Alternator on an Electric Powered Catermaran

Discussion in 'OnBoard Electronics & Controls' started by Jeremy Marker, Jul 7, 2019.

  1. Jeremy Marker
    Joined: Jul 2019
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    Jeremy Marker Junior Member

    Hi Experts,

    I am looking at building a 75 foot Cat (Bruce Roberts Design), where they don’t design for an electric motor; I will overcome this, where I plan to use clean-e-marine company, with two Trition motors and controller. My simply question is: As there is not much resistance from an alternator; can I simply hook one up to the prop-shaft to help generate power back into the battery system? There are very costly Alternators on the market that can deliver very high amps, but from research the paradigm of connecting to prop is not out there.
     
  2. alan craig
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    alan craig Senior Member

    Of course not, that would mean a 100% efficient system, not possible.
     
  3. fishwics
    Joined: Mar 2004
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    fishwics Quiet member

    Unless the boat has sails and the OP uses a prop-driven alternator to recharge the batteries when sailing. I would think though that a clutch was needed between shaft & alternator.
     
  4. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    I'd be curious what your plans are. Electric motorsailer? I dream of building a solar powered electric trimaran myself (no sails) but I'm a newbie.

    I think a DC system might minimize conversion losses and make the system simpler. But from my research the efficiency of an electric motor is about 50-60% from battery to propulsive power, and I'd estimate with about the same efficiency when charging. So charging could create drag and steal like 4 times the propulsive power you later spend on motoring.

    But motoring at e.g. 5 knots and sailing at e.g. 15 knots would require the charging to run at a different RPM. So you could connect another DC motor to the drive shaft as an alternator but you'd need a different gearing, maybe a different prop as well. Especially if you want one of those folding props to minimize drag. I'd ask the company you mentioned about this, they should be able to advice you. You could also get one of those "outboard generators" you drag behind you so you have the two systems independently.

    If you're only motoring small distances I'd look into a lot of solar panels that also supply lots of power while anchoring. Plus a small backup generator. Might be cheaper and easier.
     
  5. Jeremy Marker
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    Jeremy Marker Junior Member

    I fully understand it won't be 100% but surley it would help put some life into the power system
     
  6. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    No. The propeller would need to be turned around and the pitch and diameter changed.
    If it's a sailboat then a stand-alone, adjustable-pitch propeller designed for the purpose would work.
     
  7. tpenfield
    Joined: Dec 2016
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    tpenfield Senior Member

    Where would the power to run the alternator come from? . . . the electric motors? If so then the power consumed to run the alternator would be greater than the power put back into the batteries, because as Alan mentioned, the system would not be 100% efficient. Even if it were 100% efficient, at best it could only put back in as much power as it used.

    Might be best to use wind or solar power to help replenish the batteries.
     
    Last edited: Jul 8, 2019
  8. Jeremy Marker
    Joined: Jul 2019
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    Jeremy Marker Junior Member

    Many thanks for the reply: This has got me thinking; at the stern I will have a lift-up/down water deck, I will have a look at molding two SEAWATT type of design props with the shaft hooked-up to an altenator (two number).
     
  9. Jeremy Marker
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    Jeremy Marker Junior Member

    I am new at this, but understand that even for a car it takes a lot to recharge one battery, whereas for an eletric motor sailboat, it will have a lot more batteries. Yes solar will be used, as well as wind; I am just thinking if the propshaft is turning while under power why not hook up an alternator.
     
  10. tpenfield
    Joined: Dec 2016
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    tpenfield Senior Member

    When you say 'under power' do you mean under electric power, not 'sail' power? If so, the law conservation of energy along with less than 100% efficiency of the alternator will mean that you will actually lose electricity rather than re-claim it.

    You can Google 'conservation of energy' for more details.

    If you are meaning that the propellers may be spinning freely while under sail, and you can drive an alternator to a reasonable speed from the prop shafts, then you will be charging the batteries as desired. It is hard to say how much power you will capture as opposed to using a wind driven alternator/generator.
     
  11. Jeremy Marker
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    Jeremy Marker Junior Member

    Under motor power
     
  12. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Sigh... what motor?

    Do you want to have both an electrical motor with batteries AND an internal combustion engine?
     

  13. tpenfield
    Joined: Dec 2016
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    tpenfield Senior Member

    OK, thanks for the clarification. Adding the alternator will actually deplete power, as compared with not having the alternator, due to the law of conservation of energy.
     
    Last edited: Jul 11, 2019
    BlueBell likes this.
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