Light weight generators

Discussion in 'Hybrid' started by bushsailor, Apr 26, 2017.

  1. bushsailor
    Joined: Jan 2009
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    Location: Aus

    bushsailor Junior Member

    I have a 50' racing sailing cat that currently uses 2 x 25hp outboards as propulsion. These provide ample power and push her at 8kn cruise and 10kn flat out.
    I would like to convert to an electric drive system in the future.
    There are plenty of electric drive motors around but the large drawback for me is the significant weight of a generator large enough(roughly 30kva ) to drive the 2 10kw?? motors required to propell the boat at an acceptable cruising speed when there is no wind. (I have no wish to rely on batteries for motoring)
    Are there any light weight quiet petrol or diesel water cooled generators that could achieve this?
    Target weight for a complete system would be 250kg including 400amp hour of lithium batteries.
    The current diesel marine generators alone weigh around 400kg
    Due to high performance we sail 90% of the time.
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The equivalent system will be heavier and most likely less fuel efficient. The equivalent power is 38 Kw for the electric motors. Add the losses from the generator and batteries, and you need about 45 Kw. That is a huge generator. What is your goal for the change?
     
  3. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Hi Gonzo

    A kva is a kw, the OP wants to put in 2 10kw motors. Certainly some losses but what are the calculations to say that you need a 38 kva (kw) gen to drive 20 kw?

    To the OP, what are you trying to gain. You will not gain much in the way of a fuel consumption savings, add weight, plus quite a large installation and capex cost. Just curious
     
  4. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    It is the equivalent to 2x25HP. Otherwise, if he only wants 20Kw and can accept the loss of speed, he can go for 2x15HP outboards if the goal is less weight.
     
  5. bushsailor
    Joined: Jan 2009
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    Location: Aus

    bushsailor Junior Member

    We only require 2 x 8/10 kw electric motors. Changing the weight distribution is one of the main criteria. Weight saving is also one of the main drivers, one of the existing problems is we have the motors in wells in the hulls which also hold 140l of water. Fuel saving is not a requirement.
    I have now calculated that we require 4.5kw of power to motor at 7.5kn so I need a generator that puts out around 250amps@48v. The most cost effective voltage is 48v. A lithium battery pack of 200amp will also help in emergency situations and out of marinas etc
     
  6. DennisRB
    Joined: Sep 2004
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    Location: Brisbane

    DennisRB Senior Member

    Any details on the boat? Perhaps you could build your own from a brushless DC motor with suitable controller and industrial petrol motor. I assume you are going to use shaft drive as you planned to eliminate the outboard wells which hold a lot of water?

    Ideally the electric motor would also propel the boat and be used as a generator to save weight and money. So the petrol engine would be in series with one electric motor/gen in one hull and work to directly drive the prop shaft while also transmitting electrical power to the other hull. For anchoring docking you would just use battery power while the petrol motor is de coupled. For long distance motoring you would use petrol power from your direct drive petrol motor from one hull only.

    This setup would require a few fancy clutches. The benefits would be: That its lighter and cheaper (IE only 2 electric motors/generators). Rather than 2 electric motors AND a generator. It would be highly efficient to motor long distance as direct drive from the petrol motor is available (IE less weight again tankage). There need not be a reverse gear for the petrol motor. For reverse the petrol motor is de coupled and the 2 electric motors reverse via their controllers and battery power. The battery bank for this system need only be big enough to do some docking etc and would ideally also be the house bank.
     

  7. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    I like something similar to the DennisRB scheme, perhaps with two propeller shafts, two 5 kW PMDC motor/generators, and two 8 kW IC engines which can be clutched out. This setup would require a only two simple mechanical clutches to uncouple the IC engines from the shafts. This setup would allow 16 kW cruising shaft power, using the IC engines alone, plus some battery charging from either PMDC machine, or both. Battery power for maneuvering or short range cruising, 4 kW on each shaft is adequate for maneuvering and making about 80% of the 16 kW hull speed. High reliability, with only one IC engine in service, can get about 4 kW on each prop shaft if desired. Simple electrics too, just a few DPDT relays, no fancy controllers needed.

    An alternate, two propeller shafts, two 10 kW PMDC motors, and one 18 kW IC engine clutchable to only one shaft. This setup would require a only one mechanical clutch to uncouple the IC engine from one shaft. IC Engine puts 18 kW on portside shaft, driving 8 kW propeller, plus 10 kW PMDC generator, which feeds electric power to get 8 kW shaft power from the PMDC motor on the starboard propeller shaft. Some excess generation available to charge batteries. Simple electrics again, but not well balanced weight distribution, but overall weight similar to the first (2 IC engines, Two PMDC motor/generators) setup.

    Brushless DC motors? Why not brushed PM DC motors which function perfectly well as a generators, and are very reliable without any electronics.

    The drawback I see for the whole scheme is propeller drag. While the outboards could probably be tilted up out of the water for less drag when under sail, this may be a problem for fixed inboard shafts. Freewheeling the props helps somewhat when sailing, feathering props even better.
     
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