Need help with propulsion calculations

Discussion in 'Props' started by gerald2502, Oct 13, 2018.

  1. gerald2502
    Joined: Oct 2018
    Posts: 2
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    Location: Singapore

    gerald2502 New Member

    I'm currently designing an electric propulsion system for an Autonomous Surface Vehicle for my school project. The vessel is of a twin-hull design, and has to carry a load of 20kg. I uploaded a photo along with its rough dimensions below.
    The vessel is intended to be designed for endurance, with an operational time of 5hrs with a speed no more than 1 m/s. It must also have reverse capability. Because of this, I have opted to go for a conventional twin screw propeller design, as I have read that it is more efficient at lower speeds. I plan to enable reverse capability by reversing the polarity of the dc motors (via h-bridges) to make the propellers spin in the opposite direction.
    I am however not well-versed in any matters related to boat design (I am a computing science student) and am having difficulty selecting a suitable powerplant and propeller. I have tried reading Carlton's Marine Propellers and Propulsion but I can't make sense out of it. How do I go about calculating the power plants and propellers type/size that I need?
     

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  2. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    The weight of the boat is important. What are you building it out of ?

    That hull design is very inefficient, and will cut your running time down a lot.

    Are you restricted to a maximum of 1.3 m long ?
     
  3. gerald2502
    Joined: Oct 2018
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    Location: Singapore

    gerald2502 New Member

    for now the intention is to use inflatable PVC pontoon tubes framed with aluminium rails. There is no restriction on the boat length!
    Should the design of the hull be completed before the incorporation of the propulsion system?
     
  4. dreamingbarrierreef
    Joined: Oct 2018
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    Location: Colorado

    dreamingbarrierreef dreamingbarreef

  5. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    I would estimate a maximum engine and battery weight, then design the more efficient hulls around it. The size of the engine and the weight of the battery might be limited by practical considerations, but a huge number of batteries would be the key to winning an endurance race.

    Inflatable hulls are the not a good shape for efficient propulsion.

    The longer the hulls, the more efficient, but there must be some practical limits.
     

  6. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Nice sketch.
    Welcome to the forum.

    So max allowable speed is 1 m/s?
    5 hours endurance minimum.
    20kg payload?
    No solar panels?
    No other limits?
    Please post ALL the rules and requirements, restrictions and regulations.

    As to your original question, a single motor-prop configuration would be best.
    You'll need enough power to move ~22kg through the water at 0.99999 m/s.
    That's assuming distance counts, or is it just to remain operational through the 5 hour minimum.
    We need to see all the rules.

    But a long, skinny monohull is going to be more efficient than a catamaran.
    What is the payload, anything weighing 20 kg?
    If so, put lead shot in a bulb keel with your motor behind it driving a propeller out the back of the pod.

    A small, brushless motor, probably from the radio controlled model world and a small,
    two bladed prop from a small R/C airplane would be good.

    Rules? Budget? Timeline?
     
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