Choosing the best marine hull for an autonomous boat

Discussion in 'Boat Design' started by Anelito, Aug 6, 2018.

  1. HJS
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    HJS Member

    Since the waves can be enormously high in relation to the size of the boat, completely different hulls may be appropriate. Frequently experiences from ordinary boats might be questioned.
     
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  2. bhnautika
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    bhnautika Senior Member

    Given the deck area requirements ( for solar panel) a catamaran maybe a better fit.
     
  3. HJS
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    HJS Member

    It is difficult to make it self-righting. Better to choose a longer boat.
     
  4. Anelito
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    Anelito Junior Member

    Up to now the best (and affordable) bendable solar cells I have found can give 4.7W with an occupancy of 156x156mm.
    A square meter would give at best 193W. In any case I would not go lower than 100W, which equals to 21 cells and a deck space of half a sq. m
     
  5. tlouth7
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    tlouth7 Senior Member

    As I said above, bendy need not be a requirement for your panels.

    At 1.2m LOA you will struggle to get 0.5m^2 in a reasonable beam - a rectangular deck with full coverage (clearly not achievable) would require 0.42m beam. Therefore you need some combination of: more length (to increase area at reasonable fineness), better panels, lower power requirement.
     
  6. Anelito
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    Anelito Junior Member

    I can remove marine sensors so that my only requirements are 4W for the electronics + 3W for the navigation light. Would a wind vane be of any use?
     
  7. tlouth7
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    tlouth7 Senior Member

    And motive power....

    I wouldn't recommend a windvane, too easily damaged. Out of interest, how have you sized your motor, is this based on drag values for some prototype hullform? Also, how can you remove sensors? Surely that would result in a lot of crashing into things? If you went small enough (say 300mm LOA) I guess you could do without sensors or navigation lights on the basis that no-one will notice if they crash into you, but there probably wouldn't be room for the computers, and top speed would be very low.

    Sorry to sound so negative, but some basic back-of-the-envelope calculations show that the power needed for the motors far exceeds the generation capacity that you can fit on a hull of this size* (regardless of its exact geometry). A longer vessel gives you more deck area without an excessive drag penalty (for fixed design speed), but obviously increases cost. Also as size goes up the (fixed) sensor overhead gets smaller relative to the (increasing) motor power, so more of your solar energy is pushing the boat forward.

    * Based on the data you have given. Lower motor power or more efficient panels could solve this.

    Edit: this still sounds negative, my point is that this can be done, but until you reconcile your power requirements there is not a great deal of point trying to find a specific hull.
     
  8. Anelito
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    Anelito Junior Member

    I mean removing the temperature, speed and depth sensor and relying solely on the GPS derived speed over ground. Depth could also be useful to avoid beaching, simply finding decreasing gradients in depth values datasets over the last hour. The sensor itself consumes 60W and to have valuable data it should be activated 5-10 times per hour.

    The motor I used for planning is a speedboat one, mounted on 1m LOA racing motor boats, probably an overkill for my needs. A longer vessel can be ok whilst staying within 1.5m, then it will become very difficult to store and to transport for testing purposes.

    As I wrote previously, I am not totally sure solar power alone can provide enough reliability fur such a project, that is why at the same time I am also considering other solutions, a rigid wingsail or a mixed approach (like the C-enduro, see pic below).
    [​IMG]
    [​IMG]
     
  9. bhnautika
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    bhnautika Senior Member

    My thoughts on getting flat solar panels but getting self righting in the cat and no fin keel on the mono. Enclose panels with curved clear plastic. auto mono.jpg auto cat.jpg
     
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  10. HJS
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    HJS Member

    A boxkeel can be an effective alternative.
    Boxkeel 2.jpg
    JS
     
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  11. bhnautika
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    bhnautika Senior Member

    HJS I agree (as mentioned in post 33) but getting enough deck area on a mono hull (beam/length) for the solar panels seems to be a choke point. Maybe some cells built into the topsides of the hull?
     
  12. BlueBell
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    BlueBell Ahhhhh...

    Good point about the shallow keel,
    but his needs are minimized roll for sensor optimization.
    Also, a larger prop is needed to improve efficiency due to power issues.
    This indicates a deeper keel with perhaps a self levelling canard.

    Two rows of 156mm panels, 10 panels long (1.56m) would give a 10:1 l/b deck ratio
    and a nice 20:1 l:b ratio hull, hour glassing into a roll damping keel housing
    motor, prop, and batteries. Keeping all the electronics up in the domed,
    foam hull, close to the sensors, and above the water line.

    Placing a clear plastic dome over the panels will significantly reduce their
    power output, as much as I like the self righting catamaran idea.
    The panels could simply be mounted on the dome but
    it may not provide the roll stability his sensors are looking for...?
    His MPPT controller will take advantage of domed panels.
    (But not to the extent shown in the graphic though.)

    However, all this postulating is pointless without a prioritized SOR.
     
  13. HJS
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    HJS Member

    l / b 10/1 or 20/1 is not optimal. It is much more than just length - beam ratios that determine a ship's efficiency.
    Do not guess, calculate!
    JS
     
  14. BlueBell
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    BlueBell Ahhhhh...

    Oh, that's helpful. Not.

    I would have expected better of a Sweed.
     

  15. HJS
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    HJS Member

    Just a sketch.
    Autonom 1.jpg Autonom 2.jpg
    JS
     
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