Unmanned solar-electric

Discussion in 'Boat Design' started by tesseract, Aug 16, 2009.

  1. tesseract
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    tesseract New Member


    I am contemplating (I think that's the best word - I'm not yet to the point of design or even a serious feasibility analysis) the construction of an unmanned, computer-controlled, solar-electric boat which ideally would be capable of long-duration offshore travel. My expertise is in electronics, software, and control; I'm completely inexperienced in naval architecture so I'm posting here looking for a sanity check on that side of things.

    I'm thinking the boat would have to be easily trailerable so it can be frequently pulled in and out of a lake for testing and subsequent modification during the control system development. Also it would be nice to keep the cost of the hull itself down since there would obviously be plenty of other money sinks on this kind of project. On the other hand greater length means higher hull speed and (probably more importantly) more area for solar panels. I am thinking something like 18 to 20 ft LOA.

    A catamaran seems like the obvious way to go since it allows a slender, efficient hullform while the wide beam (10 ft?) means plenty of space for the solar array. The non-self-righting aspect worries me a bit, especially since there would be no crew to attempt to right the boat - but on the other hand if the thing capsizes there's no crew to worry about saving, either.

    How worried do I need to be about wind catching the underside of the solar array and capsizing the boat, vs. a large wave doing it? Maybe some aerodynamic modeling would be necessary if this turns into a real design project?

    One option would be to repurpose the hulls of a Hobie or similar. Are they fairly efficient and seaworthy, or are there large gains in either efficiency or seaworthiness to be had by designing (or, more likely, engaging someone to design) something specifically for the purpose? I am assuming the centerboards would be mostly unnecessary for a powerboat and will just add drag; on the other hand some of the day-sailer cats seem to have no keels at all apart from their centerboards - will they even track straight without the centerboards?

    A 10x18 ft solar array is 16 m^2. If half the power goes into the (LiFePO4?) batteries so the boat can run all night, that leaves about a kilowatt for the motor assuming 120 W/m^2 from the array. Supposing the electronics, motor, and propeller are each 85% efficient, that means around 620 watts go into propulsion. Will that be sufficient to make good 5 knots or better?

    (Might it make sense to specify the motor to run at maybe 30 to 40 percent of its maximum power output at the efficient cruising speed, so that if necessary additional speed is available to evade an obstacle or try to outrun weather? Obviously this would rely on the batteries to supply the necessary energy and could only be sustained for a short time.)

    In terms of propulsion I am thinking two motors, each with its own batteries, charger, dedicated half of the solar array, etc. This allows half of the stuff to go in each hull, which is convenient, plus it provides redundancy (remember no one is on board to fix things if they break). As for propellers, I am aware that large diameter, high-aspect designs at low rpm will be the most efficient. Putting them under the hulls kills the trailerability unless special provisions are made (also fishing nets could be a concern); how about suspending them between the hulls on saildrive-type units as is often seen on pedal craft?

    Another possibility could be to put smaller props in Kort nozzles behind/under each hull - though this might force the issue of custom built hulls in order to get something reasonable for the mechanical and hydrodynamic interplay of keel, propshaft, and nozzle. Does the extra wetted surface of the nozzle kill the propeller efficiency gain? How about if the nozzles were steerable in lieu of rudders (and their attendant wetted area)? Another way to get rid of the rudders would be to rely on differential throttle control, but then the motors aren't redundant anymore.

    Thoughts? Am I completely crazy or only slightly?
  2. CDK
    Joined: Aug 2007
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    CDK retired engineer

    At first glance I'd say that you are.
    But you didn't state the purpose of this project, so I may change my opinion.
  3. rwatson
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    rwatson Senior Member

    Yes, the purpose is definitely a big factor, but I can think of a few potential uses.

    One thought that sprang to mind is that solar on a small boat can be somewhat problematic with cloud cover, salt exncrustation on collecters etc etc etc.

    You might like to consider some ancilliary power sources like wave power, wind generating, to make the craft even more self self reliant.
  4. tesseract
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    tesseract New Member

    I considered a sailboat at first, but it seemed likely to be more difficult to control (both in terms of the algorithms and in needing more actuators) and also more problematic in a storm (no one on board to, say, decide to reef the sails). Wind turbines have disappointingly low power output unless they are huge, and I think wave generation is even worse in terms of usable power per size. Maybe something could be done with e.g. Peltier modules to exploit the temperature difference between air and seawater? I think, though, that the best way to deal with cloud cover would be to have a few hundred miles' worth of batteries and to try to use advance weather predictions to stay in the sun.

    Salt encrustation is more vexing than clouds because it doesn't go away after a few days. :) Especially because the boat will be trying its best to stay out of the rain. Maybe an active cleaning system could be devised, like a desalinator hooked up to some spray nozzles or something.
  5. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Here's a much smaller version meant for a different purpose that is completely self righting:

    The video is very interesting. May give you some ideas, anyway.

    One issue would probably be debris like weed, fishing line, etc. tangling the prop. Need an overload shutoff mechanism and some way to keep from fouling or some way of clearing the prop.

    Hope this helps.


  6. permagne
    Joined: Aug 2009
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    Location: Norway

    permagne Junior Member

    Hello tesseract.

    I'm starting on a similar project as you although a bit smaller scale.

    I plan to use an unmanned boat for depth survey of lakes by using an echosounder connected to gps.

    I plan to start out with an RC ship to make the depth logging equipment work. Then attach an autopilot, and finaly develop something like you plan, a catamaran with solar array to provide energy to the system.

    I've ordered the ardupilot to connect to my Garmin 76S gps for autopiloting the boat while surveying.

    What are you planing to use to control your ship?
    Does it need to do some tasks or shall it just travel around?

    Good luck on your ship, and let us know about your progress.

    Per Magne
  7. masalai
    Joined: Oct 2007
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    masalai masalai

    There is also an autonomous voyager somewhere in the Pacific, made of very basic materials and seeking repair and return to its voyage by whoever finds it - I think someone is data-logging by snail mail reports of encounters, as visitors are invited aboard to have a look around and there they find further info and a request to tell where and when it was encountered, by no means small, and quite whimsical but that is all I recall...

  8. bntii
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    bntii Senior Member

    The number of times I have had to change course slightly to avoid fishing boats leads me to believe that you will need to be cautious of other vessels.
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