Considerations of one or two propellers

Discussion in 'Props' started by NeilG, Jan 20, 2014.

  1. NeilG
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    NeilG Junior Member

    For a low power (e.g. human-powered) boat, are there any potential advantages of running two separate propellers instead of one from the pedals?

    Is it easy to talk about having one larger propeller being equivalent to two smaller ones, or is this a naive approach?

    Other than slight added complexity to the mechanics (and therefore a slight increase in mechanical energy losses), are there any disadvantages?

    I'm assuming it would allow operation in shallower waters. Not as shallow as a paddle wheel, but better than one large prop.
     
  2. gonzo
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    gonzo Senior Member

    Unless you have an application where extreme shallow draft is required, the loss of efficiency on multiple propellers and transmission would be a disadvantage. One large propeller is more efficient than two, not really equivalent.
     
  3. NeilG
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    NeilG Junior Member

    Thanks; this backs up what other resources seemed to be saying but it's nice to be sure.

    What pre-existing solutions are there to make a small boat with a propeller capable of being landed on and pulled up on a beach?
     
  4. FAST FRED
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    FAST FRED Senior Member

    An exposed shaft with a U joint with a SINGLE BLADE PROPELLER would only require some care.

    With limited power (you) a single blade is the most efficient.
     
  5. jehardiman
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    jehardiman Senior Member

    Yes, exposed shaft with u-joint or a cable drive (like a speedometer cable). prop is raised and lowered with a strut. Common since the early 20th century for powered dories.

    [​IMG]
     
  6. NeilG
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    NeilG Junior Member

    You mean a single-bladed propeller? I've seen it implied that less blades are most efficient for lower-power and speed situations - which is certainly what's happening here. (It's generally reckoned a person can sustain around 0.25 hp, and I certainly don't expect anything faster than 4 knots).

    An approach worth looking into; thanks.
     
  7. NeilG
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    NeilG Junior Member

    This may be stupid question:

    At this kind of power and speed, is there any chance a paddle wheel would be more efficient?
     
  8. jehardiman
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    jehardiman Senior Member

    Depends on the draft limit and operational area. If you are draft restricted to 6-7 inches a feathering paddle wheel may be better. Really depends on the hull type and sea conditions though. Paddle wheels work best in near flat calm conditions, any wave action large relative to the size of the wheel and floats messes them up.
     
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  9. FAST FRED
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    FAST FRED Senior Member

    and I certainly don't expect anything faster than 4 knots).


    My bride goes 50% faster on her 18 ft single seat skull rowboat, with OARS!

    No problem with shallow water either.
     
  10. NeilG
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    NeilG Junior Member

    I'm envisioning a fully seaworthy boat, a full set of safety equipment and 90 days of provisions. The rowing equivalents are here: http://www.oceanrowing.com/sale/sold_index2.htm

    Or some previous pedal-driven boats with similar payload:
    http://www.flickr.com/photos/rdngtchr/2190990829/

    http://upload.wikimedia.org/wikipedia/en/3/3f/Pedal_Boat_Moksha_on_River_Thames.jpg

    http://adventuresofgreg.com/blog/2010/05/12/hawaii-trip-unlikely-for-this-summer/

    And this highly engineered ocean pedal boat is only expecting a 3 knots cruise:
    http://www.torpedalo.com/about/about-the-boat/

    There are pedalled boats that have clocked 18 knots, but you don't get to take everything you need for two or more months with you, and still go fast.
     
  11. Jim Marc
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    Jim Marc New Member

    Hi, I am new here, I thought about this 1 or 2 propeller thing but the idea is a tandem style with 2 person pedaling independent simple chain drive mechanism like pedal the ocean type.
    Would the first prop increase or decrease the efficiency of the 2nd prop when both is pedaling?
    Do we get a double in speed by 2 person pedaling in tandem?
    Thanks
     
  12. jehardiman
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    jehardiman Senior Member

    Tandem props need to be matched, in effective advance (i.e. n*P), to each other; especally if counter-rotating. And, no, two props is not twice speed, but twice thrust (all things being equal). How much extra speed that can produce is a whole other discussion.
     

  13. Jim Marc
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    Jim Marc New Member

    Thanks jehardiman,
    So there are effort lost even if 2 person are pedalling the vessel (probably recumbent seated distance apart ) with 2 independent twisted chain prop of the same size say 16x16 with no double in speed?
    Is the water flow from the 1st prop affecting the 2nd prop at the recumbent seated distance away? Now lets say if it does lessen its effect then will increasing the pitch,rpm or bigger 2nd prop do any good?
    Thanks again for some clarifications.
     
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