Choosing the best marine hull for an autonomous boat

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

  1. IronPrice
    Joined: Jul 2017
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    IronPrice Senior Member

    It might be worth researching ocean gliders. They are autonomous craft with active stability wings that allow very efficient motion. You want your craft to stay on the surface, which is a difference from ocean gliders, but you may still learn something useful.
     
  2. HJS
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    Location: 59 45 51 N 019 02 15 E

    HJS Member

    Back to basic.
    Boat length 1.2 meters and desired average speed 2 knots.
    This corresponds to a relative velocity around FnL 0.31. This is an extremely high average speed for a 1.2 meter boat.
    Seacharger is twice as long and has an average speed of around 2 knots. This corresponds to FnL 0.22.
    For a 1.2 meter boat, it would be an average speed of 1.5 knots at best, never 2 knots.

    Sorry
    JS
     
  3. Anelito
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    Anelito Junior Member

    I guess I should say 1 knot for the average speed. Then I can check if the boat is not making any progress toward the next waypoint, the engine(s) will be stopped, as it means there is a strong current.
     
  4. HJS
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    HJS Member

    Do not guess, calculate! This is especially true in such a project to count on the effectiveness. The size and shape of the boat and propeller must be harmonized.
     
  5. Earl Boebert
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    Earl Boebert Senior Member

    The "Seal Flipper" keel shape, used in model yachts in the late 1940s, may be of interest. It offers the gentle sloped leading edge of a full-keel boat without as much wetted area. Proven to handle weed-filled ponds well, perhaps also good for avoiding snagging things.

    Cheers,

    Earl
     

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  6. HJS
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    HJS Member

    If the boat's length is about 1.2 meters, I would choose a Marblehead.

    Sass Marblehead.jpg
    JS
     
  7. Anelito
    Joined: Aug 2018
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    Anelito Junior Member

    Considering the boat water line length of 1.29m, the calculated boat speed is 1.3 knots.
    Surely the Marblehead class is perfectly fitted, at the moment I am following a build-plan from www.nonsolovele.com/2013Plans.shtml (Studio 2), I just fear the hull to be too small to host enough solar cells. I will redesign the keel to follow your model as it is much less subject to collecting seaweed and other garbage.
     
  8. HJS
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    HJS Member

    I do not understand the positive intent of the rounded stern in Studio 2. In all of my designs is the stern always more or less flat with a pronounced sharp edge and with the verticals almost completely straight. All to create as small stern wave as possible.

    Studio 2.jpg
    JS
     
  9. Anelito
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    Anelito Junior Member

    Yes that's strange, I wonder if those plans are properly tested or just loosely inspired by existing hulls.
    Do you have build plans of the boat you posted? Thank you
     
  10. HJS
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    HJS Member

    The boat I showed is a fifty year old design and thus outdated. When drawn it was the best I could achieve. It won the European Championship. Instead of taking an existing design, I propose to design a hull optimized for your needs.
    JS
     
  11. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    HJS
    50 year old plans can still be very relevant. There is no expiration date on great design. Newly developed materials may make options not previously available viable.

    Displacement vessels are most efficient with a transom which allows water to return softly. It is easier to get a boat planning if transom ends abruptly with shape trailing edge. Models copy styling from full-size craft to their detriment.

    Anelito

    Why worry if solar panels fit?
    Don't try to fit into existing design. First determine dementions and weight of all required equipment, including:
    +solar cells
    +motors
    +batteries
    +control electronics
    +sensors
    +antennas

    Only then can you efficiently find, design or choose appropriate hull to house them.
     
  12. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I told him the same thing but I think he wants plans in order to program a CNC machine.
    Although at 1.3 knots, it's really not going to make any difference.
    I think his real challenge is supporting the panels and battery he needs for his huge power requirements, ~145 watts.
    A custom, purpose-built hull, would better facilitate that.
    I'm not sure he's really that sincere about this project.
     
  13. Anelito
    Joined: Aug 2018
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    Anelito Junior Member

    I believe the real challenge will be to find a good balance among battery (weight) and solar panels; monocrystalline cells are quite fragile and cannot be easily bent, thus the number fitting on a curved top deck will be much less.
    The electronics fits all in two small watertight boxes measuring 8x8x14cm, while for the GPS and Lidar I will have a 5cm radius plexiglass dome placed on the centre of the boat.
    I admit from solar cell wattage calculations the boat will surely face whole days of power loss, since a simple storm or cloudy day can impact very heavily solar panels, that's why I started considering a small sail too, but I'll think about that later on.
    The battery I would like to mount onboard is a pair of 5000 mAh 4S LiPo, half a kilo each. If I use them as part of the keel, I will save a 1kg.

    It took Southampton University team one whole year to deign their boat hull given similar requirements as mine, and they were supported by very competent people and award-winning professors of marine technology. As ship design is actually a science that requires years of experience which I don't have, I prefer to stick to an already existent and proven design to slightly modify it according to my needs.
     
  14. Blueknarr
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    Blueknarr Senior Member

    Great plan. But still missing critical information. Hull most likely picked based on solar panels support requirements.
     

  15. tlouth7
    Joined: Jun 2013
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    tlouth7 Senior Member

    Why have a keel? Surely that is a carry-over from sailing designs which require significant stability to withstand sailing loads. This just needs enough to keep the solar panels facing up most of the time, which could be achieved with judicious weight placement within the hull, thus eliminating the structural challenges of a ballast keel and the catch-point. A minimally stable design that rolls over every so often might even be good as it would clear salt deposits from the deck. Note that full size motor boats do not have keels.

    Deck camber is not required to achieve good runoff - just have it in two halves angled slightly (like a very shallow pitched roof). This will be easier to construct, and well suited to mounting flat solar panels. It does result in a non-flat sheer (as does a cambered deck) which makes hull construction slightly harder.

    I would want to know:
    - displacement
    - required deck area
    - length limit if applicable
    - design speed

    Then optimise for:
    - self righting
    - protected propulsion/steering (consider that you will want your prop in line with the motor, I would favour a tunnel)
    - low steering power requirement
    - low drag
     
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