How do propellers propell a boat.

Discussion in 'Props' started by tom kane, Mar 12, 2015.

  1. powerabout
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    powerabout Senior Member

    whats hype about my comment?
     
  2. jehardiman
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    jehardiman Senior Member

    Nothing, just commenting on all the IPS claims for the tractor position.
     
  3. powerabout
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    powerabout Senior Member

    Ok got ya

    I notice ABB make their azipods that way as well although single prop.
     
  4. PAUL XAVIER
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    PAUL XAVIER Junior Member

    Propulsion Ship

    I can say about the propulsion of a ship.
    When propeller rotates water is taken from its back to the face side which creates a vacuum space at back.
    Now water surges into fill the vacuum space created by the propeller and this process continuous as long as propeller rotates.
    Now according to Newton's third law, when propeller imparts a force on the water column it imparts a force on the propeller . This force or else known as "thrust" get transferred from the propeller to its shaft all the way to "thrust block" from thrust block to the body of ship. And ship moves.
     
  5. gonzo
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    gonzo Senior Member

    If there is a vacuum at the forward face of the propeller, cavitation happens which is not good. The water should flow at relatively low negative pressure.
     
  6. tom kane
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    tom kane Senior Member

    Just so long as you remember that a vacuum does not suck, but that displaced water and air "pushes" in from all directions to fill a low pressure area.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Looks like this thread is approaching a dead end.
     
  8. tom kane
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    tom kane Senior Member

    How do propellers propel a boat

    Come on then give us your (brief) version on how a propeller propels a boat.
     
  9. daiquiri
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    daiquiri Engineering and Design

    Post #47 is my version of how a propeller propels a boat.
     
  10. markdrela
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    markdrela Senior Member

    This gets more complicated if the propeller is anywhere near the hull, as is typical, because the prop's pressure field then modifies the surface pressures on the hull. You can treat this modified force on the hull either as an added drag or a reduced thrust -- the net effect is the same.

    A more extreme case is a ducted propeller, in which case the overall thrust force on the prop+duct unit is certainly not the same as the force in the prop shaft -- the force on the duct certainly contributes a significant fraction.

    One advantage of the jet momentum-flux viewpoint of thrust production is that it doesn't care about what's "thrust" and what's "drag". The net propulsive force is accounted for regardless of where the actual pressure forces are applied, whether on the prop blades, or on the hull, or on the prop duct (if any). Furthermore, the jet kinetic energy flux plus the blade viscous losses gives the shaft power, even if the flow velocities seen by the prop, and hence the prop torque and power, are modified by a nearby hull or duct.
     
  11. tom kane
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    tom kane Senior Member

    WoW..and here I was thinking that a boat is propelled by a device in the water rotating on a motor driven boat drive shaft that has a thrust bearing transferring the thrust to the hull. The thrust is generated by this devices ability to move water in any direction or... up into the air.
    Apparently how a propeller propels a boat is not simple and needs experience in boat design to understand all the forces acting on all the parts of a boat.Lots of ideas here.
     
  12. markdrela
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    markdrela Senior Member

    Only if the propeller doesn't interact significantly with the hull.

    Common counterexample:
    On a jet boat, the force that the impeller shaft imparts to the boat via its thrust bearings is very different from the overall propulsive thrust force acting on the boat. The pressure forces on the duct walls matter a great deal in this case.
     
  13. tom kane
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    tom kane Senior Member

    Only if the propeller does not interact significantly with the hull, is why so many problems with attempts to build tunnels,pockets in hulls come to grief.
    So where is the ideal place to run a propeller, or is that as bad as asking how does a propeller propel a boat.
     
  14. markdrela
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    markdrela Senior Member

    With a diffusing inlet duct, the flow velocity at the impeller can be made significantly smaller than the boat velocity. This will postpone impeller cavitation up to a higher boat speed than for an open propeller.

    But this is a design choice. I don't know if actual jet boat drives are designed specifically for cavitation suppression or not.
     

  15. markdrela
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    markdrela Senior Member

    A bit OT, but a jet engine nacelle acts like a diffusing duct on a jet boat drive.
    The airplane cruises at Mach 0.80 to 0.85, but the engine nacelle slows down the incoming flow so that the engine front fan thinks it's flying at Mach 0.60. This allows the fan blade airfoils to be more efficient than they would be if it was operating "in the breeze" at the full airplane speed.

    And to address the current topic of thrust production...
    The thrust generated by the fan is definitely NOT equal to the tension force in the fan shaft. The pressure forces on the nacelle/duct contribute a great deal also.
     
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