1 vs 2 props for efficiency

Discussion in 'Props' started by FranzJoseph, Jan 15, 2018.

  1. FranzJoseph
    Joined: Jan 2018
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    Location: Melbourne, Austrlalia

    FranzJoseph New Member

    First up: I am not a prop expert at all.
    the design goal is do find the most efficient solution for a long narrow hull design with a LWL of 24m and a design weight of 45t
    Theory tells me that one slow turning prop is the best for efficiency.

    I believe my power options are:
    1) Either a 500HP diesel with about 1.3m diameter prop
    2) Or 2x 250HP diesel with about 0.9m diameter props
    To save weight, I would like a modern relatively high RPM engine,
    I found 260HP being available rated 3600RPM with 600NM torque at 1800RPM.
    Then there is a 6L engine with 440Hp at 2700 - 3200RPM and 1200NM torque at 2100 to 2500RPM
    Both option have about the same torque, same HP, same prop area and weigh the same. The idea is go hybrid with 1 (or 2) electrical motor(s) for slow speed and the diesel for the higher speeds via a gear box.

    Questions: Which option will be more efficient at cruising speed of about 12kn and what RPM should the prop have at that speed? Any recommendations or experience out there?

    No need to consider practicalities like maneuverability, draft and redundancy, just efficiency.
  2. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    Is that based on an analysis of propeller design charts, or is it "common knowledge"?
    Have you tried analyzing both options using the appropriate propeller design charts and procedures, and then comparing the results?

    Do you have a naval architecture or related background? Is this question related to a university project or similar?
  3. FranzJoseph
    Joined: Jan 2018
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    Location: Melbourne, Austrlalia

    FranzJoseph New Member

    Hi DCockey:
    Based on physics, I don't know if it is common knowledge.
    Haven't tried my own analysis yet since I am not an expert. I thought someone her might have done that already, why reinvent the wheel.
    It's not related to any Uni project.
  4. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Then you can forget your objective.

    Since a fixed pitch prop works best at one target speed only, any deviation and it is less efficient. Thus you need to decide what will be your most common speed and design your prop/engine etc it to that. Any other speed will be...aahh...well...it basically does the job. Unless you go for a CPP.

  5. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    There is not a great deal of difference in your case. With a single 1.3 m diameter prop, the appropriate RPM would be about 500 and the prop efficiency about 57%. For two 0.9m props the appropriate RPM would be about 700 and the efficiency about 1-2% less. So the decision can be based on other factors. Perhaps the maneuverability is a factor, or weight or cost or benefit of redundancy? There may be a drag penalty associated with have two shafts, rudders, struts, etc?
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