Fuel advantage of raked propeller

Discussion in 'Props' started by gilteva, Jun 29, 2013.

  1. gilteva
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    gilteva Junior Member

    I am looking for data on the fuel advantage of rakes blades in a propeller, compared to non rakes blades.
    Any help will be highly appreciated.
    It is for a design of a digital propeller for air.
     
  2. gonzo
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    gonzo Senior Member

    The rake on a propeller will create more lift on the stern of the boat because of the geometry of the blades. I don't know what a digital propeller for air is though. Can you explain?
     
  3. powerabout
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    powerabout Senior Member

  4. gilteva
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    gilteva Junior Member

    The rake is logical

    The rake is logical because the water speed after the propeller is higher than the speed before the propeller. The diameter of the water stream flow is smaller after the propeller compared to before the propeller. When the blade is tilted it becomes with 90 degrees with the water flow. You can read at betz law in wikipedia.
    I am looking for data on how much the angle increases the lift.
     
  5. gilteva
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    gilteva Junior Member

    See the file of the digital propeller

    I attached a file with the digital propeller.
     

    Attached Files:

  6. baeckmo
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    baeckmo Hydrodynamics

    Sorry Gil, but the rake angle that corresponds to the inclination of the streamlines would have to vary, depending on the actual load on the propeller in every operating condition. You might say, that the lower the load, the less the rake, and vice versa. Generally speaking, the rake is designed to suit the wake the propeller is operating in, but it can be used to create a downforce that is increasing the trim angle of a planing hull that is "running flat". This vertical shift of thrust is possible due to the unsymmetrical flow around a propeller approaching the surface.

    That said, you are forgetting the additional areas and consequent tip vortices that you have added to the rotating structure. These drawbacks are killing the propeller efficiency completely. The idea is a waste of time!
     
  7. gilteva
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    gilteva Junior Member

    rake for airplanes

    I need the data in case of full load. If you can help, help, if not, don't waste my time.
     
  8. powerabout
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    powerabout Senior Member

    you have already wasted your time if you think a raked propeller is more fuel efficient than a non raked on a planing boat
     
  9. Easy Rider
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    Easy Rider Senior Member

    I don't know that for sure that a raked prop is more efficient but if it is more efficient I would think it's because the blades are akin to an airplane wing w anhedral instead of dihedral.
    Fwd rake is like a Pigeon (dihedral) and aft rake is like the anhedraled wings of a Sea Gull.
    The tip losses are less w an aft rake.
    Other variables like a slightly reduced disk area may over ride that advantage though.
     
  10. gilteva
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    gilteva Junior Member

    We cannot see the fluid flow because it is transparent. The propeller increases the fluid speed, and that means that when the fluid exits the propeller the speed is higher compared the speed of entering. The diameter of fluid at entering is higher compared to the exit diameter. That means that the flow shape is conic, and not parallel. If we use a non-tilted blades, the blades hit the fluid at non-90 degrees. That means that they are not in the best performance, since the optimum angle is 90 degrees. The more the shape of the fluid flow is conic, the more it is important that the blades will have a rake. Since in big ship the rake is 20-30 degrees, it means that the fluid conic is also 20-30 degrees. Another reason for the conic is that the water come from the side of the ship and not directly in front of the propeller.
     
  11. gonzo
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    gonzo Senior Member

    I think you need to get a basic book on fluid dynamics to understand what happens. There is a combination of flow directions, vortices, cavitation, etc.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Aircraft propeller designers must be at a great disadvantage then. It's amazing they can do it all.
     
  13. powerabout
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    powerabout Senior Member

    not to mention turbine blades
     
  14. gilteva
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    gilteva Junior Member

    Maybe

    They work with the tools that they have.
    But your behavior is very natural, I don't blame you. We comes from monkeys.
     

  15. powerabout
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    powerabout Senior Member

    The last AC the guys had devices to see the wind forward of the boat.
    You might not be able to see air but technology sure can
     
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