drive impellers.
  • Some participants express uncertainty about the terminology used in the discussion, indicating a need for further clarification on the mechanics involved in propulsion systems.
  • There are references to magnetohydrodynamics as a concept for moving water directly using electromagnetic fields, with acknowledgment of its limited practical applications.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the necessity of a driveshaft for impellers, and multiple competing views regarding propulsion methods and design constraints remain. The discussion includes varying interpretations of technical terms and concepts.

Contextual Notes

Participants express a lack of familiarity with specific design terms and constraints, which may limit their ability to engage fully with the technical aspects of the discussion. There are also unresolved questions regarding the efficiency and practicality of different propulsion methods.

theycallmevirgo
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TL;DR
Is there any reason for an impeller driveshaft other than stability?
I have good basic FEA books, but all the stuff online is focused on chemical mixing.

Thanks so much

Joe
 
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Well, I just wanted to get a basic idea of what's going on. At this point I can't even select from available options on the market, and I strongly suspect that my application is going to be 50% the size of anything I can buy.

ETA: but I found a book already. I'll be posting another question shortly.
 
I'll move your thread to the ME forum for better replies. Did you look through the hits in my link?

Also, please post your design constraints. We won't be able to help you unless you post your target dimensions, power transfer, water speed range, duct characteristics, etc. Thanks.
 
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Here's a picture of an impeller from J S Carlton (Marine Propellers and Propulsion 2nd edition 2007)

1589504988903.png


If I understand correctly, the driveshaft is there for the gas motor. If I had an electric motor, could I just use coils around the impeller? Or, alternately, can I drive an electric with gas?

Thanks

Joe
 
theycallmevirgo said:
I'll be posting another question shortly.
Multiple threads merged...
 
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berkeman said:
Multiple threads merged...

Thanks for the help. I don't know half the terms you're listing as design constraints, which is why I need to do some reading. All I know for now is that I strongly suspect overall diameter is going to be 50% of anything on the market right now.
 
OK, very cool. This is now a project worth pursuing.
 
  • #10
theycallmevirgo said:
If I had an electric motor, could I just use coils around the impeller?
Yes, there are several marine propulsion designs that do basically that. You may need a much bigger diameter of that duct. Here's one that uses permanent magnets. I'm not sure what their top RPM is. I just searched "marine magnetic propulsion".

1589506005077.png
 
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  • #11
Yeah I looked at the first link, if their efficiency is as they claim it is very promising. Thanks for the help folks!
 
  • #12
Am I missing something, btw, wrt turbines? If they can be used for hydroelectric generation why not propulsion?
 
  • #14
theycallmevirgo said:
Am I missing something, btw, wrt turbines? If they can be used for hydroelectric generation why not propulsion?
Turbines being spun by what? Your example of hydroelectric implies you think a water turbine could work here...
 
  • #15
russ_watters said:
Turbines being spun by what? Your example of hydroelectric implies you think a water turbine could work here...

Yes, that's correct, driven by emagnets.
 
  • #16
anorlunda said:
recently, diesel-electric has the competitive advantage.
Why do you think that is?

Also, just to make clear, I meant specifically turbine driven by em forces pushing water, not gas turbines pushing a prop (which I've seen numerous examples of).
 
  • #17
theycallmevirgo said:
Yes, that's correct, driven by emagnets.
"Driven" by magnets? Hang on, just to make sure we have our definitions right:

Impeller: spins and pushes water.
Turbine: is spun by water.

Turbines do not get driven by magnets.
 
  • #18
russ_watters said:
"Driven" by magnets? Hang on, just to make sure we have our definitions right:

Impeller: spins and pushes water.
Turbine: is spun by water.

Turbines do not get driven by magnets.
OK, let me rephrase, because I'm guessing I'm using the wrong terms. You have a hydro power turbine, which is rotated by water to make electric power. Why can't a (nearly) identical system use electrical power to move water for propulsion?
 
  • #19
theycallmevirgo said:
OK, let me rephrase, because I'm guessing I'm using the wrong terms. You have a hydro power turbine, which is rotated by water to make electric power. Why can't a (nearly) identical system use electrical power to move water for propulsion?
You can - the other guys have given several examples. I guess I'm not seeing what is unsatisfactory in what you've been given.
 
  • #20
russ_watters said:
You can - the other guys have given several examples. I guess I'm not seeing what is unsatisfactory in what you've been given.

Well, from my (super-cursory) overview the mechanisms used for generation are much more involved (and much closer to air propulsion turbines) than what was shown. But it's entirely possible I'm mistaken.
 
  • #21
theycallmevirgo said:
OK, let me rephrase, because I'm guessing I'm using the wrong terms. You have a hydro power turbine, which is rotated by water to make electric power. Why can't a (nearly) identical system use electrical power to move water for propulsion?
Turbines are internally much like a waterwheel, whereas an impeller generally resembles a fan.

By stretching definitions just a little bit, sidewheel and sternwheel boats could be called sideturbine and sternturbine driven. (Sidewheel aka paddlewheel)

Cheers,
Tom
 
  • #22
theycallmevirgo said:
OK, let me rephrase, because I'm guessing I'm using the wrong terms. You have a hydro power turbine, which is rotated by water to make electric power. Why can't a (nearly) identical system use electrical power to move water for propulsion?
That's what your impeller is. An electric motor turns the shaft and the impeller is a water turbine. That's where you started in this thread.

EM fields directly moving water is called magnetohydrodynamics. It has been researched for decades, but few applications exist. It was even the theme of a Hollywood movie named "Hunt for Red October"

https://en.wikipedia.org/wiki/Magnetohydrodynamics
 

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