How big electric motor for a double scull?

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

  1. kerosene
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    kerosene Senior Member

    Right, that means watthours - not watts.
    Its not just being geeky about units. Watts are unit of power and relevant for batteries as pulling certain amount of power can be limited itself - ie the rate of discharge.
    anyways, your 2,4kWh battery weighs 28kg. That means in practice I guess about 2kWh of energy per cycle.
     
  2. Heimfried
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    Heimfried Senior Member

    The capacity of the batteries offered by torqeedo is given after subtraction of losses and remaining capacity. 2,7 kWh is the useable Energy.
     
  3. Jon E
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    Jon E Junior Member

    Yes, that was an impressing barge :)! I know Electric engine batteries last long, if you don not put any pressure on the motor. And specially with a big battery to!
     
  4. Jon E
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    Jon E Junior Member

    I have also bought 5 x 327 watt solarpanels, and a Morningstar mppt 60 - so i am going all green this time. A small pier for the boat will be covered With my solarpanels - and Connected to the battery inside the sculler.
     
  5. FAST FRED
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    FAST FRED Senior Member

    A long extension cord to cruise might be nice.
     
  6. peterAustralia
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    peterAustralia Senior Member

  7. philSweet
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    philSweet Senior Member

    Good news here - you are looking carefully at the charging issues early in the design.

    I think the next step as far as sizing is concerned is to run a few cost models on specific configurations.

    If you decided on a 24 volt system, then you could use #1 wire, 100 or 110 amp fuse and have 1% voltage loss over 20' of wire at 40 amps. That would let you size a motor. If you don't like the answer, consider 48 volts. That lets you run smaller wire, or longer runs, and can deliver quite a bit more power on the boat for the same cost. For example, #4 wire, 20 feet, fused at 65 or 70 amps, and 1% voltage loss at 30 amps. So 150% the power delivered at less cost and weight. You just need to look at the options as far as motors and controllers go and see what fits. 24v X 40 Amps = 960 Watts. I think that should do for cruise, with the wiring supporting double that for an occasional sprint. Controllers will cost about $1 per peak amp, so not a huge expense for a boat.

    Post three or four designs with batteries, motors, controllers, wiring sizes and lengths all speced and hang a weight and price tag each of them. And describe the actual charge and discharge cycles you envision for each design. It's really more about choosing from among options than design at this point. It would be better to run 60V from the panels onto the boat and have the charge controller next to the batteries than to have the charge controller near the panels and run 24 or 48 volts to the boat. They would be #4 wire @ 60V as well. So about $400 for wire and ends and fuses if you bought a 100ft spool of #4.
     
  8. Jon E
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    Jon E Junior Member

    This is a sketch, redesigned kookaburra scull shell as a catamaran. 4 x 327 watt solarpanel and Torqeedo engine. Since i am in the start of building this project, this might be what i could be ending up with. Thoughts? See image
     

    Attached Files:

  9. gonzo
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    gonzo Senior Member

    24V and 100Ah is not 2400 Wh. It is not possible to use 100% of the charge. Depending on the chemistry of the battery, up to 80% may be possible. Not all lithium ion batteries are the same.
     
  10. peterAustralia
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    peterAustralia Senior Member

    I dont know what my opinion is worth, but when has that ever stopped me. I think it looks quite good. Another option might be to turn those panels at 90 degrees, to make the boat 12 inches or so wider, and still use 4 panels. Would increase your sitting area a lot. I have to admit though as it is, it is pretty good.
     
  11. Heimfried
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    Heimfried Senior Member

    That depends on the "kind of" capacity given by the manufacturer. Usually the nominally capacity is given and there is a rest of 20% not usable, so the usable capacity is only 80% of the nominal one.

    Torqeedo claims to state only the in fact usable part of the capacity, so "Power 26-104" means 26 V * 104 Ah = 2.7 kWh usable amount of energy.

    http://www.torqeedo.com/en/products/catalogue

    (Catalogue, page 32)
     
  12. AnthonyW
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    AnthonyW Senior Member

    Hi Jon

    Can you tell us a bit more about the batteries and your calculations on weight?

    To get 1.2 kW out of non-lithium batteries (which are pretty darn expensive) for 2 hours our head of IT worked out he needs about 60kgs of deep cycle batteries. (In South Africa we have load shedding for two hours at a time across the country - he is setting up some lights and his TV for such periods). Thats only about 2 horse power, and a lot of weight, for a short period. Presumably you have more efficient batteries that are lighter to get the power you need for longer? Or do you only plan to cruise for short periods?

    This is a really nice project, but I would hate the battery weight to flumox the design at the end, and I am sure there are lots of people on the forum who can help you check this at this point of the design phase and give you some comfort around this.

    Another option to put out there might be to modify two K2 or K1 river racing kayaks as 1 or 2 outriggers. Might save some construction time. I looked into this for my boat build as second hand they are cheap, and once filled with foam they are quite robust. Not good enough for sailing stresses, but for a power boat with negligble output they might be great.

    Ant
     
  13. Jon E
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    Jon E Junior Member

    Hi Anthony. 24 volt 2400Wh weight 28 kilogram. The type is lifepo4. Very expensive, but the best lightweight solution.

    My double scull project will be modified above waterline for extra robustness. Everything below waterline will be kept according to Kookaburra plan drawings.

    I will try out some combustion engines first, to determine the amount of electric power i need.
     
  14. philSweet
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    philSweet Senior Member

    I thought the panels were going on the dock. Are you having both?
     

  15. Jon E
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    Jon E Junior Member

    Single hull is the plan, PhilSweet. But, but i am building the hull with that in mind that it might be possible to connect as a catamaran in the future. The above photo is only a sketch.

    If the single hull is hopeless unstable, i can still convert it to a cat. :)
     
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