clean auxiliary power charging whilst sailing

Discussion in 'Sailboats' started by bartnordwest, Jun 3, 2013.

  1. bartnordwest
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    bartnordwest Junior Member

    i am looking for a clean auxiliary power for my sailboat,a 30 ton gaffrigged cutter ,the idea is charging whilst sailing from a dragging propeller running a small compressor taking about 5 kw slowing the ship down a knot or less filling a big tank during a few days sailing that should give me the power for modest portmanouvres 30 minutes at 30kw?
     
  2. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    No way you can extract 5 kW of power with a hydro turbine dragged behind your boat. Commercial water turbines for general boating use can extract some 100-150 W at 8 kts water speed. For example, this ampair's 312 mm turbine gives 100 W at 8 kts. to obtain that, it adds 300 N of resistance to your boat, which means additional 1.2 kW (1.6 HP) at 8 kts required from your sails. It can give 2.4 kWh of energy during a 24-hr charging at that water speed, which is a pretty unreasonable scenario, imo.

    30 minutes by 30 kW gives 15 kWh of required energy, which translates into 18-20 kWh of stored energy in the batteries (depending on how you discharge them). It means that a generator like the above should work continuously for 8 days to let you store enough energy for a single port maneuver as described by you...

    And, by the way, what do you intend when you say "charging whilst sailing from a dragging propeller running a small compressor ... filling a big tank" ? What kind of energy-storage system is that? Air compression?
     
  3. boradicus
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    boradicus Senior Member

    Hi all, wouldn't you get better numbers from a wind turbine when heaved to? The power generated should be proportional the square of the length of your blades. I don't know if you could have the turbine up while sailing.
     
  4. bartnordwest
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    bartnordwest Junior Member

    thank you for anwering

    yes i think batteries are dirty, and dont last long
    but i am open to any suggstion
    but you are talking about a generator with a dragging propeller
    isnt it so that you can make propellers for any speed and torque
    the product of which would be the power
    my boat is a bit heavier than most yachts(under sail i beat my 130 hp diesel,its a 30 ton boat) ,so i think more torque ,resistance,diameter than the one you speak of
    further i am doubtfull about high pressure like the aircar,or low like a starting system for a big dieselengine?
    any idea about small high pressure compressors?
    i am used to use very little motor and generally go trough great pains to save energy so eight days sailing doesnt back me off
     
    Last edited: Jun 3, 2013
  5. daiquiri
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    daiquiri Engineering and Design

    Let's make a little hypothetical talk. :)

    The first thing you have to do is to make a realistic evaluation of the power requirements for your electric drive. 30 kW for 30 minutes time sounds like an exaggeration, and the numbers sound suspiciously well-rounded. But, for a moment, let's start with these and make a few considerations.

    A 30 kW electric motor is a beast, take a look at this model's dimensions and weight: http://www.inverterdrive.com/group/Motors-DC/30kW-DC-Motor-TT-Electric-690RPM-1000RPM-LAK4180BA/ I have a Siemens catalog, and 30 kW models are similar to that one. A water-cooled model would take less space but would introduce complications from the hydronic side of the cooling system.
    That motor works at 460 Volts, so you'll need to plug it into an array of batteries and a power converter. A typical 30-40 kW power converter is a box 50x50xH30 cm, weighing some 5-6 kg.

    How many batteries would you need? Well, from your specs: 30 kW required power for 0.5 hr. Assuming that you arrange batteries in such way to get an output of 48V (the most convenient choice in this case), your motor will draw 30000/48 = 625 A of current, for a total of 625*0.5 = 313 Ah required energy from batteries.
    Furthermore, since you will discharge them in 30 minutes, the effective capacity of your battery bank will be much lower then its rated capacity. Battery producers usually rate their batteries for 20 h discharge, which is much more efficient than a 30 minute discharge. By applying the so-called Peukert's law (see Google), it can be shown that a rated capacity to be installed is 1.62 higher than the calculated effective capacity. In other words, you'll have to install 313*1.62 = 506 Ah.
    But, if you want to give your batteries an acceptable life-span, you should discharge no more than 80% of their stored energy (and they have to be specifically made for such deep discharges). So a battery bank of 506/0.8 = 633 Ah effective capacity would be necessary, in order to use up the 313 Ah we need. That means a minimum of 6 batteries rated 115 Ah - which rounds up to 8, because you have to arrive to 48 V.
    n.8 115Ah batteries will weigh 280 kg and will cost some $2000.

    Conclusion: a 30 kW motor which works for 30 minutes at full power will require a battery bank made of 8 AEG batteries and a power converter to bring the voltage to 460 Volts. A total weight of this setup will be around 650-700 kg. A cost - somewhere between $12,000 and $16,000.

    Now, knowing this, I hope you realize how quickly the apparently tiny initial numbers can grow into significant weight and costs. Which brings us back to my initial statement - you need to evaluate precisely how much power, and for how much time, you really need for the intended use of your electric drive. After you have done this first step, we can talk and discuss other technical and economical aspects of your idea. For example - are you willing to spend the required money, and do you have the required space on board your vessel?

    Same considerations are valid for any other type of energy storage (compressed air being one of the least efficient ones).

    Cheers
     
    Last edited: Jun 4, 2013
    1 person likes this.
  6. Angélique
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    Angélique aka Angel (only by name)

    For anyone interested in the boat, Bart posted a picture of her here.


    But it seems we've lost Bart . . :confused:

    Bart, if you ever come back, also see this response.

    Cheers,
    Angel
     
  7. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

  8. bpw
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    bpw Senior Member

    If you are that concerned about not using fossil fuels just go engine-less. Everything else will be a very expensive solution that provides little in the way of useful power.

    Why carry $15,000 and a thousand pounds of stuff around to be able to motor for half an hour in calm weather? Cruising you can wait for a good wind, and day sailing you will want something that runs a couple hours to get you home for work on monday (a diesel).

    Sculling oars and yulohs work, but I can only get 1 knot out of my sculling oar on my 6 ton boat, so expect lots of work for not much go on your boat.

    Another thing to consider if you plan to sail every where and not motor, traditional boats are pigs under sail, slow and hard to maneuver. I know because I have one, We make it work but our next (engine-less) boat will be fin keel, spade rudder, and lightweight.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Solar and wind generation just not suitable?
     
  10. thudpucker
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    thudpucker Senior Member

    You need a Flettnor Rotor driving an electrical generator.
     
  11. PAR
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    PAR Yacht Designer/Builder

    Need I bring up my trained squirrel contrivance again?
     
  12. thudpucker
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    thudpucker Senior Member

    LOL belay that Squirrel!
    Solar or wind ? I wonder what the total weight and 'Sq Ft' of room in the boat might be?
    Can Solar run an AC unit?
    But some thing that would extract power from Plain, Balmy, Heat would be the ticket up here in Alabama.
     

  13. PAR
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    PAR Yacht Designer/Builder

    Yeah, don't knock it until you've run the numbers. My squirrel device (which also works with moles, gerbils and smallish ex-wives) is self contained, self reproducing and though it does take up a fair bit of room, it's entertaining to watch the little fuzzy butted varmints do their deeds. Plus you can design the system to fit every nook and cranny, that otherwise wouldn't get used for anything, in fact they like small, confined odd shaped places.
     
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