Pod motor and large prop for electric motor drive

Discussion in 'Hybrid' started by Bookmaker, Feb 13, 2010.

  1. Guest625101138

    Guest625101138 Previous Member

    With modern electronic controllers the induction motors have a wide range of efficient speed. Just that the peak power comes at over 6000rpm and that is where you would design for top boat speed. Economic cruise would be at say 4000rpm.

    Rick W
     
  2. Bookmaker
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    Bookmaker Junior Member

    Thanks for the discussion on this topic. This was what I wanted to know. Seems that at the moment the problems are primarily cost, though the diameter of a sufficiently large motor might be a drag issue. Based on this information I would not try to go this route on my next boat build.
     
  3. sandy daugherty
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    sandy daugherty Senior Member

  4. Bookmaker
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    Bookmaker Junior Member

    Well, well. So... maybe not so science fiction as I was led to believe. I'll follow the testing of that system with interest.
     
  5. Boston

    Boston Previous Member

    ya gotta love that retractable set up
     
  6. sandy daugherty
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    sandy daugherty Senior Member

    If everybody read all the blogs all the time we wouldn't reach so many firm conclusions! However.....
     
  7. Boston

    Boston Previous Member

    we are not nearly that organized
    unless sailing
    and even then some folks forget the sails
     
  8. Guest625101138

    Guest625101138 Previous Member

    That drive leg pictured is quite removed from what was originally described.

    The prop is 4-bladed with a BAR of 80% or even higher. The best efficiency that prop will give is 70% if operating at its optimum point. The props are puny in relation to size of the boat so very heavily loaded. There would also be some disturbed water flow off the motor cowling that would hamper the prop performance from the open-water data.

    For a boat of that size you would like to swing something up around 1m diameter to get high efficiency - around 90% could be achieved in this case.

    I have attached a couple of the Wageningen 'B' series curves to give you an idea of what efficiency can be achieved by reducing the blade loading.

    For my boats I am using BAR less than 10%. With props like this it is possible to get prop efficiency over 90% for cats of the size shown.

    Rick W
     

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  9. Guest625101138

    Guest625101138 Previous Member

    The attached picture is for a marine turbine but it gives an idea of the proportions that lead to the most efficient blades. In the case of a turbine the efficiency translates to minimising force for the power recovered from the stream flow:
    http://www.marineturbines.com/21/technology/

    For a propeller, high efficiency translates to minimising the power for a given force at the desired speed but the best blade profile to achieve this is the same as the turbine up to the point where cavitation becomes a constraint. Cavitation will not be a limit for relatively low power electric drive on a catamaran swinging a big prop.

    Anyhow the photo shows the proportions for an efficient prop or turbine although the ones pictured are much larger than that required to a power a 40 to 60ft cat at moderate speed.

    Rick W
     

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  10. Dave Gudeman
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    Dave Gudeman Senior Member

    Is that the advantage of those propellers that are powered by an electric-motor rim? The large diameter of the rim lets them design for efficiency at lower rpm? I remember seeing discussions of those props and never got what the advantage was.
     
  11. Guest625101138

    Guest625101138 Previous Member

    Yes, placing the coils on the perimeter makes better use of the copper and iron in the motor. It avoids the gearing needed for compact high speed electric motors to get the best from them.

    They are particularly suited to applications where ducting is advantageous such as thrusters.

    Rick W
     
  12. sandy daugherty
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    sandy daugherty Senior Member

    Some of you may not be aware of African Cats' electric pod motors. One is on its way to Europe at the moment.
     
  13. sandy daugherty
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    sandy daugherty Senior Member

    here's a wild suggestion: build the electric motor around the outside of an efficient Kort Nozzle.
     
  14. Boston

    Boston Previous Member

    I thought of that a while back but the housing ends up so large its got high drag

    another one I though of was having a ducted fan like prop but have the prop conected to the duct

    then cut away the interior part that is less efficient anyway and direct the slip and the tip vorticies inward and back sorta like a jet action

    then have the connecting ring (duct or shroud, call it want you will, be the drive

    so there is basically no central shaft but instead the ring goes into a drive chamber that simple clamps it in place on some runners and had a drive wheel that makes the prop go round

    Ill draw up a quick sketch
    but basically its just a free spinning wheel with fins going inward on it
     

  15. sandy daugherty
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    sandy daugherty Senior Member

    I too drew that up. I also considered counter-rotating rings with inward pointing blades, driven by a single center beveled gear on the end of the drive shaft. Reverse was a problem, requiring a transmission. but with the electric motor built into the rings, just some idler gears would do the trick. I wonder if it would be possible to put windings on both rings that are electrified so that you wouldn't need a fixed set. It would only turn 3 or 400 rpm, an have rope cutters on the inner circumferance. Maybe the brushes could be on the adjacent faces.

    So if anyone is inspired to patent this idea, just build me four, that put out say 25 hp equivalent thrust!
     

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