How big electric motor for a double scull?

Discussion in 'Boat Design' started by Jon E, May 18, 2015.

  1. Jon E
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    Jon E Junior Member

    I am building a 27 feet (8.20m) double scull in plywood (Kookaburra, see attachment) with small outriggers for electric cruising on the local river. The total weight including boat, batteries, engine and myself is around 200 kilogram(!). I would like to comfortably obtain a speed around 10 knots.

    I have read somewhere that the "oar-propulsion" an olympic rower can output is around 350 watt. A double scull will be around 700 watt if i have anderstod it right.

    I was considering a Torqeedo cruise 4 kw (2.24 kw propulsive power), but that might be totally overkill? Do i also need a high speed propeller, or would the original be all right?

    I would appreciate any comment on this.
    https://books.google.no/books?id=Z-l5QSB7J_gC&pg=PA30&lpg=PA30&dq=kookaburra+double+scull&source=bl&ots=A-OxU2ZhhF&sig=dUOpcpG-2AiYjfcm_SjNSIzyu7Y&hl=no&sa=X&ei=a5RZVeTMEsSnNumUgYgD&ved=0CB4Q6AEwAA#v=onepage&q=kookaburra%20double%20scull&f=false
     
  2. FAST FRED
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    FAST FRED Senior Member

    When the boat is finished I would mount a borrowed gas outboard or two, to see just what power you need to get your desired cruise speed.

    2Hp , 4HP, 6HP what ever yoy can borrow for an hour or two.

    Then convert the observation to electric power .
     
  3. gonzo
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    gonzo Senior Member

    Those figures of watts output for a rower are after all the losses are accounted for. How long do you want to cruise at 10Kt for? The reserve batteries will add weight and lower your speed.
     
  4. Jon E
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    Jon E Junior Member

    I would hope to contain 10 knots for a couple hours without to much load on the motor. It all depends on how many watt i need for that speed. I dont know. Lifepo4 battery with 2400 usable watt weight 28 kilogram. 4800 watt 56 kg. Outboard engine 18 kg.

    On Youtube there is a clip of a catamaran 4,75m long with Torqeedo Cruise 4. It will keep up with a rowing eight. It looks like displacement hulls.
     
  5. kerosene
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    kerosene Senior Member

    Are you sure? I think 350w is the human ouput, propulsive power would be less than that.

    Edit: yeahh if its expressed as oar propulsion it might mean net power. Just that 500w human output for anything but short bursts is questionable.
     
  6. kerosene
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    kerosene Senior Member

    Jon, when you talk about batteries I think you mean Wh (watt hour), am I right?
     
  7. Jon E
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    Jon E Junior Member

    I ment just the weight on total battery capasity onboard. For example 24 volt 100AH = 2400 watt and weight 28 kilogram (lifepo4).
     
  8. gonzo
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    gonzo Senior Member

    They measure olympic rowers for maybe ten minutes. The study calculated the resistance of the boat and the net power needed. It compares rowers from the 30's to today and show a considerable increase in power production. One of the biggest increases was in the 70's with the new techniques developed in Eastern Europe. I does not measure the watts lost by any losses; only the power required to move the boat at a measured speed.
     
  9. Jon E
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    Jon E Junior Member

    So, if in theory a double sculler at ten knots need 700 watt (after all the losses are accounted for) - i shoud propably use Torqeedo Cruise 2 (Propulsive power in watts: 1120). That would save me a lot of money in batteries too..!

    But as Fast Fred said; borrow a gas outboard or two - to test. That is a good advice.
     
  10. gonzo
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    gonzo Senior Member

    Calculate the actual power the propeller puts on the water (effective thrust)
     
  11. Jon E
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    Jon E Junior Member

    Propulsive power output = thrust Power to my understanding. And Torqeedo Cruise 2 output propulsive power in watts: 1120 and static thrust in lbs = 115.

    * Torqeedo static thrust measurement is based on internationally accepted ISO standards. Static thrust figures for conventional trolling motors are typically measured differently, which results in higher values. To compare Torqeedo static thrust data with conventional trolling motors, add approximately 50% to the Torqeedo static thrust values.
     
  12. gonzo
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    gonzo Senior Member

    You need to use propeller efficiency at the target speed to calculate needed power.
     
  13. peterAustralia
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    peterAustralia Senior Member

    my guess is that once you have built your boat, and your going around you will feel the desire for more stability, as there are no oars to keep the boat stable. Maybe an outrigger, or maybe make a boat 4 or 5 inches wider, maybe speed might drop to 9 knots but my guess is you will enjoy the boat a lot more
     
  14. AnthonyW
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    AnthonyW Senior Member

    I think he mentions outriggers in his first post.

    Not the most comfortable conveyance, I would have gone for single outrigger and wider main hull, but aestherically it could look very sleek and be very efficient and quiet (if bird watching part of the equation).

    Depening on the the positioning of the aka's one could put a camera mount or two next to one's sitting positions for wildlife pictures. (Or cupholders :) )
     

  15. Heimfried
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    Heimfried Senior Member

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