How does the propeller unit rotate?

Discussion in 'Pod Drives' started by Infinitus, May 16, 2021.

  1. Infinitus
    Joined: Jul 2008
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    Infinitus Junior Member

    In an azimuth thruster unit, or Z-drive, the vertical shaft rotates to change the direction of thrust - to rotate the propeller about its horizontal axis.

    The rotating elements, however, are all connected via gears and shafts to the engine. How can they rotate (to direct thrust and steer the boat) without affecting the entire drive chain in some way? For instance, if the engine is inactive, manually turning the vertical shaft (and propeller horizontally) would cause the propeller blades to rotate about the propeller's central axis, or the crankshaft of the engine (assuming they all remain connected as when the engine is powering the boat)? Is the effect negligible while in operation; is it not worth worrying about?

    I apologise for such a basic question. I've looked for this answer but only find marketing on products selling their benefits, and discussing very basic principles. If anyone could clarify this for me, and even link me to good literature on the design of these units, I'd be very grateful. To be clear, it's the ICE engine-driven type I'm asking about rather than the electric type.
     
  2. jehardiman
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    jehardiman Senior Member

  3. Infinitus
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    Infinitus Junior Member


    Thanks. So, in operation the clutch slips slightly when the boat is steered. Is the clutch an essential component, or would such a (budget version of this) system work well enough without a clutch?
     
  4. kapnD
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    kapnD Senior Member

    The housing holds bearings that allows the shafts to rotate inside of it, while the housing itself is rotated independently to steer the propellers thrust.
     
  5. jehardiman
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    jehardiman Senior Member

    No, the clutch doesn't slip, the prop changes speed (think planetary motion of sun gears). The drive leg that the drive shaft runs through rotates very slowly compared to the drive shaft so the rpm change is not that much on the prop.
    The clutch is necessary to start or idle the engine. Think outboard motor, you shift to neutral to stop thrust. A hydraulic clutch is used on large engines these days (even conventional shaft systems) to stop the inertial wringing of the shafts and gearset when the prime mover is engaged.
    You should really get some LEGO(R) TECHNIC pieces and build a model of a z-drive if you really want to understand how it all works together.
     

  6. Infinitus
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    Infinitus Junior Member


    Thanks again. Yes, I get it. It's just that in my head the system seemed somewhat sub-optimal where the prop speed is altered every time the boat is steered. You confirmed that this alteration of speed/interaction is negligible, so I'm satisfied! The system sounds very basic, and I'm thinking there's probably not too much to making one, as opposed to an outboard (where the entire engine-prop unit moves for steering), or a system comprising a static inboard linked to a vertical prop shaft with steering via a homokinetic or universal joint.



    Thanks, but I wasn't being quite that basic - almost but not quite!
     
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