Effect Rudder

Discussion in 'Hydrodynamics and Aerodynamics' started by sottorf, Jul 10, 2012.

  1. jehardiman
    Joined: Aug 2004
    Posts: 2,982
    Likes: 505, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Do not confuse what Servogear is selling, i.e. the "Ecoflow Propulsor" with the reasons, or effects, of fitting a contra-guide rudder.

    Contra-guide rudders and sternposts were mounted to reduce tortional loading and vibration on ship structures for heavily loaded blades with little or no skew. They work by interupting the wake singularity and because they slightly improve alpha i and beta they do give some small increase in efficency over a straight rudder or sternpost. However, the wake flow of the hull itself causes a tremndous increase in efficiency compared to an open wheel and they do increase overall drag so they became obsolete when greater amounts of blade skew became allowed by the rules.

    Mounting a prop in a wake effector, such as behind a body, in a tunnel or a duct perhaps with pre and/or post swirl vanes, has always been a way to manipulate the Vo and therefore eta 0, and a very fine line has to be drawn about which specific efficiency you are claiming. FWIW, I have made a propulsion set up where I could claim an efficency increase of 115% over the open water data. In fact, read the new site carefully and you will see the following quotes (note: my bolding):

    and
    So nothing new here, indeed based upon my own experience with a 88% efficient wheel, I will bet the contra-guide wake manipulation is necessary to prevent blade breakage due to bending loads brought about the thin section and extreme skew of the wheel.
     
  2. Servo Gear
    Joined: Mar 2009
    Posts: 7
    Likes: 2, Points: 0, Legacy Rep: 23
    Location: Norway

    Servo Gear Junior Member

    Hi,

    The efficiency for the other propulsion systems is taken from the attached Marintek presentation (page 29).
    Servogear efficiencies are taken from several series of model tests (SSPA) and full scale verification tests (Marintek), and the averages form the line on the graph. As the reports are proprietary information I can’t share them with you.
    If you have a specific project we are happy to calculate the efficiency for you.

    Best regards,

    Petter




     

    Attached Files:

  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I can't read the Norwegian language, but on the page 16 of Marintek's paper there is a graph which apparently shows the propulsive efficiencies of ServoGear's drive, and it gives values between 64% and 68% in the speed range between 30 and 40 kts. It's much less than what's shown in your page, and much more in line with other propulsion types in that paper. :confused: Do you perhaps have an English version of that presentation, so that we can better understand what are those figures related to?
     
  4. Servo Gear
    Joined: Mar 2009
    Posts: 7
    Likes: 2, Points: 0, Legacy Rep: 23
    Location: Norway

    Servo Gear Junior Member

    You can always phone me if you have questions, my number is our home page.
    I am sorry I don’t have an English version of the paper, maybe the author will have one?
    For the specific project on page 16 the diameter was less than optimum so the efficiency was less than what was shown in the graph on the web site. As you know there many parameters in a project that is controlled by the propeller designer:)
    Hope this helps.
     

  5. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 252, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Yes, I understand that. Though the same is valid for any type of propulsor device. Like I said, a carefully matched and maintained open prop can have over 70% efficiencies on smaller vessels and over 80% on bigger ones, so it is necessary to compare apples to apples and not to oranges when talking technical.

    What I have gathered from the discussion so far and from your last reply is that the comparative graph in your page shows the performance of your optimized propulsor (an apple) vs. non-optimized versions of other propulsors (the oranges). Which is not quite fair, but it's a common part of the marketing game - nearly everyone does it. When not optimized, even your prop arrangement will go down to 65%-68% efficiency, which is perfectly logical.

    Cheers and all the best

    D.
     
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